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rfc:rfc1507

Network Working Group C. Kaufman Request for Comments: 1507 Digital Equipment Corporation

                                                        September 1993
                                DASS
            Distributed Authentication Security Service

Status of this Memo

 This memo defines an Experimental Protocol for the Internet
 community.  It does not specify an Internet standard.  Discussion and
 suggestions for improvement are requested.  Please refer to the
 current edition of the "Internet Official Protocol Standards" for the
 standardization state and status of this protocol.  Distribution of
 this memo is unlimited.

Table of Contents

  1.   Introduction ................................................ 2
       1.1  What is DASS? .......................................... 2
       1.2  Central Concepts ....................................... 4
       1.3  What This Document Won't Tell You ..................... 11
       1.4  The Relationship between DASS and ISO Standards ....... 17
       1.5  An Authentication Walkthrough ......................... 20
  2.   Services Used .............................................. 25
       2.1  Time Service .......................................... 25
       2.2  Random Numbers ........................................ 26
       2.3  Naming Service ........................................ 26
  3.   Services Provided .......................................... 37
       3.1  Certificate Contents .................................. 38
       3.2  Encrypted Private Key Structure ....................... 40
       3.3  Authentication Tokens ................................. 40
       3.4  Credentials ........................................... 43
       3.5  CA State .............................................. 47
       3.6  Data types used in the routines ....................... 47
       3.7  Error conditions ...................................... 49
       3.8  Certificate Maintenance Functions ..................... 49
       3.9  Credential Maintenance Functions ...................... 55
       3.10 Authentication Procedures ............................. 63
       3.11 DASSlessness Determination Functions .................. 87
  4.   Certificate and message formats ............................ 89
       4.1  ASN.1 encodings ....................................... 89
       4.2  Encoding Rules ........................................ 96
       4.3  Version numbers and forward compatibility ............. 96
       4.4  Cryptographic Encodings ............................... 97
  Annex A - Typical Usage ........................................ 101
       A.1  Creating a CA ........................................ 101

Kaufman [Page 1] RFC 1507 DASS September 1993

       A.2  Creating a User Principal ............................ 102
       A.3  Creating a Server Principal .......................... 103
       A.4  Booting a Server Principal ........................... 103
       A.5  A user logs on to the network ........................ 103
       A.6  An Rlogin (TCP/IP) connection is made ................ 104
       A.7  A Transport-Independent Connection ................... 104
  Annex B - Support of the GSSAPI ................................ 104
       B.1  Summary of GSSAPI .................................... 105
       B.2  Implementation of GSSAPI over DASS ................... 106
       B.3  Syntax ............................................... 110
  Annex C - Imported ASN.1 definitions ........................... 112
  Glossary ....................................................... 114
 Security Considerations ......................................... 119
 Author's Address ................................................ 119
 Figures
  Figure 1 - Authentication Exchange Overview ....................  24

1. Introduction

1.1 What is DASS?

 Authentication is a security service. The goal of authentication is
 to reliably learn the name of the originator of a message or request.
 The classic way by which people authenticate to computers (and by
 which computers authenticate to one another) is by supplying a
 password.  There are a number of problems with existing password
 based schemes which DASS attempts to solve.  The goal of DASS is to
 provide authentication services in a distributed environment which
 are both more secure (more difficult for a bad guy to impersonate a
 good guy) and easier to use than existing mechanisms.
 In a distributed environment, authentication is particularly
 challenging.  Users do not simply log on to one machine and use
 resources there.  Users start processes on one machine which may
 request services on another.  In some cases, the second system must
 request services from a third system on behalf of the user.  Further,
 given current network technology, it is fairly easy to eavesdrop on
 conversations between computers and pick up any passwords that might
 be going by.
 DASS uses cryptographic mechanisms to provide "strong, mutual"
 authentication.  Mutual authentication means that the two parties
 communicating each reliably learn the name of the other.  Strong
 authentication means that in the exchange neither obtains any
 information that it could use to impersonate the other to a third
 party.  This can't be done with passwords alone.  Mutual
 authentication can be done with passwords by having a "sign" and a
 "counter-sign" which the two parties must utter to assure one another

Kaufman [Page 2] RFC 1507 DASS September 1993

 of their identities.  But whichever party speaks first reveals
 information which can be used by the second (unauthenticated) party
 to impersonate it.  Longer sequences (often seen in spy movies)
 cannot solve the problem in general.  Further, anyone who can
 eavesdrop on the conversation can impersonate either party in a
 subsequent conversation (unless passwords are only good once).
 Cryptography provides a means whereby one can prove knowledge of a
 secret without revealing it.  People cannot execute cryptographic
 algorithms in their heads, and thus cannot strongly authenticate to
 computers directly.  DASS lays the groundwork for "smart cards":
 microcomputers sealed in credit cards which when activated by a PIN
 will strongly authenticate to a computer.  Until smart cards are
 available, the first link from a user to a DASS node remains
 vulnerable to eavesdropping.  DASS mechanisms are constructed so that
 after the initial authentication, smart card or password based
 authentication looks the same.
 Today, systems are constructed to think of user identities in terms
 of accounts on individual computers.  If I have accounts on ten
 machines, there is no way a priori to see that those ten accounts all
 belong to the same individual.  If I want to be able to access a
 resource through any of the ten machines, I must tell the resource
 about all ten accounts.  I must also tell the resource when I get an
 eleventh account.
 DASS supports the concept of global identity and network login.  A
 user is assigned a name from a global namespace and that name will be
 recognized by any node in the network.  (In some cases, a resource
 may be configured as accessible only by a particular user acting
 through a particular node.  That is an access control decision, and
 it is supported by DASS, but the user is still known by his global
 identity).  From a practical point of view, this means that a user
 can have a single password (or smart card) which can be used on all
 systems which allow him access and access control mechanisms can
 conveniently give access to a user through any computer the user
 happens to be logged into.  Because a single user secret is good on
 all systems, it should never be necessary for a user to enter a
 password other than at initial login.  Because cryptographic
 mechanisms are used, the password should never appear on the network
 beyond the initial login node.
 DASS was designed as a component of the Distributed System Security
 Architecture (DSSA) (see "The Digital Distributed System Security
 Architecture" by M. Gasser, A. Goldstein, C. Kaufman, and B. Lampson,
 1989 National Computer Security Conference).  It is a goal of DSSA
 that access control on all systems be based on users' global names
 and the concept of "accounts" on computers eventually be replaced
 with unnamed rights to execute processes on those computers.  Until

Kaufman [Page 3] RFC 1507 DASS September 1993

 this happens, computers will continue to support the concept of
 "local accounts" and access controls on resources on those systems
 will still be based on those accounts.  There is today within the
 Berkeley rtools running over the Internet Protocol suite the concept
 of a ".rhosts database" which gives access to local accounts from
 remote accounts.  We envision that those databases will be extended
 to support granting access to local accounts based on DASS global
 names as a bridge between the past and the future.  DASS should
 greatly simplify the administration of those databases for the
 (presumably common) case where a user should be granted access to an
 account ignoring his choice of intermediate systems.

1.2 Central Concepts

1.2.1 Strong Authentication with Public Keys

 DASS makes heavy use of the RSA Public Key cryptosystem.  The
 important properties of the RSA algorithms for purposes of this
 discussion are:
  1. It supports the creation of a public/private key pair, where

operations with one key of the pair reverse the operations of

    the other, but it is computationally infeasible to derive the
    private key from the public key.
  1. It supports the "signing" of a message with the private key,

after which anyone knowing the public key can "verify" the

    signature and know that it was constructed with knowledge of
    the private key and that the message was not subsequently
    altered.
  1. It supports the "enciphering" of a message by anyone knowing

the public key such that only someone with knowledge of the

    private key can recover the message.
 With access to the RSA algorithms, it is easy to see how one could
 construct a "strong" authentication mechanism.  Each "principal"
 (user or computer) would construct a public/private key pair, publish
 the public key, and keep secret the private key.  To authenticate to
 you, I would write a message, sign it with my private key, and send
 it to you.  You would verify the message using my public key and know
 the message came from me.  If mutual authentication were desired, you
 could create an acknowledgment and sign it with your private key; I
 could verify it with your public key and I would know you received my
 message.
 The authentication algorithms used by DASS are considerably more
 complex than those described in the paragraph above in order to deal

Kaufman [Page 4] RFC 1507 DASS September 1993

 with a large number of practical concerns including subtle security
 threats.  Some of these are discussed below.

1.2.2 Timestamps vs. Challenge/Response

 Cryptosystems give you the ability to sign messages so that the
 receiver has assurance that the signer of the message knew some
 cryptographic secret.  Free-standing public key based authentication
 is sufficiently expensive that it is unlikely that anyone would want
 to sign every message of an interactive communication, and even if
 they did they would still face the threat of someone rearranging the
 messages or playing them multiple times.  Authentication generally
 takes place in the context of establishing some sort of "connection,"
 where a conversation will ensue under the auspices of the single
 peer-entity authentication.  This connection might be
 cryptographically protected against modification or reordering of the
 messages, but any such protection would be largely independent of the
 authentication which occurred at the start of the connection.  DASS
 provides as a side effect of authentication the provision of a shared
 key which may be used for this purpose.
 If in our simple minded authentication above, I signed the message
 "It's really me!" with my private key and sent it to you, you could
 verify the signature and know the message came from me and give the
 connection in which this message arrived access to my resources.
 Anyone watching this message over the network, however, could replay
 it to any server (just like a password!) and impersonate me.  It is
 important that the message I send you only be accepted by you and
 only once.  I can prevent the message from being useful at any other
 server by including your name in the message.  You will only accept
 the message if you see your name in it.  Keeping you from accepting
 the message twice is harder.
 There are two "standard" ways of providing this replay protection.
 One is called challenge/response and the other is called timestamp-
 based.  In a challenge response type scheme, I tell you I want to
 authenticate, you generate a "challenge" (generally a number), and I
 include the challenge in the message I sign.  You will only accept a
 message if it contains the recently generated challenge and you will
 make sure you never issue the same challenge to me twice (either by
 using a sequence number, a timestamp, or a random number big enough
 that the probability of a duplicate is negligible).  In the
 timestamp-based scheme, I include the current time in my message.
 You have a rule that you will not accept messages more than - say -
 five minutes old and you keep track of all messages you've seen in
 the last five minutes.  If someone replays the message within five
 minutes, you will reject it because you will remember you've seen it
 before; if someone replays it after five minutes, you will reject it

Kaufman [Page 5] RFC 1507 DASS September 1993

 as timed out.
 The disadvantage of the challenge/response based scheme is that it
 requires extra messages.  While one-way authentication could
 otherwise be done with a single message and mutual authentication
 with one message in each direction, the challenge/response scheme
 always requires at least three messages.
 The disadvantage of the timestamp-based scheme is that it requires
 secure synchronized time.  If our clocks drift apart by more than
 five minutes, you will reject all of my attempts to authenticate.  If
 a network time service spoofer can convince you to turn back your
 clock and then subsequently replays an expired message, you will
 accept it again.  The multicast nature of existing distributed time
 services and the likelihood of detection make this an unlikely
 threat, but it must be considered in any analysis of the security of
 the scheme.  The timestamp scheme also requires the server to keep
 state about all messages seen in the clock skew interval.  To be
 secure, this must be kept on stable storage (unless rebooting takes
 longer than the permitted clock skew interval).
 DASS uses the timestamp-based scheme.  The primary motivations behind
 this decision were so that authentication messages could be
 "piggybacked" on existing connection establishment messages and so
 that DASS would fit within the same "form factor" (number and
 direction of messages) as Kerberos.

1.2.3 Delegation

 In a distributed environment, authentication alone is not enough.
 When I log onto a computer, not only do I want to prove my identity
 to that computer, I want to use that computer to access network
 resources (e.g., file systems, database systems) on my behalf.  My
 files should (normally) be protected so that I can access them
 through any node I log in through.  DASS allows them to be so
 protected without allowing all of the systems that I might ever use
 to access those files in my absence.  In the process of logging in,
 my password gives my login node access to my RSA secret.  It can use
 that secret to "impersonate" me on any requests it makes on my
 behalf.  It should forget all secrets associated with me when I log
 off.  This limits the trust placed in computer systems.  If someone
 takes control of a computer, they can impersonate all people who use
 that computer after it is taken over but no others.
 Normally when I access a network service, I want to strongly
 authenticate to it.  That is, I want to prove my identity to that
 service, but I don't want to allow that service to learn anything
 that would allow it to impersonate me.  This allows me to use a

Kaufman [Page 6] RFC 1507 DASS September 1993

 service without trusting it for more than the service it is
 delivering.  When using some services, for example remote login
 services, I may want that service to act on my behalf in calling
 additional services.  DASS provides a mechanism whereby I can pass
 secrets to such services that allow them to impersonate me.
 Future versions of this architecture may allow "limited delegation"
 so that a user may delegate to a server only those rights the server
 needs to carry out the user's wishes.  This version  can limit
 delegation only in terms of time.  The information a user gives a
 server (other than the initial login node) can be used to impersonate
 the user but only for a limited period of time.  Smart cards will
 permit that time limitation to apply to the initial login node as
 well.

1.2.4 Certification Authorities

 A flaw in the strong authentication mechanism described above is that
 it assumes that every "principal" (user and node) knows the public
 key of every other principal it wants to authenticate.  If I can fool
 a server into thinking my public key is actually your public key, I
 can impersonate you by signing a message, saying it is from you, and
 having the server verify the message with what it thinks is your
 public key.
 To avoid the need to securely install the public key of every
 principal in the database of every other principal, the concept of a
 "Certification Authority" was invented.  A certification authority is
 a principal trusted to act as an introduction service.  Each
 principal goes to the certification authority, presents its public
 key, and proves it has a particular name (the exact mechanisms for
 this vary with the type of principal and the level of security to be
 provided).  The CA then creates a "certificate" which is a message
 containing the name and public key of the principal, an expiration
 date, and bookkeeping information signed by the CA's private key.
 All "subscribers" to a particular CA can then be authenticated to one
 another by presenting their certificates and proving knowledge of the
 corresponding secret.  CAs need only act when new principals are
 being named and new private keys created, so that can be maintained
 under tight physical security.
 The two problems with the scheme as described so far are "revocation"
 and "scaleability".

1.2.4.1 Certificate Revocation

 Revocation is the process of announcing that a key has (or may have)
 fallen into the wrong hands and should no longer be accepted as proof

Kaufman [Page 7] RFC 1507 DASS September 1993

 of some particular identity.  With certificates as described above,
 someone who learns your secret and your certificate can impersonate
 you indefinitely - even after you have learned of the compromise.  It
 lacks the ability corresponding to changing your password.  DASS
 supports two independent mechanisms for revoking certificates. In the
 future, a third may be added.
 One method for revocation is using timeouts and renewals of
 certificates.  Part of the signed message which is a certificate may
 be a time after which the certificate should not be believed.
 Periodically, the CA would renew certificates by signing one with a
 later timeout.  If a key were compromised, a new key would be
 generated and a new certificate signed.  The old certificate would
 only be valid until its timeout.  Timeouts are not perfect revocation
 mechanisms because they provide only slow revocation (timeouts are
 typically measured in months for the load on the CA and communication
 with users to be kept manageable) and they depend on servers having
 an accurate source of the current time.  Someone who can trick a
 server into turning back its clock can use expired certificates.
 The second method is by listing all non-revoked certificates in the
 naming service and believing only certificates found there.  The
 advantage of this method is that it is almost immediate (the only
 delay is for name service "skulking" and caching delays).  The
 disadvantages are: (1) the availability of authentication is only as
 good as the availability of the naming service and (2) the security
 of revocation is only as good as the security of the naming service.
 A third method for revocation - not currently supported by DASS - is
 for certification authorities to periodically issue "revocation
 lists" which list certificates which should no longer be accepted.

1.2.4.2 Certification Authority Hierarchy

 While using a certification authority as an introduction service
 scales much better than having every principal learn the public key
 of every other principal by some out of band means, it has the
 problem that it creates a central point of trust.  The certification
 authority can impersonate any principal by inventing a new key and
 creating a certificate stating that the new key represents the
 principal.  In a large organization, there may be no individual who
 is sufficiently trusted to operate the CA.  Even if there were, in a
 large organization it would be impractical to have every individual
 authenticate to that single person.  Replicating the CA solves the
 availability problem but makes the trust problem worse.  When
 authentication is to be used in a global context - between companies
 - the concept of a single CA is untenable.

Kaufman [Page 8] RFC 1507 DASS September 1993

 DASS addresses this problem by creating a hierarchy of CAs.  The CA
 hierarchy is tied to the naming hierarchy.  For each directory in the
 namespace, there is a single CA responsible for certifying the public
 keys of its members.  That CA will also certify the public keys of
 the CAs of all child directories and of the CA of the parent
 directory.  With this cross-certification, it is possible knowing the
 public key of any CA to verify the public keys of a series of
 intermediate CAs and finally to verify the public key of any
 principal.
 Because the CA hierarchy is tied to the naming hierarchy, the trust
 placed in any individual CA is limited.  If a CA is compromised, it
 can impersonate any of the principals listed in its directory, but it
 cannot impersonate arbitrary principals.
 DASS provides mechanisms for every principal to know the public key
 of its "parent" CA - the CA controlling the directory in which it is
 named.  The result is the following rules for the implications of a
 compromised CA:
  a) A CA can impersonate any principal named in its directory.
  b) A CA can impersonate any principal to a server named in its
     directory.
  c) A CA can impersonate any principal named in a subdirectory to
     any server not named in the same subdirectory.
  d) A CA can impersonate to any server in a subdirectory any
     principal not named in the same subdirectory.
 The implication is that a compromise low in the naming tree will
 compromise all principals below that directory while a compromise
 high in the naming tree will compromise only the authentication of
 principals far apart in the naming hierarchy.  In particular, when
 multiple organizations share a namespace (as they do in the case of
 X.500), the compromise of a CA in one organization can not result in
 false authentication within another organization.
 DASS uses the X.500 directory hierarchy for principal naming.  At the
 top of the hierarchy are names of countries.  National authorities
 are not expected to establish certification authorities (at least
 initially), so an alternative mechanism must be used to authenticate
 entities "distant" in the naming hierarchy.  The mechanism for this
 in DASS is the "cross-certificate" where a CA certifies the public
 key for some CA or principal not its parent or child.  By limiting
 the chains of certificates they will use to parent certificates
 followed by a single "cross certificate" followed by child

Kaufman [Page 9] RFC 1507 DASS September 1993

 certificates, a DASS implementation can avoid the need to have CAs
 near the root of the tree or can avoid the requirement to trust them
 even if they do exist.  A special case can also be supported whereby
 a global authority whose name is not the root can certify the local
 roots of independent "islands".

1.2.5 User vs. Node Authentication

 In concept, DASS mechanisms support the mutual authentication of two
 principals regardless of whether those principals are people,
 computers, or applications.  Those mechanisms have been extended,
 however, to deal with a common case of a pair of principals acting
 together (a user and a node) authenticating to a single principal (a
 remote server).  This is done by having optionally in each
 credentials structure two sets of secrets - one for the user and one
 for the node.  When authentication is done using such credentials,
 both secrets sign the request so the receiving party can verify that
 both principals are present.
 This setup has a number of advantages.  It permits access controls to
 be enforced based on both the identity of the user and the identity
 of the originating node.  It also makes it possible to define users
 of systems who have no network wide identities who can access network
 resources on the basis of node credentials alone.  The security of
 such a setup is less because a node can impersonate all of its users
 even when they are not logged in, but it offers an easier transition
 from existing global identities for all users.

1.2.6 Protection of User Keys

 DASS mechanisms generally deal with authentication between principals
 each knowing a private key.  For principals who are people, special
 mechanisms are provided for maintaining that private key.  In
 particular, it many cases it will be most convenient to keep
 passwords as secrets rather than private keys.  This architecture
 specifies a means of storing private keys encrypted under passwords.
 This would provide security as good as hiding a private key were it
 not that people tend to choose passwords from a small space (like
 words in a dictionary) such that a password can be more easily
 guessed than a private key.  To address this potential weakness, DASS
 specifies a protocol between a login node and a login agent whereby
 the login agent can audit and limit the rate of password guesses.
 Use of these features is optional.  A user with a smart card could
 store a private key directly and bypass all of these mechanisms.  If
 users can be forced to choose "good" passwords, the login agent could
 be eliminated and encrypted credentials could be stored directly in
 the naming service.

Kaufman [Page 10] RFC 1507 DASS September 1993

 Another way in which user keys are protected is that the architecture
 does not require that they be available except briefly at login.
 This reduces the threat of a user walking away from a logged on
 workstation and having someone take over the workstation and extract
 his key.  It also makes the use of RSA based smart cards practical;
 the card could keep the user's private key and execute one signature
 operation at login time to authenticate an entire session.

1.3 What This Document Won't Tell You

 Architecture documents are by their nature difficult to read.  This
 one is no exception. The reason is that an architecture document
 contains the details sufficient to build interoperable
 implementations, but it is not a design specification. It goes out of
 its way to leave out any details which an implementation could choose
 without affecting interoperability. It also does not specify all the
 uses of the services provided because these services are properly
 regarded as general purpose tools.
 The remainder of this section includes information which is not
 properly part of the authentication architecture, but which may be
 useful in understanding why the architecture is the way it is.

1.3.1 How DASS is Embedded in an Operating System

 While architecturally DASS does not require any operating system
 support in order to be used by an application (other than the
 services listed in Section 2), it is expected that actual
 implementations of DASS will be closely tied to the operating systems
 of host computers.  This is done both for security and for
 convenience.
 In particular, it is expected that when a user logs into a node, a
 set of credentials will be created for that user and then associated
 by the operating system with all processes initiated by or on behalf
 of the user.  When a user delegates to a service, the remote
 operating system is expected to accept the delegation and start up
 the remote process with the delegated credentials.  Most nodes are
 expected to have credentials of their own and support the concept of
 user accounts.  When user credentials are created, the node is
 expected to verify them in its own context, determine the appropriate
 user account, and add node credentials to the created credentials
 set.

1.3.2 Forms of Credentials

 In the DASS architecture, there is a single data structure called
 "Credentials" with a large number of optional parts.  In an

Kaufman [Page 11] RFC 1507 DASS September 1993

 implementation, it is possible that not all of the architecturally
 allowed subsets will be supported and credentials structures with
 different subsets of the data may be implemented quite differently.
 The major categories of credentials likely to be supported in an
 implementation are:
  1. Claimant credentials - these are the credentials which would

normally be associated with a user process in order that it be

    able to create authentication tokens.  It would contain the
    user's name, login ticket, session private key, and (at least
    logically) local node credentials and cached outgoing
    contexts.
  1. Verifier credentials - these are the credentials which would

normally be associated with a server which must verify tokens

    and produce mutual authentication response tokens.  Since
    servers may be started by a node on demand, some
    representation of verifier credentials must exist independent
    of a process.  If an operating system wishes to authenticate a
    request before starting a server process, the credentials must
    exist in usable form.  An implementation may choose to have
    all services on a "node" share a verifier credentials
    structure, or it may choose to have each service have its own.
  1. Combined credentials - architecturally, a server may have a

structure which is both claimant credentials and verifier

    credentials combined so that the server may act in either role
    using a single structure.  There is some overlap in the
    contents.  There is no requirement, however, that an
    implementation support such a structure.
  1. Stub credentials - In the architecture, a credentials

structure is created whenever a token is accepted. If delegation

    took place, these are claimant credentials usable by their
    possessor to create additional tokens.  If no delegation took
    place, this structure exists as an architectural place holder
    against which an implementation may attempt to authenticate
    user and node names.  An implementation might choose to
    implement  stub credentials  with a different mechanism than
    claimant or verifier credentials.  In particular, it might do
    whatever user and node authentication is useful itself and not
    support this structure at all.

Kaufman [Page 12] RFC 1507 DASS September 1993

1.3.3 Support for Alternative Certification Authority

    Implementations
 A motivating factor in much of the design of DASS is the need to
 protect certification authorities from compromise. CAs are only used
 to create certificates for new principals and to renew them on
 expiration (expiration intervals are likely to be measured in
 months). They therefore do not need to be highly available. For
 maximum security, CAs could be implemented on standalone PCs where
 the hardware, software, and keys can be locked in a safe when the CA
 is not in use. The certificates the CA generates must be delivered to
 the naming service to be registered, and a possible mechanism for
 this is for the CA to have an RS232 line to an on-line component
 which can pass certificates and related information but not login
 sessions. The intent would be to make it implausible to mount a
 network attack against the CA.  Alternatively, certificates could be
 carried to the network on a floppy disk.
 For CAs to be secure, a whole host of design details must be done
 right. The most important of these is the design of user and system
 manager interfaces that make it difficult to "trick" a user or system
 manager into doing the wrong thing and certifying an impostor or
 revealing a key. Mechanisms for generating keys must also be
 carefully protected to assure that the generated key cannot be
 guessed (because of lack of randomness) and is not recorded where a
 penetrator can get it. Because a certificate contains relatively
 little human intelligible information (its most important components
 are UIDs and public keys), it will be a challenge to design a user
 interface that assures the human operator only authorizes the signing
 of intented certificates. Such considerations are beyond the scope of
 the architecture (since they do not affect interoperability), but
 they did affect the design in subtle ways.  In particular, it does
 not assume uniform security throughout the CA hierarchy and is
 designed to assure that the compromise of a CA in one part of the
 hierarchy does not have global implications.
 The architecture does not require that CAs be off-line. The CA could
 be software that can run on any node when the proper secret is
 installed.  Administrative convenience can be gained by integrating
 the CA with account registration utilities and naming service
 maintenance. As such, the CA would have to be on-line when in use in
 order to register certificates in the naming service.  The CA key
 could be unlocked with a password and the password could be entered
 on each use both to authenticate the CA operator and to assure that
 compromise of the host node while the CA is not in use will not
 compromise the CA.  This design would be subject to attacks based on
 planting Trojan horses in the CA software, but is entirely
 interoperable with a more secure implementation.  Realistic tradeoffs

Kaufman [Page 13] RFC 1507 DASS September 1993

 must be made between security, cost, and administrative convenience
 bearing in mind that a system is only as secure as its weakest link
 and that there is no benefit in making the CA substantially more
 secure than the other components of the system.

1.3.4 Services Provided vs. Application Program Interface

 Section 3 of this document specifies "abstract interfaces" to the
 services provided by DASS. This means it tells what services are
 provided, what parameters are supplied by the caller, and what data
 is returned. It does not specify the calling interfaces.  Calling
 interfaces may be platform, operating system, and language dependent.
 They do not affect interoperability; different implementations which
 implement completely different calling interfaces can still
 interoperate over a network. They do, however, affect portability. A
 program which runs on one platform can only run on another which
 implements an identical API.
 In order to support portability of applications - not just between
 implementations of DASS but between implementations of DASS and
 implementations of Kerberos - a "Generic Security Service API" has
 been designed and is outlined in Annex B. This API could be the only
 "published" interface to DASS services.  This interface does not,
 however, give access to all the functions provided by DASS and it
 provides some non-DASS services. It does not give access to the
 "login" service, for example, so the login function cannot be
 implemented in a portable way. Clearly an implementation must provide
 some implementation of the login function, though perhaps only to one
 system program and the implementation need not be portable.
 Similarly, the Generic API provides no access to node authentication
 information, so applications which use these services may not be
 portable.
 The Generic API provides services for encryption of user data for
 integrity and possibly privacy. These services are not specified as a
 part of the DASS architecture. This is because we envisioned that
 such services would be provided by the communications network and not
 in applications. These services are provided by the Generic API
 because these services are provided by Kerberos, there exist
 applications which use these services, and they are desired in the
 context of the IETF-CAT work. The DASS architecture includes a Key
 Distribution service so that the encryption functions of the Generic
 API can be supported and integrated. Annex B specifies how those
 services can be implemented using DASS services.
 The Services Provided also differ from the GSSAPI because there are
 important extensions envisioned to the API for future applications
 and it was important to assure that architecturally those services

Kaufman [Page 14] RFC 1507 DASS September 1993

 were available.  In particular, DASS provides the ability for a
 principal to have multiple aliases and for the receiver of an
 authentication token to verify any one of them.  We want DASS to
 support the case where a server only learns the name it is trying to
 validate in the course of evaluating an ACL.  This may be long after
 a connection is accepted.  The Services Provided section therefore
 separates the Accept_token function from the Verify Principal Name.
 The other motivation behind a different interface is that DASS
 provides node authentication - the ability to authenticate the node
 from which a request originates as well as the user.  Because
 Kerberos provides no such mechanism, the capability is missing from
 the GSSAPI, but we expect some applications will want to make use of
 it.

1.3.5 Use of a Naming Service

 With the exception of the syntactical representation of names, which
 is tied to X.500, the DASS architecture is designed to be independent
 of the particular underlying naming service.  While the intention is
 that certificates be stored in an X.500 naming service in the fields
 architecturally reserved for this purpose in the standard, this
 specification allows for the possibility of different forms of
 certificate stores.  The SPX implementation of DASS implements its
 own certificate distribution service because we did not want to
 introduce a dependency on an X.500 naming service.

1.3.6 Key Hiding - Credentials

 The abstract interfaces described in section 3 specify that
 "credentials" and "keys" are the inputs and outputs of various
 routines.  Credentials structures in particular contain secret
 information which should not be made available to the calling
 application.  In most cases, keeping this information from
 applications is simply a matter of prudence - a misbehaving
 application can do nearly as much damage using the credentials as it
 can by using the secrets directly.  Having access to the keys
 themselves may allow an application to bypass auditing or leak a key
 to an accomplice who can use it on another node where a large amount
 of activity is less likely to be noticed.  In some cases, most
 dramatically where a "node key" is present in user credentials, it is
 vital that the contents of the credentials be kept out of the hands
 of applications.
 To accomplish this, a concrete interface is expected to create
 "credentials handles" that are passed in and out of DASS routines.
 The credentials themselves would be kept in some portion of memory
 where unprivileged code can't get at them.

Kaufman [Page 15] RFC 1507 DASS September 1993

 There is another aspect of the way credentials are used which is
 important to the design of real implementations.  In normal use, a
 user will create a set of credentials in the process of logging on to
 a system and then use them from many processes or jobs.  When many
 processes share a set of credentials, it is important for the sake of
 performance that they share one set of credentials rather than having
 a copy of the credentials made for each.  This is because information
 is cached in credentials as a side effect of some requests and for
 good performance those caches should be shared.
 As an example, consider a system executing a series of copy commands
 moving files from one system to another.  The credentials of the user
 will have been established when the user logged on.  The first time a
 copy is requested, a new process will start up, open a connection to
 the destination system, and create a token to authenticate itself.
 Creating that token will be an expensive operation, but information
 will be computed and "cached" in the credentials structure which will
 allow any subsequent tokens on behalf of that user to that server to
 be computed cheaply.  After the copy completes, the connection is
 closed and the process terminates.  In response to a second copy
 request, another new process will be created and a new token
 computed.  For this operation to get a performance benefit from the
 caching, the information computed by the first process must somehow
 make it to the second.
 A model for how this caching might work can be seen in the way
 Kerberos caches credentials.  Kerberos keeps credentials in a file
 whose name can be computed from the name of the local user.  This
 file is initialized as part of the login process and its protection
 is set so that only processes running under the UID of the user may
 read and write the file.  Processes cache information there; all
 processes running on behalf of the user share the file.
 There are two problems with this scheme: first, on a diskless node
 putting information in a file exposes it to eavesdroppers on the
 network; second, it does not accomplish the "key hiding" function
 described earlier in this section.  In a more secure implementation,
 the kernel or a privileged process would manage some "pool" of
 credentials for all processes on a node and would grant access to
 them only through the DASS calls.  Credentials structures are complex
 and varying length; DASS may organize them as a set of pools rather
 than as contiguous blocks of data.  All such design issues are
 "beyond the scope of the architecture".  Implementations must decide
 how to control access to credentials.  They could copy the Kerberos
 scheme of having credentials available to processes with the UID of
 the login session which created them and to privileged processes or
 there may be a more elaborate mechanism for "passing" credentials
 handles from process to process.  This design should probably follow

Kaufman [Page 16] RFC 1507 DASS September 1993

 the operating system mechanisms for passing around local privileges.

1.3.7 Key Hiding - Contexts

 The "GSSAPI" has a concept of a security context which has some of
 the same key hiding problems as a credentials structure.  Security
 contexts are used in calls to cryptographically protect user data
 (from modification or from disclosure and modification) using keys
 established during authentication.  The "services provided"
 specification says that create_ and accept_token return a "shared
 key" and "instance identifier".  The GSSAPI says that a context
 handle is returned which is an integer.  A secure implementation
 would keep the key and instance identifier in protected memory and
 only allow access to them through provided interfaces.
 Unlike credentials, there is probably no need to provide mechanisms
 for contexts to be shared between processes.  Contexts will normally
 be associated with some notion of a communications "connection" and
 ends of a connection are not normally shared.  If an implementation
 chooses to provide additional services to applications like message
 sequencing or duplicate detection, contexts will have to contain
 additional fields.  These can be created and maintained without any
 additional authentication services.

1.4 The Relationship between DASS and ISO Standards

 This section provides an introduction to DASS authentication in terms
 of the ISO Authentication Framework (DP10181-2).   The purpose of
 this introduction is to give the reader an intuitive understanding of
 the way DASS works and how its mechanisms and terminology relate to
 standards.  Important details have been omitted here but are spelled
 out in section 3.

1.4.1 Concepts

 The primary goal of authentication is to prevent impersonation, that
 is, the pretense to a false identity. Authentication always involves
 identification in some form. Without authentication, anyone could
 claim to be whomever they wished and get away with it.
 If it didn't matter with whom one was communicating, elaborate
 procedures for authentication would be unnecessary. However, in most
 systems, and in timesharing and distributed processing environments
 in particular, the rights of individuals are often circumscribed by
 security policy. In particular, authorization (identity based access
 control) and accountability (audit) provisions could be circumvented
 if masquerading attempts were impossible to prevent or detect.

Kaufman [Page 17] RFC 1507 DASS September 1993

 Almost all practical authentication mechanisms suitable for use in
 distributed environments rely on knowledge of some secret
 information. Most differences lie in how one presents evidence that
 they know the secret. Some schemes, in particular the familiar simple
 use of passwords, are quite susceptible to attack. Generally, the
 threats to authentication may be classified as:
  1. forgery, attempting to guess or otherwise fabricate evidence;
  1. replay, where one can eavesdrop upon another's authentication

exchange and learn enough to impersonate them; and

  1. interception, where one slips between the communicants and is

able to modify the communications channel unnoticed.

 Most such attacks can be countered by using what is known as strong
 authentication. Strong authentication refers to techniques that
 permit one to provide evidence that they know a particular secret
 without revealing even a hint about the secret. Thus neither the
 entity to whom one is authenticating, nor an eavesdropper on the
 conversation can further their ability to impersonate the
 authenticating principal at some future time as the result of an
 authentication exchange.
 Strong authentication mechanisms, in particular those used here, rely
 on cryptographic techniques. In particular, DASS uses public key
 cryptography. Note that interception attacks cannot be countered by
 strong authentication alone, but generally need additional security
 mechanisms to secure the communication channel, such as data
 encryption.

1.4.2 Principals and Their Roles

 All authentication is on behalf of principals. In DASS the following
 types of principals are recognized:
  1. user principals, normally people with accounts who are

responsible for performing particular tasks. Generally it is

    users that are authorized to do things by virtue of having
    been granted access rights, or who are to be held accountable
    for specific actions subject to being audited.
  1. server principals, which are accessed by users.
  1. node principals, corresponding to locations where users and

servers, or more accurately, processes acting on behalf of

    principals can reside.

Kaufman [Page 18] RFC 1507 DASS September 1993

 Principals can act in one of two capacities:
  1. the claimant is the active entity seeking to authenticate

itself, and

  1. the verifier is the passive entity to whom the claimant is

authenticating.

 Users normally are claimants, whereas servers are usually verifiers,
 although sometimes servers can also be claimants.
 There is another kind of principal:
  1. certification authorities (CA's) issue certificates which

attest to another principal's public key.

1.4.3 Representation, Delegation and Representation Transfer

 Of course, although it is users that are responsible for what the
 computer does, human beings are physically unable to directly do
 anything within a computer system. In point of fact, it is a
 process executing on behalf of a user that actually performs
 useful work. From the point of view of performing security
 controlled functions, the process is the agent, or
 representative, of the user, and is authorized by that user to do
 things on his behalf. In the terms used in the ISO Authentication
 Framework, the user is said to have a representation in the
 process.
 The representation has to come into existence somehow.  Delegation
 refers to the act of creating a representation. A user is said to
 create a representation for themselves by delegating to a process. If
 the user creates another process, say by doing an rlogin on a
 different computer, a representation may be needed there as well. This
 may be accomplished automatically by a process known as representation
 transfer.  DASS uses the term delegation to also mean the act of
 creating additional representations on a remote systems.
 A representation is instantiated in DASS as credentials.  Credentials
 include the identity of the principal as well as the cryptographic
 "state" needed to engage in strong authentication procedures. Claimant
 information in credentials enable principals to authenticate
 themselves to others, whereas verifier information in credentials
 permit principals to verify the claims of others.  Credentials
 intended primarily for use by a claimant will be referred to as
 claimant credentials in the text which follows.  Credentials intended
 primarily for use in verification will be referred to as verifier
 credentials.  A particular set of credentials may or may not contain

Kaufman [Page 19] RFC 1507 DASS September 1993

 all of the data necessary to be used in both roles.  That will depend
 on the mechanisms by which the credentials were created.
 In some contexts, but not here, the concept of representation
 and/or delegation is sometimes referred to as proxy. This term is
 used in ECMA TR/46.  We avoid use of the term because of possible
 confusion with an unrelated use of the term in the context of
 DECnet.

1.4.4 Key Distribution, Replay, Mutual Authentication and Trust

 Strong authentication uses cryptographic techniques. The
 particular mechanisms used in DASS result in the distribution of
 cryptographic keys as a side effect. These keys are suitable for
 use for providing a data origin authentication service and/or a
 data confidentiality service between a pair of authenticated
 principals.
 Replay detection is provided using timestamps on relevant
 authentication messages, combined with remembering previously
 accepted messages until they become "stale". This is in contrast
 to other techniques, such as challenge and response exchanges.
 Authentication can be one-way or mutual. One-way authentication is
 when only one party, in DASS the claimant, authenticates to the other.
 Mutual authentication provides, in addition, authentication of the
 verifier back to the claimant. In certain communications schemes, for
 example connectionless transfer, only one-way authentication is
 meaningful. DASS supports mutual authentication as a simple extension
 of one-way authentication for use in environments where it makes
 sense.
 DASS potentially can allow many different "trust relationships"
 to exist. All principals trust one or more CA's to safeguard the
 certification process. Principals use certificates as the basis
 for authenticating identities, and trust that CA's which issue
 certificates act responsibly. Users expect CA's to make sure that
 certificates (and related secrets) are only made for principals
 that the CA knows or has properly authenticated on its own.

1.5 An Authentication Walkthrough

 The OSI Authentication Framework characterizes authentication as
 occurring in six phases. This section attempts to describe DASS
 in these terms.

Kaufman [Page 20] RFC 1507 DASS September 1993

1.5.1 Installation

 In this phase, principal certificates are created, as is the
 additional information needed to create claimant and verifier
 credentials. OSI defines three sub-phases:
  1. Enrollment. In DASS, this is the definition of a principal in

terms of a key, name and UID.

  1. Validation, confirmation of identity to the satisfaction of

the CA, after which the CA generates a certificate.

  1. Confirmation. In DASS, this is the act of providing the user

with the certificate and with the CA's own name, key and UID,

    followed up by the user creating a  trusted authority for that
    CA. A trusted authority is a certificate for the CA signed by
    the user.
 Included in this step in DASS is the posting of the certificate so as
 to be available to principals wishing to verify the principal's
 identity. In addition, the user principal saves the trusted authority
 so as to be available when it creates credentials.

1.5.2 Distribution

 DASS distributes certificates by placing them in the name service.

1.5.3 Acquisition

 Whenever principals wish to authenticate to one another, they access
 the Name Service to obtain whatever public key certificates they need
 and create the necessary credentials. In DASS, acquisition means
 obtaining credentials.
 Claimant credentials implement the representation of a principal in a
 process, or, more accurately, provide a representation of the
 principal for use by a process. In making this representation, the
 principal delegates to a temporary delegation key. In this fashion
 the claimant's long term principal key need not remain in the system.
 Claimant credentials are made by invoking the get credentials
 primitive. Claimant credentials are a DASS specific data structure
 containing:
  1. a name
  1. a ticket, a data structure containing

Kaufman [Page 21] RFC 1507 DASS September 1993

    .  a validity interval,
    .  UID, and
    .  (temporary) delegation public key, along with a
    .  digital signature on the above made with the principal
       private key
  1. the delegation private key
 Optionally in addition, there may be credential information relating
 to the node on which the user is logged in and the account on that
 node.  A detailed description of all the information found in
 credentials can be found in section 3.  Verifier credentials are made
 with initialize_server. Verifier credentials consist of a principal
 (long term) private key. The rationale is that these credentials are
 usually needed by servers that must be able to run indefinitely
 without re-entry of any long term key.
 In addition, claimants and verifiers have a trusted authority, which
 consists of information about a trusted CA.  That information is its:
  1. name (this will appear in the "issuer" field in principal

certificates),

  1. public key (to use in verifying certificates issued by that

CA), and

  1. UID.
 Trusted authorities are used by principals to verify certificates for
 other principals' public keys.  CAs are arranged in a hierarchy
 corresponding to the naming hierarchy, where each directory in the
 naming hierarchy is controlled by a single CA.  Each CA certifies the
 CA of its parent directory, the CAs of each of its child directories,
 and optionally CAs elsewhere in the naming hierarchy (mainly to deal
 with the case where the directories up to a common ancestor lack
 CAs).  Even though a principal has only a single CA as a trusted
 authority, it can securely obtain the public key of any principal in
 the namespace by "walking the CA hierarchy".

1.5.4 Transfer

 The DASS exchange of authentication information is illustrated in
 Figure 1-1. During the transfer phase, the DASS claimant sends an
 authentication token  to the verifier. Authentication tokens are made
 by invoking the create_token primitive. The authentication token is

Kaufman [Page 22] RFC 1507 DASS September 1993

 cryptographically protected and specified as a DASS data structure in
 ASN.1. The authentication token includes:
  1. a ticket,
  1. a DES authenticating key encrypted using the intended

verifier's public key

  1. one of the following:
    . if delegation is not being performed, a digital signature on
      the encrypted DES key using the delegation private key, or
    . if delegation is being performed, sending the delegation
      private key, DES encrypted using the DES authenticating key
  1. an authenticator, which is a cryptographic checksum made using

the DES authenticating key over a buffer containing

    . a timestamp
    . any application supplied "channel bindings". For example,
      addresses or other context information. The purpose of this
      field is to thwart substitution and replay attacks.
  1. additional optional information concerning node authentication

and context.

 As a side effect, after init_authentication_context, the caller
 receives a local authentication context, a data structure containing:
  1. the DES key, and
  1. if mutual authentication is being requested, the expected

response.

 In order to construct an authentication token, the claimant needs to
 access the verifier's public key certificate from the Name Service
 (labeled CDC, for Certificate Distribution Center, in the figure).
 Note that while an authenticator can only be used once, it is
 permissible to re-establish the same local authentication context
 multiple times. That is, the ticket and DES key establishment
 components of the authentication token may have a relatively long
 lifetime. This permits a performance improvement in that repeated
 applications of public key operations can be alleviated if one caches
 authentication contexts, along with other components from a
 successfully used authentication token and the associated verified

Kaufman [Page 23] RFC 1507 DASS September 1993

 principal public key value. It is a relatively inexpensive operation
 to create (and verify) "fresh" authenticators based on cached
 authentication context.
    Claimant Actions      | Communications |  Verifier Actions
                          |                |
         verifier name    |                |
                 |        |                |
                 |        |           +---+|
                 \------------------->|   ||
   trusted                |           |   ||
 authorities              |           |CDC||
      |    +-----------+  |certificate|   ||
      |    |  Verify   |<-------------|   ||
      \--->|Certificate|  |           +---+|
           +-----------+  |                |
   Claimant        |      |                |
 credentials    Verifier  |                |   Verifier
      |       Public Key  |                | Credentials
      |            |      |                |       |
      |            V      |                |       V
      |    +-----------+  | Authentication | +-----------+
      |    |   Make    |  |     Token      | |   Check   |   Replay
      \--->|  Token    |-------------------->|   Token   |<-->Cache
           +-----------+  |                | +-----------+
    DES <---/      |      |                |  |   |    \----->DES
    key            |      |                | /Claimant        key
                   |      |                |/Public Key
                   |      |                /      |        trusted
                   |      |      Claimant /|      V     authorities
                   |      |+---+   Name  / | +-----------+     |
          authentication  ||   |<-------/  | |  Verify   |<----/
             context      ||   |certificate| |Certificate|
                   |      ||CDC|------------>|           |-->accept/
                   |      ||   |           | +-----------+   reject
                   |      ||   |           |      |      \
                   |      |+---+           |authentication\
                   V      |     mutual     |   context     V
           +-----------+  | authentication |      |      claimant
        /--|  Accept   |  |    response    | +----------+credentials
       V   |  Mutual   |<--------------------|  Make    |(delegation)
   accept/ +-----------+  |                | | Response |
   reject                 |                | +----------+
                          |                |
            Figure 1 - Authentication Exchange Overview

Kaufman [Page 24] RFC 1507 DASS September 1993

1.5.5 Verification

 Upon receipt of an authentication token, the verifier extracts the
 DES key using its verifier credentials, accesses the Name Service
 (labeled CDC for Certificate Distribution Center) to obtain the
 certificates needed to perform cryptographic checks on the incoming
 information, and verifies all of the signatures on the received
 certificates and the authentication token.  Verification can result
 in creation of new claimant credentials if delegation is performed.
 As part of this process, verified authenticators are retained for a
 suitable timeout period.

1.5.6 Unenrolment

 This is the removal of information from the Name Service. The only
 other form of revocation supported by DASS is certificate timeout.
 Every certificate contains an expiration time (expected in ordinary
 use to be about a year from its signing date).  DASS does not
 currently support the revocation lists in X.509.

2. Services Used

 Aside from operating system services needed to maintain its internal
 state, DASS relies on a global distributed database in which to store
 its certificates, a reliable source of time, and a source of random
 numbers for creating cryptographic keys.

2.1 Time Service

 DASS requires access to the current time in several of its
 algorithms.  Some of its uses of time are security critical.  In
 others, network synchronization of clocks is required.  DASS does
 not, however, depend on having a single source of time which is both
 secure and tightly synchronized.
 The requirements on system provided time are:
  1. For purposes of validating certificates and tickets, the

system needs access to know the date and time accurate to

    within a few hours with no particular synchronization
    requirements.  If this time is inaccurate, then valid requests
    may be rejected and expired messages may be accepted.
    Certificate expiration is a backup revocation mechanism, so
    this can only cause a security compromise in the event of
    multiple failures.  In theory, this could be provided by
    having a local clock on every node accurate to within a few
    hours over the life of the product to provide this function.

Kaufman [Page 25] RFC 1507 DASS September 1993

    If an insecure network time service is used to provide this
    time, there are theoretical security threats, but they are
    expected to be logistically impractical to exploit.
  1. For purposes of detecting replay of authentication tokens, the

system needs access to a strictly monotonic time source which

    is reasonably synchronized across the network (within a few
    minutes) for the system to work, but inaccuracy does not
    present a security threat except as noted below. It may
    constitute an availability threat because valid requests may
    be rejected.  In order to get strict monotonicity in the
    presence of a rapid series of requests, time must be returned
    with high precision.  There is no requirement for a high
    degree of accuracy.  Inaccurate time could present a security
    threat in the following scenario: if a client's clock is made
    sufficiently fast that its tokens are rejected, someone
    harvesting those tokens from the wire could replay them later
    and impersonate the client.  In some environments, this might
    be an easier threat than harvesting tokens and preventing
    their delivery.
  1. For purposes of aging stale entries from caches, DASS requires

reasonably accurate timing of intervals. To the extent that

    intervals are reported as shorter than the actually were,
    revocation of certificates from the naming service may not be
    as timely as it should be.

2.2 Random Numbers

 In order to generate keys, DASS needs a source of "cryptographic
 quality" random numbers.  Cryptographic quality means that
 knowing any of the "random numbers" returned from a series and
 knowing all state information which is not protected, an attacker
 cannot predict any of the other numbers in the series.  Hardware
 sources are ideal, but there are alternative techniques which may
 also be acceptable. A 56 bit "truly random" seed (say from a
 series of coin tosses) could be used as a DES key to encrypt an
 infinite length known text block in CBC mode to produce a pseudo-rand
 sequence provided the key and current point in the sequence were
 adequately protected.  There is considerable controversy
 surrounding what constitutes cryptographic quality random
 numbers, and it is not a goal of this document to resolve it.

2.3 Naming Service

 DASS stores creates and uses "certificates" associated with every
 principal in the system, and encrypted credentials associated
 with most.  This information is stored in an on-line service

Kaufman [Page 26] RFC 1507 DASS September 1993

 associated with the principal being certified.  The long term
 vision is for DASS to use an X.500 naming service, and DASS will
 from its inception authenticate X.500 names.  To avoid a
 dependence on having an X.500 naming service available (and to
 gain the benefits of a "login agent" that controls password
 guessing), an alternative certificate  distribution center
 protocol is also described.
 The specific requirements DASS places on the naming service are:
  1. It must be highly available. A user's naming service entry

must be available to any node where the user is to obtain

    services (or service will be denied).  A server's naming
    service entry must be available from any node from which the
    service is to be invoked (or service will be denied).
  1. It must be timely. The presence of "stale" information in the

naming service may cause some problems. When a password

    changes, the old password may remain valid (and the new
    password invalid) to the extent the naming service provides
    stale information.  When a user or server is added to the
    network, it will not be able to participate in authentication
    until the information added to the naming service is available
    at the node doing the authentication.  In the unusual
    circumstance that a key changes, the entity whose key has
    changed will not be able to use the new key until the new
    certificate is uniformly available.
  1. It must be secure with regard to certain specific properties.

In general, the security of DASS protected applications does

    not depend on the security of the naming service.  It is
    expected that the availability needs of the naming service
    will prevent it from being as secure as some applications need
    to be.  There are two aspects of DASS security which do depend
    on the security of the naming service: timely revocation of
    certificates and protection of user secrets against dictionary
    based password guessing. DASS depends on the removal of
    certificates from the naming service in order to revoke them
    more quickly than waiting for them to time out.  For this
    mechanism to provide any actual security, it must not be
    possible for a network entity to "impersonate" the naming
    service and the naming service must be able to enforce access
    controls which prevent a revoked certificate from being
    reinstated by an unauthorized entity.  In the long run, it is
    expected that DASS itself will be used to secure the naming
    service, which presents certain potential recursion problems
    (to be addressed in the naming service design).  If the naming
    service is not authenticated (as is expected in early

Kaufman [Page 27] RFC 1507 DASS September 1993

    versions) attacks where a revoked certificate is "reinstated"
    through impersonation of the naming service are possible.
 The specific functions DASS requests of the naming service are
 simple:
  1. Given an X.500 name, store a set of certificates associated

with that name.

  1. Given an X.500 name, retrieve the set of certificates

associated with that name.

  1. Given an X.500 name, store a set of encrypted credentials

associated with that name.

  1. Given and X.500 name, retrieve a set of encrypted credentials

associated with that name.

 Implementation over a particular naming service may implement more
 specialized functions for reasons of efficiency.  For example, the
 certificates associated with a name may be separated into several
 sets (child, parent, cross, self) so that only the relevant ones may
 be retrieved.  In order that access to the naming service itself be
 secure, the protocols should be authenticated.  Certificates should
 generally be readable without authentication in order to avoid
 recursion problems.  Requests to read encrypted credentials should be
 specialized and should include proof of knowledge of the password in
 order that the naming service can audit and slow down false password
 guesses.
 The following sections describe the interfaces to specific naming
 services:

2.3.1 Interface to X.500

 Certificates associated with a particular name are stored as
 attributes of the entry as specified in X.509.  X.509 defines
 attributes appropriate for parent and cross certificates
 (CrossCertificatePair, CACertificate) for some principals; we will
 have to define a DASSUserPrincipal object class including these
 attributes in order to properly use them with ordinary users.
 Retrieval is via normal X.500 protocols.  Certificates should be
 world readable and modifiable only by appropriate authorities.
 Encrypted credentials are stored with the entry of the principal
 under a yet to be defined attribute.  The credentials should be
 encoded as specified in section 4.  In the absence of extensions to
 the X.500 protocol to control password guessing, the encrypted

Kaufman [Page 28] RFC 1507 DASS September 1993

 credentials should be world readable and updatable only by the named
 principal and other appropriate authorities.

2.3.2 Interface to CDC

 The CDC (Certificate Distribution Center) is a special purpose name
 server created to service DASS until an X.500 service is available in
 all of the environments where DASS needs to operate.  The CDC uses a
 special purpose protocol to communicate with DASS clients.  The
 protocol was designed for efficiency, simplicity, and security.  CDCs
 use DASS as an authentication mechanism and to protect encrypted
 credentials from unaudited password guessing.
 Each DASS client maintains a list of CDCs and the portion of the
 namespace served by that CDC.  Each directory has a master replica
 which is the only one which will accept updates.  The CDCs maintain
 consistency with one another using protocols beyond the scope of this
 document.  When a DASS client wishes to make a request of a CDC, it
 opens a TCP or DECnet connection to the CDC and sends an ASN.1 (BER)
 encoded request and receives a corresponding ASN.1 (BER) encoded
 response.  Clients are expected to learn the IP or DECnet address and
 port number of the CDC supporting a particular name from a local
 configuration file.  To maximize performance, the requests bundle
 what would be several requests if made in terms of requests for
 individual certificates.  It is intended that all certificates needed
 for an authentication operation be retrievable with at most two CDC
 requests/responses (one to the CDC of the client and one to the CDC
 of the server).
 Documented here is the protocol a DASS client would use to retrieve
 certificates and credentials from a CDC and update a user password.
 This protocol does not provide for updates to the certificate and
 credential databases.  Such updates must be supported for a practical
 system, but could be done either by extensions to this protocol or by
 local security mechanisms implemented on nodes supporting the CDC.
 Similarly, availability can be enhanced by replicating the CDC.
 Automating the replication of updates could be implemented by
 extensions to this protocol or by some other mechanism.  This
 specification assumes that updates and replication are local matters
 solved by individual CA/CDC implementations.
 Requests and responses are encoded as follows:

2.3.2.1 ReadPrinCertRequest

 This request asks the CDC to return the child certificates and
 selected incoming cross certificates for the specified object.  The
 format of the request is:

Kaufman [Page 29] RFC 1507 DASS September 1993

      ReadPrinCertRequest ::= [4] IMPLICIT SEQUENCE {
           flags [0] BIT STRING DEFAULT {},
           index [1] IMPLICIT INTEGER DEFAULT 0,
           resolveFrom [2] Name OPTIONAL,
           principal Name,
           crossCertIssuers ListOfIssuers OPTIONAL
           }
      ListOfIssuers ::= SEQUENCE OF Name
 The first 24 bits of flags, if present, contain a protocol version
 number.  Clients following this spec should place the value 2.0.0 in
 the three bytes.  Servers following this spec should accept any value
 of the form 1.x.x or 2.x.x.  flags bits beyond the first 24 are
 reserved for future use (should not be supplied by clients and should
 be ignored by servers).
 index is only used if the response exceeds the size of a single
 message; in that case, the query is repeated with index set to the
 value that was returned by ReadPrinCertResponse.  resolveFrom and
 principal imply a set of entities for which certificates should be
 retrieved.  resolveFrom (if present) must be an ancestor of principal
 and child certificates will be retrieved for principal and all names
 which are ancestors of principal but descendants of resolveFrom.  The
 encoding of names is per X.500 and is specified in more detail in
 section 4.  The CDC returns the certificates in order of the object
 they came from, parents before children.
 crossCertIssuers is a list of cross certifiers that would be believed
 in the context of this authentication.  If supplied, the CDC may
 return a chain of certificates starting with one of the named
 crossCertIssuers and ending with the named principal.  One of
 resolveFrom or crossCertIssuers must be present in any request; if
 both are present, the CDC may return either chain.

2.3.2.2 ReadPrinCertResponse

 This is the response a CDC sends to a ReadPrinCertRequest.  Its
 syntax is:
      ReadPrinCertResponse ::= [5] IMPLICIT SEQUENCE {
           status [0] IMPLICIT CDCstatus DEFAULT success,
           index [1] INTEGER OPTIONAL,
           resolveTo [2] Name OPTIONAL,
           certSequence [3] IMPLICIT CertSequence,
           indexInvalidator [4] OCTET STRING (SIZE(8)) OPTIONAL,
           flags [5] BIT STRING OPTIONAL
           }
      CertSequence ::= SEQUENCE OF Certificate

Kaufman [Page 30] RFC 1507 DASS September 1993

 status indicates success or the cause of the failure.
 index if present indicates that the request could not be fully
 satisfied in a single request because of size limitations.  The
 request should be repeated with this index supplied in the request to
 get more.
 resolveTo will be present if index is present and should be supplied
 in the request for more certificates.  certSequence contains
 certificates found matching the search criteria.
 indexInvalidator may be present and indicates the version of the
 database being read.  If a set of certificates is being read in
 multiple requests (because there were too many to return in a single
 message), the reader should check that the value for indexInvalidator
 is the same on each request.  If it is not, the server may have
 skipped or duplicated some certificates.  This field must not be
 present if the version number in the request was missing or version
 1.x.x.
 The first 24 bits of flags, if present, indicate the protocol version
 number.  Implementers of this version of the spec should supply 2.0.0
 and should accept any version number of the form 1.x.x or 2.x.x.

2.3.2.3 ReadOutgoingCertRequest

 This requests from the CDC a list of all parent and outgoing cross
 certificates for a specified object.  A CDC is capable of storing
 cross certificates either with the subject or the issuer of the cross
 certificate.  In response to this request, the CDC will return all
 parent and cross certificates stored with the issuer for the named
 principal and all of its ancestors. Its syntax is:
      ReadOutgoingCertRequest ::= [6] IMPLICIT SEQUENCE {
           flags [0] BIT STRING DEFAULT {},
           index [1] IMPLICIT INTEGER DEFAULT 0,
           principal Name
           }
 The first 24 bits of flags is a protocol version number and should
 contain 2.0.0 for clients implementing this version of the spec.
 Servers implementing this version of the spec should accept any
 version number of the form 1.x.x or 2.x.x.  The remaining bits are
 reserved for future use (they should not be supplied by clients and
 they should be ignored by servers).
 index is used for continuation (see ReadPrinCertRequest).

Kaufman [Page 31] RFC 1507 DASS September 1993

 principal is the name for which certificates are requested.

2.3.2.4 ReadOutgoingCertResponse

 This is the response to a ReadOutgoingCertRequest.  Its syntax is:
      ReadOutgoingCertResponse::= [7] IMPLICIT SEQUENCE {
           status [0] IMPLICIT CDCStatus DEFAULT success,
           index [1] INTEGER OPTIONAL,
           certSequence [2] IMPLICIT CertSequence,
           indexInvalidator [3] OCTET STRING (SIZE(8))
      OPTIONAL,
           flags [4] BIT STRING OPTIONAL
           }
      CertSequence ::= SEQUENCE OF Certificate
 status indicates success of the cause of failure of the operation.
 index is used for continuation; see ReadPrinCertRequest.
 certSequence is the list of parent and outgoing cross certificates.
 indexInvalidator is used for continuation; see ReadPrinCertResponse
 (the same rules apply with respect to version numbers).
 The first 24 bits of flags, if present, contain the protocol version
 number.  Clients implementing this version of the spec should supply
 the value 2.0.0.  Servers should accept any values of the form 1.x.x
 or 2.x.x.  The remaining bits are reserved for future use (they
 should not be supplied by clients and should be ignored by servers).

2.3.2.5 ReadCredentialRequest

 This request is made to retrieve an principal's encrypted
 credentials.  To prevent unaudited password guessing, this structure
 includes an encrypted value that proves that the requester knows the
 password that will decrypt the structure.  The syntax of the request
 is:
      ReadCredentialRequest ::= [2] IMPLICIT SEQUENCE {
           flags [0] BIT STRING DEFAULT {}
           principal Name,
           logindata [2] BIT STRING DEFAULT {},
           token [3] BIT STRING OPTIONAL
           }

Kaufman [Page 32] RFC 1507 DASS September 1993

 The first 24 bits of flags contains the version number of the
 protocol.  The value 2.0.0 should be supplied. Any value of the form
 1.x.x or 2.x.x should be accepted. Any additional bits are reserved
 for future use (should not be supplied by clients and should be
 ignored by servers).
 principal is the name of the principal for whom encrypted credentials
 are desired.
 logindata is an encrypted value.  It may only be present if the
 version number is 2.0.0 or higher.  It must be present to read
 credentials which are protected by the login agent functionality of
 the CDC.  It is constructed as a single RSA block encrypted under the
 public key of the CDC.  The public key of the CDC is learned by some
 local means.  Possibilities include a local configuration file or by
 using DASS to read and verify a chain of certificates ending with the
 CDC [the CDC serving a directory should have its public key listed
 under a name consisting of the directory name with the RDN
 "CSS=X509"; the OID for the type CSS is 1.3.24.9.1].  The contents of
 the block are as follows:
  1. The low order eight bytes contain a randomly generated DES key

with the last byte of the DES key placed in the last byte of

    the RSA block.  This DES key will be used by the CDC to
    encrypt the response.  Key parity bits are ignored.
  1. The next to last eight bytes contain a long Posix time with

the integer time encoded as a byte string using big endian

    order.
  1. The next eight bytes (from the end) contain a hash of the

password. The algorithm for computing this hash is listed in

    section 4.4.2.  The CDC never computes this hash; it simply
    compares the value it receives with the value associated with
    the credentials.
  1. The next sixteen bytes (from the end) contain zero.
  1. The remainder of the RSA block (which should be the same size

as the public modulus of the CDC) contains a random number.

    The first byte should be chosen to be non-zero but so the
    value in the block does not exceed the RSA modulus.  Servers
    should ignore these bits.  This random number need not be of
    cryptographic strength, but should not be the same value for
    all encryptions.  Repeating the DES key would be adequate.
  1. The byte string thus constructed is encrypted using the RSA

algorithm by treating the string of bytes as a "big endian"

Kaufman [Page 33] RFC 1507 DASS September 1993

    integer and treating the integer result as "big endian" to
    make a string of bytes.
 token will not be present in the initial implementation but a space
 is reserved in case some future implementation wants to authenticate
 and audit the node from which a user is logging in.

2.3.2.6 ReadCredentialProtectedResponse

 This is the second possible response to a ReadPrinLoginRequest.  It
 is returned when the encrypted credentials are protected from
 password guessing by the CDC acting as a login agent.  Its syntax is:
 ReadCredentialProtectedResponse::=[16] IMPLICIT SEQUENCE {
         status [0] IMPLICIT CDCStatus DEFAULT success,
         encryptedCredential [1] BIT STRING,
         flags [2] BIT STRING OPTIONAL
         }
 status indicates that the request succeeded or the cause of the
 failure.
 encryptedCredential contains the DASSPrivateKey structure (defined in
 section 4.1) encrypted under a DES key computed from the user's name
 and password as specified in section 4.4.2 and then reencrypted under
 the DES key provided in the ReadPrinLoginRequest.
 The first 24 bits of flags, if present, contains the version number
 of the protocol.  Implementers of this version of the spec should
 supply 2.0.0 and should accept any version number of the form 2.x.x.
 Other bits are reserved for future use (they should not be supplied
 and they should be ignored).

2.3.2.7 WriteCredentialRequest

 This is a request to update the encrypted credential structure.  It
 is used when a user's key or password changes.  The syntax of the
 request is:
      WriteCredentialRequest ::= [17] IMPLICIT SEQUENCE {
           flags [0] BIT STRING DEFAULT {},
           authtoken [2] BIT STRING OPTIONAL,
           principal [3] Name,
           logindata [4] BIT STRING DEFAULT {},
           furtherSensitiveStuff [5] BIT STRING
           }
 The first 24 bits of flags is a version number.  Clients implementing

Kaufman [Page 34] RFC 1507 DASS September 1993

 this version of the spec should supply 2.0.0.  Servers should accept
 any value of the form 2.x.x.  Additional bits are reserved for future
 use (clients should not supply them and servers should ignore them).
 token, if present, authenticates the entity making the request.  A
 request will be accepted either from a principal proving knowledge of
 the password (see logindata below) or a principal presenting a token
 in this field and satisfying the authorization policy of the CDC.
 This field need not be present if logindata includes the hash2 of the
 password (anyone knowing the old password may set a new one).
 principal is the name of the object for which encrypted credentials
 should be updated.
 logindata is encrypted as in ReadPrinLoginRequest.  It proves that
 the requester knows the old password of the principal to be updated
 (unless the token supplied is from the user's CA) and includes the
 key which encrypts furtherSensitiveStuff.
 furtherSensitiveStuff is an encrypted field constructed as follows:
  1. The first eight bytes consist of the hash2 defined in section

4.4.2 with the last byte of the hash2 value stored first. The

    CDC stores this value and compares it with the values supplied
    in future requests of ReadCredentialRequest and
    WriteCredentialRequest.
  1. The next (variable number of) bytes contains a DASSPrivateKey

structure (defined in section 4.1). This is the new

    credential structure that will be returned by the CDC on
    future ReadCredentialRequests.
  1. The result is padded with zero bytes to a multiple of eight

bytes.

  1. The entire padded string is encrypted using the key from

logindata or token using DES in CBC mode with zero IV.

 the new eight byte "hash2" defined in section 4.4.2 concatenated with
 the DASSPrivateKey structure encrypted under the new "hash1" all
 encrypted under the DES key included in logindata.

2.3.2.8 HereIsStatus

 This is the response message to ill-formed requests and requests that
 only return a status and no data.  It's syntax is:

Kaufman [Page 35] RFC 1507 DASS September 1993

      HereIsStatus ::= [1] IMPLICIT SEQUENCE {
           status [0] IMPLICIT CDCStatus DEFAULT success
           }
 status indicates success or the cause of the failure.

2.3.2.9 Status Codes

 The following are the CDCStatus codes that can be returned by
 servers.  Not all of these values are possible with all calls, and
 some of the status codes are not possible with any of the calls
 described in this document.
      CDCStatus ::= INTEGER {
           success(0),
           accessDenied(1),
           wrongCDC(2),     --this CDC does not store the
                            --requested information
           unrecognizedCA(3),
           unrecognizedPrincipal(4),
           decodeRequestError(5),--invalid BER
           illegalRequest(6),    --request not recognised
           objectDoesNotExist(7),
           illegalAttribute(8),
           notPrimaryCDC(9),--write requests not accepted
                            --at this CDC replica
           authenticationFailure(11),
           incorrectPassword(12),
           objectAlreadyExists(13),
           objectWouldBeOrphan(15),
           objectIsPermanent(16),
           objectIsTentative(17),
           parentIsTentative(18),
           certificateNotFound(19),
           attributeNotFound(20),
           ioErrorOnCertifDatabase(100),

Kaufman [Page 36] RFC 1507 DASS September 1993

           databaseFull(101),
           serverInternalError(102),
           serverFatalError(103),
           insufficientResources(104)
           }

3. Services Provided

 This section specifies the services provided by DASS in terms of
 abstract interfaces and a model implementation.  A particular
 implementation may support only a subset of these services and may
 provide them through interfaces which combine functions and supply
 some parameters implicitly. The specific calling interfaces are in
 some cases language and operating system specific.  An actual
 implementation may choose, for example, to structure interfaces so
 that security contexts are established and then passed implicitly in
 calls rather than explicitly including them in every call.  It might
 also bundle keys into opaque structures to be used with supplied
 encryption and decryption routines in order to enhance security and
 modularity and better comply with export regulations. Annex B
 describes a Portable API designed so that applications using a
 limited subset of the capabilities of DASS can be easily ported
 between operating systems and between DASS and Kerberos based
 environments.  The model implementation describes data structures
 which include cached values to enhance performance.  Implementations
 may choose different contents or different caching strategies so long
 as the same sequence of calls would produce the same output for some
 caching policy.
 DASS operates on four kinds of data structures: Certificates,
 Credentials, Tokens, and Certification Authority State.  Certificates
 and Tokens are passed between implementations and thus their exact
 format must be architecturally specified. This detailed bit-for-bit
 specification is in section 4. Credentials generally exist only
 within a single node and their format is therefore not specified
 here. The contents of all of these data structures is listed below
 followed by the algorithms for manipulating them.
 There are three kinds of services provided by DASS: Certificate
 Maintenance, Credential Maintenance, and Authentication. The first
 two kinds exist only in support of the third. Certificate maintenance
 functions maintain the database of public keys in the naming service.
 These functions tend to be fairly specialized and may not be
 supported on all platforms. Before authentication can take place,
 both authenticating principals must have constructed credentials
 structures. These are built using the Credential Maintenance calls.

Kaufman [Page 37] RFC 1507 DASS September 1993

 The Authentication functions use credential information and
 certificates, produce and consume authentication tokens and tell the
 two communicating parties one another's names.

3.1 Certificate Contents

 For purposes of this architecture, a certificate is a data structure
 posted in the naming service which proclaims that knowledge of the
 private key associated with a stated public key authenticates a named
 principal. Certificates are "signed" by some authority, are readable
 by anyone, and can be verified by anyone knowing the public key of
 the authority.  DASS organizes the CA trust hierarchy around the
 naming hierarchy. There exists a trusted authority associated with
 each directory in the naming hierarchy. Generally, each authority
 creates certificates stating the public keys of each of its children
 (in the naming hierarchy) and the public key of its parent (in the
 naming hierarchy). In this way, anyone knowing the public key of any
 authority can learn the public key of any other by "walking the
 tree". In order that principals may authenticate even when all of
 their ancestor directories do not participate in DASS, authorities
 may also create "cross-certificates" which certify the public key of
 a named entity which is not a descendent.  Rules for finding and
 following these cross-certificates are described in the Get_Pub_Keys
 routines.  Every principal is expected to know the public key of the
 CA of the directory in which it is named. This must be securely
 learned when the principal is initialized and may be maintained in
 some form of local storage or by having the principal sign a
 certificate listing the name and public key of its parent and posting
 that certificate in the naming service.
 The syntax and content of DASS certificates are defined in terms of
 X.509 (Directory - Authentication Framework).  While that standard
 prescribes a single syntax for certificates, DASS considers
 certificates to be of one of six types:
  1. Normal Principal certificates are signed by a CA and certify

the name and public key of a principal where the name of the

    CA is a prefix of the name of the principal and is one
    component shorter.
  1. Trusted Authority certificates are signed by an ordinary

principal and certify the name and public key of the

    principal's CA (i.e., the CA whose name is a prefix of the
    principal's name and is one component shorter).
  1. Child certificates are signed by a CA and certify the name and

public key of a CA of a descendent directory (i.e., where the

    name of the issuing CA is a prefix of the name of the subject

Kaufman [Page 38] RFC 1507 DASS September 1993

    CA and is one component shorter).
  1. Parent certificates are signed by a CA and certify the name

and public key of the CA of its parent directory (i.e., whose

    name is a prefix of the name of the issuer and is one
    component shorter).
  1. Cross certificates are signed by a CA and certify the name and

public key of a CA of a directory where neither name is a

    prefix of the other.
  1. Self certificates are signed by a principal or a CA and the

issuer and subject name are the same. They are not used in

    this version of the architecture but are defined as a
    convenient data structure in which in which implementations
    may insecurely pass public keys and they may be used in the
    future in certain key roll-over procedures.
 It is intended that some future version of the architecture relax the
 restrictions above where prefixes must be one component shorter.
 Being able to handle certificates where prefixes are two or more
 components shorter complicates the logic of treewalking somewhat and
 is not immediately necessary, so such certificates are disallowed for
 now.
 The syntax of certificates is defined in section 4. For purposes of
 the algorithms which follow, the following is the portion of the
 content which is used (names in brackets refer to the field names in
 the ASN.1 encoded structure):
  1. UID of the issuer (optional)
  1. Full name of the issuer (the authority or principal signing)

[issuer]

  1. UID of the subject (optional)
  1. Full name of the subject (the authority or principal whose key

is being certified) [subject]

  1. Public Key of the subject [subjectPublicKey]
  1. Period of validity (effective date and expiration date)

[valid]

  1. Signature over the entire content of the certificate created

using the private key of the issuer.

Kaufman [Page 39] RFC 1507 DASS September 1993

 When parsing a certificate, the reader compares the two name fields
 to determine what type of certificate it is. For Parent and Trusted
 Authority certificates, the names are ignored for purposes of all
 further processing. For Child and Normal Principal certificates, only
 the suffix by which the child's name is longer than the parent's is
 used for further processing. The reason for this is so that if a
 branch of the namespace is renamed, all of the certificates in the
 moved branch remain valid for purposes of DASS processing. The only
 purposes of having full names in these certificates are (1) to comply
 with X.509, (2) for possible interoperability with other
 architectures using different algorithms, and (3) to allow principals
 to securely store their own names in trusted authority certificates
 in the case where they do not have enough local storage to keep it.

3.2 Encrypted Private Key Structure

 In order that humans need only remember a password rather than a full
 set of credentials, and also to make installation of nodes and
 servers easier, there is a defined format for encrypting RSA secrets
 under a password and posting in the naming service. This structure
 need only exist when passwords are used to protect RSA secrets; for
 servers which keep their secrets in non-volatile memory or users who
 carry smart cards, they are unnecessary.
 This structure includes the RSA private/public key pair encrypted
 under a DES key. The DES key is computed as a one-way hash of the
 password.  This structure also optionally includes the UID of the
 principal.  It is needed only if a single RSA key is shared by
 multiple principals (with multiple UIDs).
 Since this structure is posted in the name service and may be used by
 multiple implementations, its format must be architecturally defined.
 The exact encoding is listed in section 4.

3.3 Authentication Tokens

 This section of the document defines the contents of the
 authentication tokens which are produced and consumed by Create_token
 and Accept_token. With DASS, the token passed from the client to the
 server is complex, with a large number of optional parts, while the
 token passed from server to client (in the case of mutual
 authentication only) is small and simple.
 The authentication token potentially contains a large number of
 parts, most of which are optional depending on the type of
 authentication. The following defines the content and purpose of each
 of the parts, but does not describe the actual encoding (in the
 belief that such details would be distracting). The encoding is in

Kaufman [Page 40] RFC 1507 DASS September 1993

 section 4.
 The authentication process begins when the initiator calls
 Create_token with the name of the target. This routine returns an
 authentication token, which is sent to the target. The target calls
 Accept_token passing it the token. Both routines produce a second
 "mutual authentication token". The target returns this to the
 initiator to prove that it received the token.

3.3.1 Initial Authentication Token

 The components of the initial authentication token are (names in
 brackets refer to the field names within the ASN.1 encoded structures
 defined in section 4):
  a) Encrypted Shared Key - [authenticatingKey] - This is a Shared
     (DES) key encrypted under the public key of the target. Also
     included in the encrypted structure is a validity interval and
     a recognizable pattern so that the receiver can tell whether
     the decryption was successful.
  b) Login Ticket - [sourcePrincipal.userTicket] - This is a
     "delegation certificate" signed by a principal's long term
     private key delegating to a short term public key. Its "active
     ingredients" are:
    1) UID of delegating principal [subjectUID]
    2) Period of validity [validity]
    3) Delegation public key [delegatingPublicKey]
    4) Signature by private key of principal
       The existence of this signature is testimony that the
       private key corresponding to the delegation public key
       speaks for the user during the validity interval.
       This data structure is optional and will be missing if the
       authentication is only on behalf of a Local Username on a
       node (i.e., proxy) rather than on behalf of a real principal
       with a real key.
  c) Shared Key Ticket - [sourcePrincipal.sharedKeyTicketSignature]
     - This is a signature of the Encrypted Shared Key by the
     Delegation Public key in the Login Ticket.  The existence of
     this signature is testimony that  the DES key in the encrypted
     shared key speaks for the user.
     This data structure is optional and will be missing if the

Kaufman [Page 41] RFC 1507 DASS September 1993

     authentication is only on behalf of a Local Username on a node
     (i.e., proxy) rather than on behalf of a real principal with a
     real key. It will also be missing if delegation is taking
     place.
  d) Node Ticket - [sourceNode.nodeTicketSignature] - This is a
     signature of the Encrypted Shared key and a "Local Username"
     on the host node by the node's private key.  The existence of
     this signature is testimony by the node that the DES key in
     the encrypted shared key speaks for the named account on that
     node.
  e) Delegator - [sourcePrincipal.delegator] - This data structure
     contains the private login key encrypted under the Shared key.
     It is optional and is present only if the initiator is
     delegating to the destination.
  f) Authenticator - [authenticatorData] - This data structure
     contains a timestamp and a message digest of the channel
     bindings signed by the Shared Key. It is always present.
  g) Principal name - [sourcePrincipal.userName] - This is the name
     of the initiating principal. It is optional and will be
     missing for strong proxy where bits on the wire are at a
     premium and where the destination is capable of independently
     constructing the name.
  h) Node name - [sourceNode.nodeName] - This is the name of the
     initiating node. It is optional and will be missing for strong
     proxy where bits on the wire are at a premium and the name is
     present elsewhere in the message being passed.
  i) Local Username - [sourceNode.username] - This is the local
     user name on the initiating node. It is optional and will be
     missing for strong proxy where bits on the wire are at a
     premium and where the name is present elsewhere in the message
     being passed.

3.3.2 Mutual Authentication Token

 The authentication buffer sent from the target to the initiator (in
 the case of mutual authentication) is much simpler. It contains only
 the timestamp taken from the authenticator encrypted under the Shared
 Key.  It is ASN.1 encoded to allow for future extensions.

Kaufman [Page 42] RFC 1507 DASS September 1993

3.4 Credentials

 DASS organizes its internal state with Credentials structures.  There
 are many kinds of information which can be stored in credentials.
 Rather than making a different kind of data structure for each kind
 of data, DASS provides a single credentials structure where most of
 its fields are optional.  Operating systems must provide some
 mechanism for having several processes share credentials. An example
 of a mechanism for doing this would be for credentials to be stored
 in a file and the name of the file is used as a "handle" by all
 processes which use those credentials. Some of the calls which follow
 cause credentials structures to be updated. It is important to the
 performance of a system that updates to credentials (such as occur
 during the routines Verify_Principal_Name and Verify_Node_Name, where
 the caches are updated) be visible to all processes sharing those
 credentials.
 In many of the calls which follow, the credentials passed may be
 labeled: claimant credentials, verifier credentials or some such.
 This indicates whose credentials are being passed rather than a type
 of credentials. DASS supports only one type of credentials, though
 the fields present in the credentials of one sort of principal may be
 quite different from those present in the credentials of another.
 An implementation may choose to support multiple kinds of credentials
 structures each of which will support only a subset of the functions
 available if it is not implementing the full architecture.  This
 would be the case, for example, if an implementation did not support
 the case where a server both received requests from other principals
 and made requests on its own behalf using a single set of
 credentials.
 The following are a list of the fields that may be contained in a
 credentials structure. They are grouped according to common usage.

3.4.1 Claimant information

 This is the information used when the holder of these credentials is
 requesting something. It includes:
  a) Full X.500 name of the principal
  b) Public Key of the principal
  c) Login Ticket - a login ticket contains:
    1) the UID of the principal

Kaufman [Page 43] RFC 1507 DASS September 1993

    2) a period of validity (effective date & expiration date)
    3) a delegation public key
    4) a signature of the ticket contents by the principal's long
       term key
  d) Delegation Private Key (corresponding to the public key in c3)
  e) Encrypted Shared Key (present only when credentials were
     created by accept_token; this information is needed to verify
     a node ticket after credentials are accepted)

3.4.2 Verifier information

 This is the information needed by a server to decrypt incoming
 requests. It is also used by generate_server_ticket to generate a
 login ticket.
  a) RSA private key.

3.4.3 Trusted Authority

 This is information used to seed the walk of the CA hierarchy to
 reliably find the public key(s) associated with a name.
 Normally, the trusted authority in a set of credentials will be
 the directory parent of the principal named in Claimant
 information.  In some circumstances, it may instead be the
 directory parent of the node on which the credentials reside.
  a) Full X.500 name of a CA
  b) Corresponding RSA Public Key
  c) Corresponding UID

3.4.4 Remote node authentication

 This information is present only for credentials generated by
 "Accept_token". It includes information about any remote node which
 vouched for the request.
  a) Full X.500 name of the node
  b) Local Username on the node
  c) Node ticket.

Kaufman [Page 44] RFC 1507 DASS September 1993

3.4.5 Local node credentials

 This information is added by Combine_credentials, and is used by
 Create_token to add a node signature to outbound requests.
  a) Full X.500 name of the node
  b) Local Username on the node
  c) RSA private key of the node

3.4.6 Cached outgoing contexts

 There may be one (or more) such structures for each server for which
 this principal has created authentication tokens. These represent a
 cache: they may be discarded at any time with no effect except on
 performance. For each association, the following information is kept:
  a) Destination RSA Public Key (index)
  b) Encrypted Shared key
  c) Shared Key Ticket (optional, included if there has been a
     non-delegating connection)
  d) Node Ticket
  e) Delegator (optional, included if there has been a delegating
     connection)
  f) Validity interval
  g) Shared Key

3.4.7 Cached Incoming Contexts

 There may be one such structure for each client from which this server
 has received an authentication token.  These represent a cache: they
 may be discarded at any time with no effect except on performance. (An
 implementation may choose to keep one System-wide Cache (and list of
 incoming timestamps). While it is unlikely that the same Encrypted
 Shared Key will result from encryption of Shared keys generated by
 different clients or for different servers, an implementation must
 ensure that an entry made for one client/server can not be reused by
 another client/server.  Similarly an implementation may choose to keep
 separate caches for the Shared Key/Validity Interval/Delegation Public
 Key, the Nodename/UID/key/username and the Principal name/UID/key.)
 For each association, the following information is kept:

Kaufman [Page 45] RFC 1507 DASS September 1993

  a) Encrypted Shared key (index)
  b) Shared Key
  c) Validity Interval
  d) Full X.500 name of Client Principal
  e) UID of Client Principal
  f) Public Key of Client Principal
  g) Name of Client Node
  h) UID of Client Node
  i) Public Key of Client Node
  j) Local Username on Client node
  k) Delegation Public key of Client Principal's Login Ticket
 The Name, UID and Public key of the Principal are all entered
 together once the Login Ticket has been verified. Similarly the Node
 name, Node key and Username are entered together once the Node Ticket
 has been verified. These pieces of information are only present if
 they have been verified.

3.4.8 Received Authenticators

 A record of all the authenticators received is kept. This is used to
 detect replayed messages. (This list must be common to all targets
 that could accept the same authenticator (channel bindings will
 prevent other targets from accepting the same authenticator). This
 includes different `servers' sharing the same key.)  The entries in
 this list may be deleted when the timestamp is old enough that they
 would no longer be accepted. This list is kept separate from the
 Cached incoming context in order that the information in the cached
 incoming context can be discarded at any time. An implementation
 could choose to save these timestamps with the cached incoming
 context if it ensures that it can never purge entries from the cache
 before the timestamp has aged sufficiently. This list is accessed
 based on an extract from the signature from the Authenticator. The
 extract must be at least 64 bits, to ensure that it is very unlikely
 that 2 authenticators will be received with matching signatures.
  a) Extract from Signature from Authenticator

Kaufman [Page 46] RFC 1507 DASS September 1993

  b) Timestamp
 If an implementation runs out of space to store additional
 authenticators, it may either reject the token which would have
 overflowed the table or it may temporarily narrow the allowed clock
 skew to allow it to free some of the space used to hold "old"
 authenticators.  The first strategy will always falsely reject
 tokens; the second may cause false rejection of tokens if the allowed
 clock skew gets narrowed beyond the actual clock skew in the network.

3.5 CA State

 The CA needs to maintain some internal state in order to generate
 certificates. This internal state must be protected at all times, and
 great care must be taken to prevent its being disclosed. A CA may
 choose to maintain additional state information in order to enhance
 security.  In particular, it is the responsibility of the CA to
 assure that the same UID is not serially reused by two holders of a
 single name.  In most cases, this can be done by creating the UID at
 the time the user is registered.  To securely permit users to keep
 their UIDs when transferring from another CA, the CA must keep a
 record of any UIDs used by previous holders of the name. Since
 actions of a CA are so security sensitive, the CA should also
 maintain an audit trail of all certificates signed so that a history
 can be reconstructed in the event of a compromise.  Finally, for the
 convenience of the CA operator, the CA should record a list of the
 directories for which it is responsible and their UIDs so that these
 need not be entered whenever the CA is to be used.  The state
 includes at least the following information:
  1. Public Key of CA
  1. Private Key of CA
  1. Serial number of next certificate to be issued

3.6 Data types used in the routines

 There are several abstract data types used as parameters to the
 routines described in this section. These are listed here
  a) Integer
  b) Name
     Names unless otherwise noted are always X.500 names.  While
     most of the design of DASS is naming service independent, the
     syntax of certificates and tokens only permits X.500 names to
     be used.  If DASS is to be used in an environment where some

Kaufman [Page 47] RFC 1507 DASS September 1993

     other form of name is used, those names must be translated
     into something syntactically compliant with X.500 using some
     mechanism which is beyond the scope of this architecture.  The
     only other form of name appearing in this architecture is a
     "local user name", which corresponds to the simple name of an
     "account" on a node.  As a type, such names appear in
     parameter lists as "Strings".
  c) String
     A String is a sequence of printable characters.
  d) Absolute Time
     A UTC time. The precision of these Times is not stated. A
     precision of the order of one second in all times is
     sufficient.
  e) Time Interval
     A Time interval is composed of 2 times. A Start Time and an
     End Time, both of which are Absolute Times
  f) Timestamp
     A Timestamp is a time in POSIX format. I.e., two 32 bit
     Integers. The first representing seconds, and the second
     representing nanoseconds.
  g) Duration
     A Duration is the length of a time interval.
  h) Octet String
     A sequence of bytes containing binary data
  i) Boolean
     A value of either True or False
  j) UID
     A UID is an bit string of 128 bits.
  k) OID
     An OID is an ISO Object Identifier.
  l) Shared key
     A Shared key is a DES key, a sequence of 8 bytes
  m) CA State
     A structure of the form described in '3.5
  n) Credentials
     A structure of the form described in '3.4

Kaufman [Page 48] RFC 1507 DASS September 1993

  o) Certificate
     An ASN.1 encoding of the structure described in '3.1
  p) Authentication Token
     An ASN.1 encoding of the structure described in '3.3.1
  q) Mutual Authentication Token
     An ASN.1 encoding of the structure described in '3.3.2
  r) Encrypted Credentials
     An ASN.1 encoding of  the  structure described in '3.2
  s) Public key
     A representation of an RSA Public key, including all the
     information needed to encode the public key in a certificate.
  t) Set of Public key/UID pairs
     A set of Public key/UID pairs. This Data type is only used
     internally in DASS - it does not appear in any interface used
     to other architectures.

3.7 Error conditions

 These routines can return the following error conditions (an
 implementation may indicate errors with more or less precision):
  a) Incomplete chain of trustworthy CAs
  b) Target has no keys which can be trusted.
  c) Invalid Authentication Token
  d) Login Ticket Expired
  e) Invalid Password
  f) Invalid Credentials
  g) Invalid Authenticator
  h) Duplicate Authenticator

3.8 Certificate Maintenance Functions

 Authentication services depend on a set of data structures maintained
 in the naming service. There are two kinds of information:
 Certificates, which associate names and public keys and are signed by
 off-line Certification Authorities; and Encrypted Credentials, which

Kaufman [Page 49] RFC 1507 DASS September 1993

 contain RSA Private Keys and certain context information encrypted
 under passwords. Encrypted Credentials are only necessary in
 environments where passwords are used. Credentials may alternatively
 be stored in some other secure manner (for example on a smart card).
 The certificate maintenance services are designed so that the most
 sensitive - the actual signing of certificates - may be done by an
 off-line authority.  Once signed, certificates must be posted in the
 naming service to be believed.  The precise mechanisms for moving
 certificates between off-line CAs and the on-line naming service are
 implementation dependent.  For the off-line mechanisms to provide any
 actual security, the CAs must be told what to sign in some reliable
 manner.  The mechanisms for doing this are implementation dependent.
 The abstract interface says that the CA is given all of the
 information that goes into a certificate and it produces the signed
 certificate.  There are requirements surrounding the auditing of a
 CA's actions. The details of what actions are audited, where the
 audit trail is maintained, and what utilities exist to search that
 audit trail are not specified here. The functions a CA must provide
 are:

3.8.1 Install CA

 Install_CA(
                     keysize               Integer,   --inputs
                     CA_state              CA State,  --outputs
                     CA_Public_Key         Public Key)
 This routine need only generate a public/private key pair of the
 requested size. Keysize is likely to be in implementation constant
 rather than a parameter.  The value is likely to be either 512 or
 640.  Key sizes throughout will have to increase over time as
 factoring technology and CPU speeds improve.  Both keys are stored as
 part of the CA_state; the public key is returned so that other CAs
 may cross-certify this one. The `Next Serial number' in the CA state
 is set to 1.

3.8.2 Create Certificate

 Create_certificate(
                                                  --inputs
                     Renewal               Boolean,
                     Include_UID           Boolean,
                     Issuer_name           Name,
                     Issuer_UID            UID,
                     Effective_date        Absolute Time,
                     Expiration_date       Absolute Time,
                     Subject_name          Name,

Kaufman [Page 50] RFC 1507 DASS September 1993

                     Subject_UID           UID,
                     Subject_public_key    Public Key,
                                                  --updated
                     CA_state              CA State,
                                                  --outputs
                     Certificate           Certificate)
 This procedure creates and signs a certificate.  Note that the
 various contents of the certificate must be communicated to the CA in
 some reliable fashion.  The Issuer_name and UID are the name and UID
 of the directory on whose behalf the certificate is being signed.
 This routine formats and signs a certificate with the private key in
 CA_state. It audits the creation of the certificate and updates the
 sequence number which is part of CA_state. The Issuer and Subject
 names are X.500 names.  If the CA state includes a history of what
 UIDs have previously been used by what names, this call will only
 succeed in the collision case if the Renewal boolean is set true.  If
 the Include_UID boolean is set true, this routine will generate a
 1992 format X.509 certificate; otherwise it will generate a 1988
 format X.509 certificate.

3.8.3 Create Principal

 Create_principal(
                                                  --inputs
                     Password              String,
                     keysize               Integer,
                     Principal_name        Name,
                     Principal_UID         UID,
                     Parent_Public_key     Public Key,
                     Parent_UID            UID,
                                                  --outputs
                     Encrypted_Credentials Encrypted Credentials,
                     Trusted_authority_certificate Certificate)
 This procedure creates a new principal by generating a new
 public/private key pair, encrypting the public and private keys under
 the password, and signing a trusted authority certificate for the
 parent CA.  In an implementation not using passwords (e.g., smart
 cards), an alternative mechanism must be used for initially creating
 principals.  If a principal has protected storage for trusted
 authority information, it is not necessary to create a trusted
 authority certificate and store it in the naming service.  Some
 procedure analogous to this one must be executed, however, in which
 the principal learns the public key and UID of its CA and its own
 name.

Kaufman [Page 51] RFC 1507 DASS September 1993

 This routine creates two output structures with the following steps:
  a) Generate a public/private key pair using the indicated
     keysize. An implementation will likely fix the keysize as an
     implementation constant, most likely 512 or 640 bits, rather
     than accepting it as a parameter.  Key sizes generally will
     have to increase over time as factoring technology and CPU
     speeds improve.
  b) Form the encrypted credentials by using the public key,
     private key, and Principal_UID and encrypting them using a
     hash of the password as the key.
  c) Generate a trusted authority certificate (which is identical
     in format to a "parent" certificate) getting fields as
     follows:
    1) Certificate version is X.509 1992.
    2) Issuer name is the Principal name (which is an X.500 name).
    3) Issuer UID is the Principal UID.
    4) Validity is for all time.
    5) Subject name is constructed from the Principal name by
       removing the last simple name from the hierarchical name.
    6) Subject UID is the CA_UID.
    7) Subject Public Key is the CA_Public_Key
    8) Sequence number is 1.
    9) Sign the certificate with the newly generated private key of
       the principal.

3.8.4 Change Password

 Change_password(                                 --inputs
                     Encrypted_credentials Encrypted Credentials,
                     Old_password          String,
                     New_password          String,
                                                  --outputs
                     Encrypted_credentials Encrypted Credentials)
 If credentials are stored encrypted under a password, it is possible
 to change the password if the old one is known.  Note that it is

Kaufman [Page 52] RFC 1507 DASS September 1993

 insufficient to just change a user's password if the password has
 been disclosed.  Anyone knowing the old password may have already
 learned the user's private key.  If a password has been disclosed,
 the secure recovery procedure is to call create_principal again
 followed by create_certificate to certify the new key.
 Using DASS, it may not be appropriate for users to periodically
 change their passwords as a precaution unless they also change their
 private keys by the procedure above.  The only likely use of the
 change_password procedure is to handle the case where an
 administrator has chosen a password for the user in the course of
 setting up the account and the user wishes to change it to something
 the user can remember.  A future version of the architecture may
 smooth key roll-over by having the change_password command also
 generate a new key and sign a "self" certificate in which the old key
 certifies the new one.  As a separate step, a CA which notices a self
 certificate posted in the naming service could certify the new key
 instead of the old one when the user's certificate is renewed.  While
 this procedure is not as rapid or as reliable as having the user
 directly interact with the CA, it offers a reasonable tradeoff
 between security and convenience when there is no evidence of
 password compromise.
 This routine simply decrypts the encrypted credentials structure
 supplied using the password supplied. It returns a bad status if the
 format of the decrypted information is bad (indicating an incorrect
 password). Otherwise, it creates a new encrypted credentials
 structure by encrypting the same data with the new password. It would
 be highly desirable for the user interface to this function to
 provide the capability to randomly generate passwords and prohibit
 easily guessed user chosen passwords using length, character set, and
 dictionary lookup rules, but such capabilities are beyond the scope
 of this document.  If encrypted credentials are stored in some local
 secure storage, the above function is all that is necessary (in fact,
 if the storage is sufficiently secure, no password is needed;
 credentials could be stored unenciphered).  If they are stored in a
 naming service, this function must be coupled with one which
 retrieves the old encrypted credentials from the naming service and
 stores the new.  The full protocol is likely to include access
 control checks that require the principal to acquire credentials and
 produce tokens.  For best security, the encrypted credentials should
 be accessible only through a login agent.  The role of the login
 agent is to audit and limit the rate of password guessing.  If
 passwords are well chosen, there is no significant threat from
 password guessing because searching the space is computationally
 infeasible.  In the context of a login agent, change password will be
 implemented with a specialized protocol requiring knowledge of the
 password and (for best security) a trusted authority from which the

Kaufman [Page 53] RFC 1507 DASS September 1993

 public key of the login agent can be learned.  See section 2.3.2 for
 the plans for the non-X.500 credential storage facility.

3.8.5 Change Name

 Change_name(
                                                  --inputs
                     Claimant_Credentials  Credentials,
                     New_name              Name,
                     CA_Public_Key         Public Key,
                     CA_UID                UID,
                                                  --outputs
                     Trusted_Authority_Certificate Certificate)
 DASS permits a principal to have many current aliases, but only one
 current name.  A principal can authenticate itself as any of its
 aliases but verifies the names of others relative to the name by
 which it knows itself.  Aliases can be created simply by using the
 create_certificate function once for each alias.  To change the name
 of a principal, however, requires that the principal securely learn
 the public key and UID of its new parent CA.  As with
 create_principal, if a principal has secure private storage for its
 trusted authority information, it need not create a certificate, but
 some analogous procedure must be able to install new naming
 information.
 This routine produces a new Trusted Authority Certificate with
 contents as follows:
  a) Issuer name is New_name (an X.500 name)
  b) Issuer_UID is Principal UID from Credentials.
  c) Validity is for all time.
  d) Subject name is constructed from the Issuer name by removing
     the last simple name from the hierarchical name, and
     converting to an X.500 name.
  e) Subject UID is CA_UID
  f) Subject Public Key is CA_Public_Key
  g) Sequence number is 1.
  h) The certificate is signed with the private key of the
     principal from the credentials. Note that this call will only
     succeed if the principal's private key is in the credentials,

Kaufman [Page 54] RFC 1507 DASS September 1993

     which will only be true if the credentials were created by
     calling Create_server_credentials.

3.9 Credential Maintenance Functions

 DASS credentials can potentially have information about two
 principals.  This functionality is included to support the case
 where a user on a node has two identities that might be
 recognized for purposes of managing access controls.  First,
 there is the user's network identity; second, there is an
 identity as controlling a particular "account" or "username" on
 that node.  There are two reasons for recognizing this second
 identity: first, access controls might be specified such that
 only a user is only permitted access to certain resources when
 coming through certain trusted nodes (e.g., files that can't be
 accessed from a terminal at home); and second, before the
 transition strategy to global identities is complete, as a way to
 refer to USER@NODE in a way analogous to existing mechanisms but
 with greater security.
 The mapping of global usernames to local user names on a node is
 outside the scope of DASS.  This is done via a "proxy database"
 or some analogous local mechanism.  What DASS provides are
 mechanisms for adding node oriented credentials into a user's
 credentials structure, carrying the dual authentication
 information in authentication tokens, and extracting the
 information from the credentials structure created by
 Accept_token.
 Some applications of DASS will not make use of the node
 authentication related extensions.  In that case, they will never
 use the Combine_credentials, Create_credentials, Get_node_info,
 or Verify_node_name functions.
 The "normal" sequence of events surrounding a user logging into a
 node are as follows:
  a) When the user logs in, he types either a local user ID known
     to the node or a global name (the details of the user
     interface are implementation specific).  Through some sort of
     local mapping, the node determines both a global name and a
     local account name.  The user also enters a password
     corresponding to the global name.
  b) The node calls network_login specifying the user's global name
     and the supplied password.  The result is credentials which
     can be used to access network services but which have not yet
     been verified to be valid.

Kaufman [Page 55] RFC 1507 DASS September 1993

  c) The node calls verify_principal_name using its own credentials
     to verify the authenticity of the user's credentials (these
     node credentials must have previously been established by a
     call to initialize_server during node initialization).
  d) If that test succeeds, the node adds its credentials to those
     of the user by calling combine_credentials.
 The set of facilities for manipulating credentials follow:

3.9.1 Network login

 Network_login(
                                                  --inputs
                     Name                  Name,
                     password              String,
                     keysize               Integer,
                     expiration            Time interval,
                     TA_credentials        Credentials,--optional
                                                  --outputs
                     Claimant_credentials  Credentials)
 This function creates credentials for a principal when the principal
 "logs into the network".
 Name is the X.500 name of the principal.
 Password is a secret which authenticates the principal to the
 network.
 Keysize specifies the size of the temporary "login" or "delegation"
 key.  In a real implementation, it is expected to be an
 implementation constant (most likely 384 or 512 bits).
 Expiration sets a lifetime for the credentials created.  For a normal
 login, this is likely to be an implementation constant on the order
 of 8-72 hours.  Some mechanism for overriding it must be provided to
 make it possible (for example) to submit a background job that might
 run days or even months after they are submitted.
 TA_credentials   are used if the encrypted credentials are protected
 by a login agent. If they are missing, the password will be less well
 protected from guessing attacks.
 This routine does not (as one might expect) securely authenticate the
 principal to the calling procedure.  Since the password is used to
 obtain the principal's private key, this call will normally fail if
 the principal supplies an invalid password.  A penetrator who has

Kaufman [Page 56] RFC 1507 DASS September 1993

 compromised the naming service could plant fake encrypted credentials
 under any name and impersonate that name as far as this call is
 concerned. A caller that wishes to authenticate the user in addition
 to obtaining credentials to be able to act on the user's behalf
 should call Verify_principal_name (below) with the created
 credentials and the credentials of the calling process.
 This routine constructs a credentials structure from information
 found in the naming service encrypted using the supplied password.
  a) If the encrypted credentials structure is protected with a
     login agent, retrieve the public key of the login agent:
    1) If TA_credentials are available, use them in a call to
       Get_Pub_Keys to get the public key of the login agent (whose
       name is derived from the name of the principal by truncating
       the last element of the RDN and adding CSS=X509).
    2) If TA_credentials are not available, look up the public key
       of the login agent in the naming service.
     Login agents limit and audit password guesses, and are
     important when passwords may not be well chosen (as when users
     are allowed to choose their own).  To fully prevent the
     password guessing threat, principals may only log onto nodes
     that already have TA_credentials which can be used to
     authenticate the login agent.  To support nodes which have no
     credentials of their own and to allow this procedure to
     support node initialization, it is possible to network login
     without TA credentials.
     A principal who logs into a node that lacks TA credentials is
     subject to the following subtle security threat:  A penetrator
     who impersonates the naming service could post his own public
     key and address as those of the login agent.  This procedure
     would then in the process of logging in reveal the the
     penetrator enough information for the penetrator to mount an
     unaudited password guessing attack against the principal's
     credentials.
  b) Retrieve the encrypted credentials from the naming service or
     login agent.  In the case of the login agent, the password is
     one-way hashed to produce proof of knowledge of the password
     and the hashed value is supplied to the login agent encrypted
     under its public key as part of the request.
  c) Decrypt the encrypted credentials structure using a the
     supplied password. Verify that the decryption was successful

Kaufman [Page 57] RFC 1507 DASS September 1993

     by verifying that the resulting structure can be parsed
     according the the ASN.1 rules for Encrypted_Credentials and
     that the two included primes when multiplied together produce
     the included modulus. If the decryption was unsuccessful then
     the routine returns the `Invalid password' error status. The
     decryption results in both the Private Key and the Public Key.
  d) Generate a public/private key pair for the Delegation Key,
     using the indicated keysize. Key size is likely to be an
     implementation constant rather than a supplied parameter, with
     likely values being 384 and 512 bits.  Key sizes generally
     will have to increase over time as factoring technology and
     CPU speeds improve.  Delegation keys can be relatively shorter
     than long term keys because DASS is designed so that
     compromise of the delegation key after it has expired does not
     result in a security compromise.  An important advantage of
     making key size an implementation constant is that nodes can
     generate key pairs in advance, thus speeding up this procedure.
     Key generation is the most CPU intensive RSA procedure and
     could make login annoyingly slow.
  e) Construct a Login Ticket by signing with the user's private
     key a combination of the public key, a validity period
     constructed from the current time and the expiration passed in
     the call, and the principal UID found in the encrypted-key
     structure.
  f) Forget the user's private key.
  g) Retrieve from the naming service any trusted authority
     certificates stored with the user's entry. Discard any that
     are not signed by the user's public key and UID.  An
     implementation in which the login node has credentials of its
     own may choose its trusted authority information instead of
     retrieving and verifying trusted authority certificates from
     the naming service.  This will have a subtle effect on the
     security of the resulting system.
  h) Construct a credentials structure from:
    1) Claimant credentials:
      (i)  Name of the principal from calling parameter
      (ii) Login Ticket as constructed in (e)
      (iii)Delegation Private key as constructed in (d)
      (iv) Public key from the encrypted credentials structure
    2) No verifier credentials

Kaufman [Page 58] RFC 1507 DASS September 1993

    3) Trusted Authorities: for the most recently signed trusted
       authority certificate (There is normally only one Trusted
       Authority Certificate.  If there is more than one then an
       implementation may choose to maintain a list of all the valid
       keys. They should all refer to the same CA (UID and name).):
      (i)  Name of the CA from the subject field of the certificate
      (ii) Public Key of the CA from the subject public key field
      (iii)UID of the CA from the subject UID field
    4) no remote node credentials
    5) no local node credentials
    6) no cached outgoing associations
    7) no cached incoming associations

3.9.2 Create Credentials

 Create_credentials(
                                                    --outputs
                     Claimant_credentials  Credentials)
 This routine creates an "empty" credentials structure.  It is needed
 in the case of a user logging into a node and obtaining node oriented
 credentials but no global username credentials.  Because the
 "combine_credentials" call wants to modify a set of user credentials
 rather than create a new set, this call is needed to produce the
 "shell" for combine_credentials to fill in.
 It is unlikely that any real implementation would support this
 function, but rather would have some functions which combine
 network_login, create_credentials, and combine_credentials in
 whatever ways are supported by that node.

3.9.3 Combine Credentials

 Combine_credentials(
                                                  --inputs
                     node_credentials      Credentials,
                     localusername         String,
                                                  --updated
                     user_credentials      Credentials)
 This routine is provided by implementations which support the notion
 of local node credentials.  After the node has verified to its own

Kaufman [Page 59] RFC 1507 DASS September 1993

 satisfaction that the user_credentials are entitled to access to a
 particular local account, this call adds node credential information
 to the user_credential structure.  This function may be applied to
 user_credentials created by network_login, create_credentials, or
 accept_token.
  a) Fill in the local node credentials substructure of
     user_credentials as follows:
    1) Full name of the node: from Full name of the Principal in
       node_credentials
    2) Local username on the node: from proxy lookup
    3) RSA private key of the node: from verifier credentials in
       node_credentials
  b) Optionally,  change the trusted authorities to match the
     trusted authorities from the node credentials.  This is an
     implementation option, done most likely as a performance
     optimization.  The only case where this option is required is
     where no trusted authorities existed in the user credentials
     (because they were created by create_credentials of
     accept_token).  Server credentials should generally keep their
     own trusted authorities.
 It is likely that an implementation will choose not to replicate its
 node credentials in every credentials structure that it supports, but
 rather will maintain some sort of pointer to a single copy.  This
 algorithm is stated as it is only for ease of specification.

3.9.4 Initialize_server

 initialize_server(
                                                  --inputs
                     Name                  Name,
                     password              String,
                     TA_credentials        Credentials, --optional
                                                  --outputs
                     Server_credentials    Credentials)
 Somehow a server must get access to its credentials. One way is for
 the credentials to be stored in the naming service like user
 credentials encrypted under a service password. The service then
 needs to gain at startup time access to a service password. This may
 be easier to manage and is not insecure so long as the service
 password is well chosen. Alternately, the service needs some
 mechanism to gain access directly to its credentials. The credentials

Kaufman [Page 60] RFC 1507 DASS September 1993

 created by this call are intended to be very long lived. They do not
 time out, so a node or server might store them in Non-Volatile memory
 after "initial installation" rather than calling this routine at each
 "boot". These credentials are shared between all servers which use
 the same key. This routine works as follows:
  a) Retrieve from the naming service or login agent the encrypted
     credentials structure corresponding to the supplied name. See
     Network_login for a discussion of the use of TA_credentials
     and login agents.
  b) Decrypt that structure using a one-way hash of the supplied
     password. Verify that the decryption was successful. Verify
     that the public key in the structure matches the private key.
  c) Retrieve from the naming service any trusted authority
     certificates stored under the supplied name. Discard any which
     do not contain the UID from the encrypted credentials
     structure or are not signed by the key in the encrypted
     credentials structure.
  d) Construct a credentials structure from:
    1) Claimant credentials:
      (i)   Name of the principal from the calling parameter
      (ii)  UID of the principal from the encrypted-key structure
      (iii) No login ticket
      (iv)  No login secret key
    2) Verifier credentials:
      (i)   Server secret key from the encrypted-key structure
    3) Trusted Authorities: from the most recently signed Trusted
       Authority Certificate:
      (i)   Name of CA from the Subject Name field
      (ii)  UID of the CA from the Subject UID field
      (iii) Public Key of the CA from the Subject Public Key field
    4) no node credentials
    5) no cached outgoing associations
    6) no cached incoming associations

Kaufman [Page 61] RFC 1507 DASS September 1993

3.9.5 Generate Server Ticket

 generate_server_ticket(
                                                  --inputs
                     expiration            Time interval,
                                                  --updated
                     Server_credentials    Credentials)
 Server credentials created by initialize_server can be used to accept
 incoming authentication tokens and can act as node_credentials for
 outgoing authentications, but cannot create user_credentials of their
 own. If a server initiates connections on its own behalf, it must
 have a ticket just like any other user might have. That ticket has
 limited lifetime and the right to act on behalf of the server can be
 delegated. The server cannot, however, delegate the right to receive
 connections intended for it. An implementation must come up with a
 policy for the expiration of server tickets and how long before
 expiration they are renewed.  A likely policy is for this procedure
 to be implicitly called by Create_token if there is no current ticket
 present in the credentials.  If so, this interface need not be
 exposed.
 This routine is implemented as follows:
  a) Generate an RSA public/private key pair.
  b) Compute a validity interval from the current time and the
     expiration supplied.
  c) Construct a login ticket from the RSA public key (from a),
     validity interval (from b), the UID from the credentials, and
     signed with the server key in the credentials. (Discard
     previous Login Ticket if there was one).
  d) Discard all information in the  Cached Outgoing Contexts.

3.9.6 Delete Credentials

 delete_credentials(
                                                  --updated
                     credentials           Credentials)
 Erases the secrets in the credentials structure and deallocates the
 storage.

Kaufman [Page 62] RFC 1507 DASS September 1993

3.10 Authentication Procedures

 The guts of the authentication process takes place in the next two
 calls. When one principal wishes to authenticate to another, it calls
 Create_token and sends the token which results to the other. The
 recipient calls Accept_token and creates a new set of credentials.
 The other calls in this section manipulate the received credentials
 in order to retrieve its contents and verify the identity of the
 token creator.

3.10.1 Create Token

 Create_token(
                                                  --inputs
                     target_name            Name,
                     deleg_req_flag         Boolean,
                     mutual_req_flag        Boolean,
                     replay_det_req_flag    Boolean,
                     sequence_req_flag      Boolean,
                     chan_bindings          Octet String,
                     Include_principal_name Boolean,
                     Include_node_name      Boolean,
                     Include_username       Boolean,
                                                    --updated
                     claimant_credentials   Credentials,
                                                  --outputs
                     authentication_token   Authentication token,
                     mutual_authentication_token
                                 Mutual Authentication token,
                     Shared_key             Shared Key,
                     instance_identifier    Timestamp)
 This routine is used by the initiator of a connection to create an
 authentication token which will prove its identity. If the claimant
 credentials includes node/account information, the token will include
 node authentication.
 target_name is the X.500 name of the intended recipient of the token.
 Only an entity with access to the private key associated with that
 name will be able to verify the created token and generate the
 mutual_authentication_token.
 deleg_req_flag indicates whether the caller wishes to delegate to the
 recipient of the token. If it is set, the delegated_credentials
 returned by Accept_token will be capable of generating tokens on
 behalf of the caller. Node based authentication information cannot be
 delegated. The mutual_req_flag, replay_det_req_flag , and
 sequence_req_flag are put in the authentication token and passed to

Kaufman [Page 63] RFC 1507 DASS September 1993

 the target.  This information is included in the token to make it
 easier to implement the GSSAPI over DASS.  DASS itself makes no use
 of this information.
 In most applications, the purpose of a token exchange is to
 authenticate the principals controlling the two ends of a
 communication channel.  chan_bindings contains an identifier of the
 channel which is being authenticated, and thus its format and content
 should be tied to the underlying communication protocol.  DASS only
 guarantees that the information has been communicated reliably to the
 named target. If DASS is used with a cryptographically protected
 channel (such as SP4), this data should contain a one-way hash of the
 key used to encrypt the channel. If that channel is multiplexed, the
 data should also include the ID of the subchannel.  If the channel is
 not encrypted, the network must be trusted not to modify data on a
 connection.  The source and target network addresses and a connection
 ID should be included in the chan_bindings at the source and checked
 at the target.  A token exchange also results in the two ends sharing
 a key and an instance identifier.  If that key and instance
 identifier are used to cryptographically protect subsequent
 communications, then chan_bindings need not have any cryptographic
 significance but may be used to differentiate multiple entities
 sharing the public keys of communicating principals.  For example, if
 a service is replicated and all replicas share a public key,
 chan_bindings should include something that identifies a single
 instance of the service (such as current address) so that the token
 cannot be successfully presented to more than one of the servers.
 include_principal_name, include_node_name, and include_username are
 flags which determine whether the principal name, node name, and/or
 username from the credentials structure are to be included in the
 token.  This information is made optional in a token so that
 applications which communicate this information out of band can
 produce "compressed" tokens.  If this information is included in the
 token, it will be used to populate the corresponding fields in the
 credentials structure created by Accept_token.  claimant_credentials
 are the credentials of the calling procedure.  The secrets contained
 therein are used to sign the token and the trusted authorities are
 used to securely learn the public key of the target.  The cached
 outgoing contexts portion of the credentials may be updated as a side
 effect of this call.
 The major output of this routine is an  authentication_token which
 can be passed to the target in order to authenticate the caller.
 In addition to returning an authentication token, this routine
 returns a mutual_authentication_token,  a shared_key, and an
 instance_identifier. The mutual authentication token is the same as

Kaufman [Page 64] RFC 1507 DASS September 1993

 the one generated by the Accept_token call at the target. If the
 protocol using DASS wishes mutual authentication, the target should
 return this token to the source. The source will compare it to the
 one returned by this routine using Compare_Mutual_Token (below) and
 know that the token was accepted at its proper destination.
 The DES key and instance identifier can be used to encrypt or sign
 data to be sent to this target. The key and instance will be given to
 the target by Accept_token, and the key will only be known by the two
 parties to the authentication. If a single set of credentials is used
 to authenticate to the same target more than once, the same DES key
 is likely to be returned each time.  If the parties wish to protect
 against the possibility of an outside agent mixing and matching
 messages from one authenticated session with those of another, they
 should include the instance identifier in the messages. The instance
 identifier is a timestamp and it is guaranteed that the DES
 key/instance identifier pair will be unique.
 An implementation may wish to "hide" the DES key from calling
 applications by placing it in system storage and providing calls
 which encrypt/decrypt/sign/verify using the key.
 The primary tasks of this routine are to create its output
 parameters. As a side effect, it may also update claimant_credentials
 It's algorithm is as follows:
  a) The login ticket is checked. If it has passed the end of its
     lifetime an `Login Ticket Expired' error is returned. If there
     is a login ticket, but no corresponding private key then an
     `Invalid credentials' error is returned (this is the case if
     the credentials were created by an authentication-without-
     delegation operation).  If there is no login ticket or an
     expired one and if the long term private key is present in the
     credentials, an implementation may choose to automatically call
     create_server_ticket to renew the ticket.
  b) Create new timestamp using the current time.  (This timestamp
     must be unique for this Shared Key. The timestamp is a 64 bit
     POSIX time, with a resolution of 1 nanosecond An implemen tation
     must ensure that timestamps cannot be reused.)
  c) The public key and UID of target_name are looked up by calling
     get_pub_keys, using the target_name and the Trusted Authority
     section of the claimant_credentials structure. If none is
     found, an error status is returned. Otherwise, the cached
     outbound connections portion of credentials are searched
     (indexed by target Public Key) for a cached Shared key with a
     validity interval which has not expired. If a suitable one is

Kaufman [Page 65] RFC 1507 DASS September 1993

     found skip to step g, else create a cache entry as follows:
  d) Destination Public Key is the one found looking up the target.
     A Shared Key is generated at random. A validity interval is
     chosen according to node policy but not to exceed the validity
     interval of the ticket in the credentials (if any).
  e) Create the Encrypted Shared Key, using the public key of the
     Target, and place in the cache.
  f) If node authentication credentials are available in the
     credentials structure, create a "Node Ticket" signature using
     the node secret and include it in the cache.
  g) If delegation is requested and no delegator is present in the
     cache, create one by encrypting the delegation private key
     under the Shared key. The delegation private key is
     represented as an ASN.1 data structure containing only one of
     the primes (p).
  h) If delegation is not requested and no Shared Key Ticket is in
     the cache, create one by signing the requisite information
     with the delegation private key.
  i) Create the Authenticator.  The contents of the Authenticator
     (including the channel bindings) are encoded into ASN.1, and
     the signature is computed. The Authenticator is then
     re-encoded, without including the Channel Bindings but using
     the same signature.
  j) Create output_token as follows:
    1) Encrypted Shared Key from cache
    2) Login Ticket from Claimant Credentials (if present)
    3) Shared Key Ticket from cache (if no delegation and if
       present)
    4) Node Ticket from cache (if present)
    5) Delegator from cache (if delegation and if present)
    6) Authenticator
    7) Principal name from credentials (if present and parameter
       requests this)
    8) Node name from credentials (if present and parameter request
       this)
    9) Local Username from credentials (if present and parameter
       requests this)
  k) Compute Mutual_authentication_token by encrypting the
     timestamp from the authenticator using the Shared key.

Kaufman [Page 66] RFC 1507 DASS September 1993

  l) The instance_identifier is the timestamp. This and the Shared
     key are returned for use by the caller for further encryption
     operations (if these are supported).

3.10.2 Accept_token

 Accept_token(
                                                  --inputs
                     authentication_token  Authentication Token,
                     chan_bindings         Octet String,
                                                   --updated
                     verifying_credentials Credentials,
                                                  --outputs
                     accepted_credentials  Credentials,
                     deleg_req_flag        Boolean,
                     mutual_req_flag       Boolean,
                     replay_det_req_flag   Boolean,
                     sequence_req_flag     Boolean,
                     mutual_authentication_token
                                      Mutual authentication token
                     shared_key            Shared Key,
                     instance_identifier   Timestamp)
 This routine is used by the recipient of an authentication token to
 validate it.  authentication_token is the token as received;
 chan_bindings is the identifier of the channel being authenticated.
 See the description of Create_token for information on the
 appropriate contents for chan_bindings.  DASS does not enforce any
 particular content, but checks to assure that the same value is
 supplied to both Create_token and Accept_token.
 Verifying_credentials are the credentials of the recipient of the
 token.  They must include the private key of the entity named as the
 target in Create_token or the call will fail.  The cached incoming
 contexts section of the verifying credentials may be modified as a
 side effect of this call.
 Accepted_credentials will contain additional information about the
 token creator. If delegation was requested, these credentials can be
 used to make additional calls to Create_token on the creator's
 behalf. Whether or not delegation was requested, they can also be
 used in the calls which follow to gain additional information about
 the token creator.
 The deleg_req_flag indicates whether the accepted_credentials include
 delegation which can be used by the recipient to act on behalf of the
 principal.  Mutual_req_flag, replay_det_req_flag, and
 sequence_req_flag are passed through from Create_token in support of

Kaufman [Page 67] RFC 1507 DASS September 1993

 the GSSAPI.  DASS makes no use of these fields.
 The mutual_authentication_token can be returned to the token creator
 as proof of receipt. In many protocols, this will be used by a client
 to authenticate a server. Only the genuine server would be able to
 compute the mutual_authentication_token from the token.
 The shared_key and instance_identifier can be used to encrypt or sign
 data between the two authenticating parties. See Create_token.
 This routine verifies the contents of the authentication token in the
 context of the verifying credentials (In particular, the Private Key
 of the server is used.  Also, the Cached Incoming Contexts and
 Incoming Timestamp list is used.) and returns information about it.
 The algorithm updates a cache of information. This cache is not
 updated if the algorithm exits with an error. The algorithm is as
 follows:
  a) If there is a Login Ticket, but no Shared Key Ticket or
     Delegator then exit with error `Invalid Authenticator'. If
     there is a Shared Key Ticket or Delegator, but no Login Ticket
     then exit with error `Invalid Authentication Token'.
     Look up the Encrypted Shared key in the Cached Incoming Contexts
     of the credentials structure. (This cache entry is used during
     the execution of this routine. An implementation must ensure that
     references to the cache entry can not be affected by other users
     modifying the cache.  One way is to use a copy of the cache entry,
     and update it at exit.)  If it is not found then create
     a new cache entry as follows:
    1) Encrypted Shared Key, from the Authentication Token.
    2) Shared Key and Validity Interval, by decrypting the
       Encrypted Shared Key using the server private key in
       credentials. If the decryption fails then exit with error
       `Invalid Authentication Token'.
  b) Check that the Validity Interval (in the cache entry) includes
     the current time; return `Invalid Authentication Token' if not.
     Check the Timestamp is within max-clock-skew of the current
     time, return `invalid Authentication Token' if not.
     Reconstruct the Authenticator including the Channel Bindings
     passed as a parameter.

Kaufman [Page 68] RFC 1507 DASS September 1993

     Check that the reconstructed Authenticator is signed by the
     Shared key. If not then exit with error `Invalid
     Authentication Token'.
     Look up the Authenticator Signature in the Received
     Authenticators. If the same Signature is found in the list
     then exit with error `Duplicate Authenticator'. Otherwise add
     the Signature and timestamp to the list.
     If there is a Login Ticket and the Delegation Public key is in
     the cache entry, then check that the same key is specified in
     the Login Ticket, if not then exit with error `Invalid
     Authentication Token'. Place the Delegation Public key in the
     cache if it is not already there.
     If there is a Login Ticket, the Delegation Public key was not
     previously in the cache entry, and there is a Shared Key
     Ticket in the Authentication Token, then check that the Shared
     Key Ticket is signed by the Delegation Public Key in the Login
     Ticket. If not then exit with error `Invalid Authentication
     Token'.
     If a delegator is present in the message then decrypt the
     delegator using the Shared key. If the private key does not
     match the Delegation Public key then exit with error
     `Invalid Authentication Token' (The prime in the delegator
     is used to find the other prime (from the modulus). The division
     must not have a remainder.  Neither prime may be 1. The two
     primes are then used to reconstruct any other information
     needed to perform cryptographic operations.).
     Build the delegation credentials data structure as follows:
     1) Claimant credentials:
      (i)  Login Ticket from the Authentication token
      (ii) Delegation Private key from the decrypted delegator if
            the token is delegating.
      (iii)Encrypted Shared Key from the Authentication token.
     2) There are no verifier credentials.
     3) Trusted authorities are copied from the verifying_credentials
        passed to this routine (If an implementation is able to
        obtain the original Trusted Authorities of the Principal then
        it may do so instead of using the server's Trusted
        Authorities.).
     4) Remote node credentials (Node name, Username, Node Ticket)
     5) There are no local node credentials.
     6) There are no cached contexts.

Kaufman [Page 69] RFC 1507 DASS September 1993

  c) The returned boolean values are obtained from the
     Authenticator.
  d) Mutual_authentication_token is computed by encrypting the
     timestamp from the Authenticator with the Shared key from the
     cache.
  e) Instance_identifier is the timestamp from the Authenticator.
     This and the Shared key are returned to the caller for further
     encryption operations (if these are supported).

3.10.3 Compare Mutual Token

 Compare_mutual_token(
                                                  --inputs
                     Generated_token    Mutual authentication token,
                     Received_token     Mutual authentication token,
                                                   --outputs
                     equality_flag         Boolean)
 This routine compares two mutual authentication tokens and tells
 whether they match.  In the expected use, the first is the token
 generated by Create_token at the initiating end and the second is the
 token generated by Accept_token at the accepting end and returned to
 the initiating end.  This routine can be implemented as a byte by
 byte comparison of the two parameters.

3.10.4 Get Node Info

 get_node_info(
                                                  --inputs
                     accepted_credentials  Credentials,
                                                  --outputs
                     nodename              Name,
                     username              String)
 This routine extracts from accepted credentials the name of the node
 from which the authentication token came and the named account on
 that node. Because this information is not cryptographically
 protected within the token, this information can only be regarded as
 a "hint" by the receiving application.  It can, however, be verified
 using Verify_node_name in a cryptographically secure manner.  This
 information will only be present if these are accepted credentials
 and it the caller of Create_token set the include_node_name and/or
 include_username flags.
 An actual implementation is not likely to have get_node_info and
 verify_node_name as separate calls.  They are specified this way

Kaufman [Page 70] RFC 1507 DASS September 1993

 because there are different ways this information might be used.  For
 most applications, the nodename and username will be included in the
 token, and a single function might extract and verify them (it might
 in fact be part of accept token).  For other applications, the
 nodename and username will not be in the token but rather will be
 computed from other information passed during connection initiation
 so a call would have to take these as inputs.  Still other
 applications such as ACL evaluators that want to support the renaming
 and aliasing capabilities of DASS would defer verifying node
 information until they came upon an ACL which allowed access only
 from a particular node.  They would then verify that the name on the
 ACL was an authenticatable alias for the node which created the
 token.  All of these uses can be defined in terms of calls to
 get_node_info and verify_node_name.

3.10.5 Get Principal UID

 get_principal_uid(
                                                  --inputs
                     accepted_credentials  Credentials,
                                                  --outputs
                     uid                   UID)
 This routine extracts a principal UID from a set of credentials.
 As with Get_Node_Info, this interface is not likely to appear in an
 actual implementation, but rather will be bundled with other
 routines.  It is specified this way because there might be a variety
 of algorithms by which credentials are evaluated and all of them can
 be defined in terms of these primitives.
 In DASS, it is possible for a principal to have many aliases.  This
 can happen either because the principal was given multiple names to
 limit the number of CAs that need to be trusted when authenticating
 to different servers or because the principal's name has changed and
 the old name remains behind as an alias.  Accept_token returns the
 name by which the principal identified itself when creating its
 credentials. A service may know the user by some alias. The normal
 way to handle this is for the service to know the principal's UID
 (which is constant over name changes) and to compare it with the UID
 in the token to identify a likely alias situation. It gets the UID
 from the token using this routine. It then confirms the alias by
 calling verify_principal_name.
 The UID is in a signed portion of accepted credentials, but the
 signature may not have been verified at the time this call is issued.
 The information returned by this routine must therefore be regarded
 as a hint.  If a call to Verify_principal_name succeeds, however,

Kaufman [Page 71] RFC 1507 DASS September 1993

 then the caller can securely know that the name given to that routine
 and the UID returned by this one are the authenticated source of the
 token.

3.10.6 Get Principal Name

 get_principal_name(
                                                  --inputs
                     accepted_credentials  Credentials,
                                                  --outputs
                     name                  Name)
 This routine extracts a principal name from a set of credentials.
 This name is the name most recently associated with the principal. It
 may be the name that the principal supplied when the credentials were
 created (in which case it may not have been verified yet) or it may
 be a different name that has been verified.
 As with Get_Node_Info and Get_Principal_UID, this routine is not
 likely to appear in an actual implementation, but will be bundled in
 some fashion with related procedures.  The name returned by this
 procedure is not guaranteed to have been cryptographically verified.
 Verify_Principal_Name performs that function.

3.10.7 Get Lifetime

 get_lifetime(
                                                  --inputs
                     Claimant_credentials  Credentials,
                                                  --outputs
                     lifetime              Duration)
 This routine computes the life remaining in a set of credentials.
 Its most common use would be to know to renew credentials before they
 expire.
 Returns the remaining lifetime of the login ticket in the
 credentials. This can either be the done on the node where the
 original login took place, or at a server which has been delegated
 to. It indicates how much longer these credentials can be used for
 further delegations. This routine will return 0 if the login ticket
 has passed the end of its life, if there is no login ticket, or if
 the credentials do not contain the private key certified by the
 ticket (i.e., where they were created by an authentication-without-
 delegation operation).

Kaufman [Page 72] RFC 1507 DASS September 1993

3.10.8 Verify Node Name

 Verify_node_name(
                                                  --inputs
                     nodename              Name,
                     username              String,
                                                   --updated
                     verifying_credentials Credentials,
                     accepted_credentials  Credentials,
                                                  --outputs
                     Name matches          Boolean)
 This routine tests whether the originating node of an authentication
 token can be authenticated as having the provided name. Like a
 principal, a node may have multiple aliases. One of them may be
 returned by Get_node_info, but this call allows a suspected alias to
 be verified.  The verifying credentials supplied with this call must
 be the same credentials as were used in the Accept_token call. The
 procedure for completing this request is as follows:
  a) If there is no Node Ticket in the claimant credentials then
     return False.
  b) Search the incoming context cache of the verifying credentials
     for an entry containing the same encrypted shared key as the
     encrypted shared key subfield of the claimant information of
     the accepted credentials.  In the steps which follow,
     references to "the cache" refer to this entry.  If none is
     found, initialize such an entry as follows:
    1) Encrypted shared key from the encrypted shared key subfield
       of the claimant information of the accepted credentials.
    2) The shared key and validity interval are determined by
       decrypting the encrypted shared key using the RSA private
       key in the verifier information of the server credentials.
       If this procedure is called after a call to Accept_token
       using the same server credentials (as is required for
       correct use), the shared key and validity interval must
       correctly decrypt.  If called in some other context, the
       results are undefined.  The validity interval is not
       checked.
    3) Initialize all other entries in the cache to missing.
  c) If there is a "local username on client node" in the cache and
     it does not match the username supplied as a parameter, return
     False.

Kaufman [Page 73] RFC 1507 DASS September 1993

  d) If there is a "name of client node" in the cache and it
     matches the nodename supplied as a parameter:
    1) Set the "Full name of the node" subfield of the remote node
       authentication field of the accepted credentials to be the
       nodename supplied as a parameter.
    2) Set the "Local Username on the node" subfield of the remote
       node authentication field of the accepted credentials to be
       the username supplied as a parameter.
    3) return True.
  e) Call the Get_Pub_Keys subroutine with the server_credentials,
     the nodename supplied as a parameter, and Try_Hard=False.
  f) If "Public Key of Client Node" is missing from the cache,
     check all of the Public keys returned to see if one verifies
     the node ticket.  If one does, set the "Public Key of Client
     Node" and "UID of Client Node" fields in the cache to be the
     PK/UID pair that verified the ticket and set the "Local
     Username on Client node" field to be the username supplied as
     a parameter..
  g) If any of the Public Key/UID pairs match the "Public Key of
     Client Node" and "UID of Client Node" fields in the cache,
     then:
    1) Set the "name of client node" in the cache equal to the
       nodename supplied as a parameter.
    2) Set the "Full name of the node" subfield of the remote node
       authentication field of the accepted credentials to be the
       nodename supplied as a parameter.
    3) Set the "Local Username on the node" subfield of the remote
       node authentication field of the accepted credentials to be
       the username supplied as a parameter.
    4) Return True.
  h) If none of them match, call Get_Pub_Keys again with
     Try_Hard=True and repeat steps 6 & 7.  If Step 7 fails a
     second time, return False.

Kaufman [Page 74] RFC 1507 DASS September 1993

3.10.9 Verify Principal Name

 Verify_principal_name(
                                                  --inputs
                     principal_name        Name,
                                                   --updated
                     verifier_credentials  Credentials,
                     claimant_credentials  Credentials,
                                                  --outputs
                     Name matches          Boolean)
 This routine tests (in the context of the verifier credentials)
 whether the claimant credentials are authenticatable as being those
 of the named principal.  This procedure is called with a set of
 accepted credentials to authenticate their source, or with a set of
 credentials produced by network_login to authenticate the creator of
 those credentials.  If the claimant credentials were created by
 Accept_token, then the verifier credentials supplied in this call
 must be the same as those used in that call.  The procedure for
 completing this request is as follows:
  a) If there is no Login Ticket in the claimant credentials, then
     return False.
  b) If the current time is not within the validity interval of the
     Login Ticket, then return False.
  c) If there is an Encrypted Shared Key present in the Claimant
     information field of the claimant credentials, then find or
     create a matching cache entry in the Cached Incoming Contexts
     of the verifier credentials.  In the description which
     follows, references to "the cache" refer to this entry.  If
     the cache entry must be created, its contents is set to be as
     follows:
    1) Encrypted shared key from the encrypted shared key subfield
       of the claimant information of the accepted credentials.
    2) The shared key and validity interval are determined by
       decrypting the encrypted shared key using the RSA private
       key in the verifier information of the server credentials.
       If this procedure is called after a call to Accept_token
       using the same server credentials (as is required for
       correct use), the shared key and validity interval must
       correctly decrypt.  If called in some other context, the
       results are undefined.  The validity interval is not
       checked.

Kaufman [Page 75] RFC 1507 DASS September 1993

    3) Initialize all other entries in the cache to missing.
  d) If there is a cache entry and if the "Public Key of Client
     Principal" field is present and if the "UID of Client
     Principal" field is present and matches the UID in the Login
     Ticket, then:
    1) Set the Public Key of the principal field in the Claimant
       information to be the Public Key of Client Principal.
    2) If the "Full name of the principal" field is missing from
       the claimant information of the claimant credentials, then
       set it to the "Name of Client Principal" field from the
       cache.
  e) If there is a cache entry and if the "Name of Client
     Principal" field is present and if it matches the principal
     name supplied to this routine and if the UID in the cache
     matches the UID in the Login Ticket, return True.
  f) Call the Get_Pub_Keys subroutine with the name and verifier
     credentials supplied to this routine and Try_Hard=FALSE.
     Ignore any keys retrieved where the corresponding UID does not
     match the UID in the claimant credentials.
  g) If the Public Key of the principal is missing from the
     claimant information of the claimant credentials, then attempt
     to verify the signature on the login ticket with each public
     key returned by Get_Pub_Keys.  If verification succeeds:
    1) Set the Public Key of the principal in the claimant
       information of the claimant credentials to be the Public Key
       that verified the ticket.
    2) If the Full name of the principal in the claimant
       information of the claimant credentials is missing, set it
       to the name supplied to this routine.
    3) If there is a cache entry, set the Name of Client Principal
       to be the name supplied to this routine, the UID of Client
       Principal to be the UID from the Login Ticket, and the
       Public Key of Client Principal to be the Public Key that
       verified the ticket.
    4) Return True.
  h) If the Public Key of the principal is present in the claimant
     information of the claimant credentials, then see if it

Kaufman [Page 76] RFC 1507 DASS September 1993

     matches any of the public keys returned by Get_Pub_Keys.  If
     one of them matches:
    1) If the Full name of the principal in the claimant
       information of the claimant credentials is missing, set it
       to the name supplied to this routine.
    2) If there is a cache entry, set the Name of Client Principal
       to be the name supplied to this routine, the UID of Client
       Principal to be the UID from the Login Ticket, and the
       Public Key of Client Principal to be the Public Key that
       verified the ticket.
    3) Return True.
  i) If steps 7 & 8 fail, retry the call to Get_Pub_Keys with
     Try_Hard=TRUE, and retry steps 7 & 8.  If they fail again,
     return false.

3.10.10 Get Pub Keys

 Get_Pub_Keys(
                                                  --inputs
                     TA_credentials     Credentials
                     Try_Hard           Boolean,
                     Target Name        Name,
                                                  --outputs
                     Pub_keys           Set of Public key/UID pairs
 This common subroutine is used in the execution of Create_Token,
 Verify_Principal_Name, and Verify_Node_Name.  Given the name of a
 principal, it retrieves a set of public key/UID pairs which
 authenticate that principal (normally only one pair).  It does this
 by retrieving from the naming service a series of certificates,
 verifying the signatures on those certificates, and verifying that
 the sequence of certificates constitute a valid "treewalk".
 The credentials structure passed into this procedure represent a
 starting point for the treewalk.  Included in these credentials will
 be the public key, UID, and name of an authority that is trusted to
 authenticate all remote principals (directly or indirectly).
 The "Try_Hard" bit is a specification anomaly resulting from the fact
 that caches maintained by this routine are not transparent to the
 calling routines.  It tells this procedure to bypass caches when
 doing all name service lookups because the information in caches is
 believed to be stale.  In general, a routine will call Get_Pub_Keys
 with Try_Hard set false and try to use the keys returned.  If use of

Kaufman [Page 77] RFC 1507 DASS September 1993

 those keys fails, the calling routine may call this routine again
 with Try_Hard set true in hopes of getting additional keys.
 Routinely calling this routine with Try_Hard set true is likely to
 have adverse performance implications but would not affect the
 correctness or the security of the operation.
 The name supplied is the full X.500 name of the principal for whom
 public keys are needed as part of some authentication process.
 This procedure securely learns the public keys and UIDs of foreign
 principals by constructing a valid chain of certificates between its
 trusted TA and the certificate naming the foreign principal.  In the
 simplest case, where the TA has signed a certificate for the foreign
 principal, the chain consists of a single certificate.  Otherwise,
 the chain must consist of a series of certificates where the first is
 signed by the TA, the last is a certificate for the foreign
 principal, and the subject of each principal in the chain is the
 issuer of the next.  What follows is first a definition of what
 constitutes a valid chain of certificates followed by a model
 algorithm which constructs all of (and only) the valid chains which
 exist between the TA and the target name.
 In order to limit the implications of the compromise of a single CA,
 and also to limit the complexity of the search of the certificate
 space, there are restrictions on what constitutes a valid chain of
 certificates from the TA to the Name provided.  The only CAs whose
 compromise should be able to compromise an authentication are those
 controlling directories that are ancestors of one of the two names
 and that are not above a common ancestor.  Therefore, only
 certificates signed by those CAs will be considered valid in a
 certificate chain.  Normally, the CA for a directory is expected to
 certify a public key and UID for the CA of each child directory and
 one parent directory.  A CA may also certify another CA for some
 remote part of the naming hierarchy, and such certificates are
 necessary if there are no CAs assigned to directories high in the
 naming hierarchy.
 A certificate chain is considered valid if it meets the following
 criteria:
  a) It must consist of zero or more  parent certificates, followed
     by zero or one   cross certificates, followed by zero or more
     child certificates.
  b) The number of parent certificates may not exceed the number of
     levels in the naming hierarchy between the TA name and the
     name of the least common ancestor in the naming hierarchy
     between the TA name and the target name.

Kaufman [Page 78] RFC 1507 DASS September 1993

  c) Each  parent certificate must be stored in the naming service
     under the entry of its issuer.
  d) The subject of the cross certificate (if any) must be an
     ancestor of the target name but must be a longer name than the
     least common ancestor of the TA name and the target name.
  e) The cross certificate (if any) must have been stored in the
     naming service under the entry of its issuer or there must
     have been an indication in the naming service that
     certificates signed by this issuer may be stored with their
     subjects.
  f) The issuer of each parent certificate does not have stored
     with it in the naming service a cross certificate with the
     same issuer whose subject is an ancestor of the target name.
  g) Each child certificate must be stored in the naming service
     under the entry of its subject.
  h) The subject of each child certificate does not have associated
     with it in the naming service a cross certificate with the
     same subject whose issuer is the same as the issuer of any of
     the parent certificates or the cross certificate of the chain.
  i) The subject of each certificate must be the issuer of the
     certificate that follows in the chain.  The equality test can
     be met by either of two methods:
    1) The public key of the subject in the earlier certificate
       verifies the signature of the later and the subject UID in
       the earlier certificate is equal to the issuer UID in the
       later; or
    2) The public key of the subject in the earlier certificate
       verifies the signature of the later, the earlier lacks a
       subject UID and/or the later lacks an issuer UID and the
       name of the subject in the earlier certificate is equal to
       the name of the issuer in the later.
  j) The Public Key of the TA verifies the signature of the first
     certificate.
  k) The UID of the TA equals the UID of the issuer of the first
     certificate  or the UID is missing on one or both places and
     the name of the TA equals the name of the issuer of the first
     certificate.

Kaufman [Page 79] RFC 1507 DASS September 1993

  l) All of the certificates are valid X.509 encodings and the
     current time is within all of their validity intervals.
 If a chain is valid, the name which it authenticates can be
 constructed as follows:
  a) If the chain contains a cross certificate, the name
     authenticated can be constructed by taking the subject name
     from the cross certificate and appending to it a relative name
     for each child certificate which follows.  The relative name
     is the extension by which the subject name in the child
     certificate extends the issuer name.
  b) If the chain does not contain a cross certificate, the name
     authenticated can be constructed by taking the TA name,
     truncating from it the last  n name components where  n is the
     number of  parent certificates in the chain, and appending to
     the result a relative name for each child certificate.  The
     relative name is the extension by which the subject name in
     the child certificate extends the issuer name.
 In the common case, the authenticated name will be the subject
 name in the last certificate.  The authenticated name is
 constructed by the rules above to deal with namespace
 reorganization.  If a branch of the namespace is renamed (due to,
 for example, a corporate acquisition or reorganization), only the
 certificates around the break point need to be regenerated.
 Certificates below the break will continue to contain the old
 names (until renewed), but the algorithms above assure the
 principals in that branch will be able to authenticate as their
 new names.  Further, if the certificates at the branch point are
 maintained for both the old and new names for an interim period,
 principals in the moved branch will be able to authenticate as
 either their old or new names for that interim period without
 having duplicate certificates.
 A final complication that the algorithm must deal with is the
 location of  cross certificates.  If a key is compromised or for
 some other reason it is important to revoke a certificate ahead
 of its expiration, it is removed from the naming service.  This
 algorithm will only use certificates that it has recently
 retrieved from the naming service, so revocation is as effective
 as the mechanisms that prevent impersonation of the naming
 service.   There are plans to eventually use DASS mechanisms to
 secure access to the naming service; until they are in place,
 name service impersonation is a theoretical threat to the
 security of revocation.  Opinions differ as to whether it is a
 practical threat.   Child certificates are always stored with the

Kaufman [Page 80] RFC 1507 DASS September 1993

 subject and will not be found unless stored in the name server of
 the subject.    Parent  certificates are always stored with the
 issuer and will not be found unless stored in the name server of
 the issuer.  For best security, cross certificates should be
 stored with the issuer because the name server for the subject
 may not be adequately trustworthy to perform revocation.  There
 are performance and availability penalties, however, in doing so.
 The architecture and the algorithm therefore support storing
 cross certificates with either the issuer or the subject.  There
 must be some sort of flag in the name service associated with the
 issuer saying whether cross certificates from that issuer are
 permitted to be stored in the subject's name service entry, and
 if that flag is set such certificates will be found and used.
 In order to make revocation effective, DASS must assure that
 naming service caches do not become arbitrarily stale (the
 allowed age of a cache entry is included in the sum of times with
 together make up the revocation time).  If DASS uses a naming
 service such as DNS that does not time out cache entries, it must
 bypass cache on all calls and (to achieve reasonable performance)
 maintain its own naming service cache.  It may be advantageous to
 maintain a cache in any case so the that the fact that the
 certificates have been verified can be cached as well as the fact
 that they are current.

3.10.10.1 Basic Algorithm

 For ease of exposition, this first description will ignore the
 operation of any caches.  Permissible modifications to take
 advantage of caches and enhance performance will be covered in
 the next section.  This path will be followed if the Try_Hard bit
 is set True on the call.
 Rather than trying construct all possible chains between the TA
 and the name to be authenticated (in the event of multiple
 certificates per principal, there could be exponentially many
 valid chains), this algorithm computes a set of PK/UID/Name
 triples that are valid for each principal on the path between the
 TA and the name to be authenticated.  By doing so, it minimizes
 the processing of redundant information.
  a) Determining path and initialization
     Several state variables are manipulated during the tree walk.
     These are called:

Kaufman [Page 81] RFC 1507 DASS September 1993

    1) Current-directory-name
       This is the name indicating the current place in the tree
       walk.  Initially, this is the name of the TA.
    2) Least-Common-Ancestor-Name
       This is the portion of the names which is common to both the
       CA and the Target.  This is computed at initialization and
       does not change during the treewalk.
    3) Trusted-Key-Set
       For each name which is an ancestor of either the TA or the
       Target but not of the Least-Common-Ancestor, a list of
       PK/UID/Name triples.  This is initialized to a single triple
       from the TA information in the supplied credentials.
    4) Search-when-descending
       This is a list of PK/UID/Name triples of issuers that will
       be trusted when descending the tree.  This set is initially
       empty.
    5) Saved-RDNs
       This is a sequence of Relative Distinguished Names (RDNs)
       stripped off the right of the target name to form
       Least-common-ancestor-name.  This "stack" is initially empty
       and is populated during Step 3.
  b) Ascending the "TA side" of the tree
     While Current-directory-name is not identical to
     Common-point-Name the algorithm moves up the tree. At each
     step it does the following operations.
    1) Find all cross certificates stored in the naming service
       under Current-directory-name in which the subject is an
       ancestor of the principal to be authenticated or an
       indication that cross certificates from this issuer are
       stored in the subject entry.  If there is an indication that
       such certificates are stored in the subject entry, copy all
       triples in Trusted-Key-Set for Current-directory-name into
       the "Search-when-descending" list.  If any such certificates
       are found, filter them to include only those which meet the
       following criteria:
      (i)  For some triple in the Trusted-Key-Set corresponding to
           the Current-directory-name, the public key in the triple
           verifies the signature on the certificate  and either the
           UID in the triple matches the issuer UID in the
           certificate  or the UID in the triple and/or the

Kaufman [Page 82] RFC 1507 DASS September 1993

           certificate is missing and the name in the triple matches
           the issuer name in the certificate.
      (ii) No certificates were found signed by this issuer in which
           the subject name is longer than the subject name in this
           certificate (i.e., if there are cross certificates to two
           different ancestors, accept only those which lead to the
           closest ancestor).
      (iii)The current time is within the validity interval of the
           certificate.
    2) If any cross certificates were found (whether or not they
       were all eliminated as part of the filtering process), set
       Current-directory-name to the longest name that was found in
       any certificate and construct a set of PK/UID/Name triples
       for that name from the certificates which pass the filter
       and place them in the Trusted Key Set associated with their
       subject.  Exit the ascending tree loop at this point and
       proceed directly to step 3.  Note that this means that if
       there are cross certificates to an ancestor of the target
       but they are all rejected (for example if they have
       expired), the treewalk will   not construct a chain through
       the least common ancestor and will ultimately fail unless a
       crosslink from a lower ancestor is found stored with its
       subject.  This is a security feature.
    3) If no cross certificates are found, find all the parent
       directory certificates for the directory whose name is in
       the Current-directory-name.  Filter these to find only those
       which meet the following criteria:
      (i)  The current time is within the validity interval.
      (ii) For some triple corresponding to the
           Current-directory-name, the public key in the triple
           verifies the signature on the certificate  and either  the
           UID in the triple matches the issuer UID in the
           certificate  or the UID in the triple and/or the
           certificate is missing and the name in the triple matches
           the issuer name in the certificate.
    4) Construct PK/UID/Name triples from the remaining
       certificates for the directory whose name is constructed by
       stripping the rightmost simple name from the
       Current-directory-name and place them in the Trusted-Key-Set.

Kaufman [Page 83] RFC 1507 DASS September 1993

    5) Strip the rightmost simple name of the
       Current-directory-name.
    6) Repeat from step (a) (testing to see if
       current-directory-name is the same as Common-point-Name).
  c) Searching the "target side" of the tree for a crosslink:
    1) Initialization: set Current-directory-name to the name
       supplied as input to this procedure.
    2) Retrieve from the naming service all cross certificates
       associated with Current-directory-name.  Filter to only
       those that meet the following criteria:
      (i)  The current time is within their validity interval.
      (ii) The subject name is equal to Current-directory-name.
      (iii)For some PK/UID/Name triple in the
           "Search-when-descending" list compiled while ascending
           the tree, the Public Key verifies the signature on the
           certificate and  either the UID matches the issuer UID in
           the certificate   or a UID is missing from the triple
           and/or the certificate and the Name in the triple matches
           the issuer name in the certificate.
      (iv) There are no certificates found meeting criteria (ii) and
           (iii) matching a PK/UID/Name triple in the
           Search-when-descending list whose subject is a directory
           lower in the naming hierarchy.
    3) If any qualifying certificates are found, construct
       PK/UID/Name triples for each of them; these should replace
       rather than supplement any triples already in the
       Trusted-key-set for that directory.
    4) If after steps (b) and (c), there are no PK/UID/Name triples
       corresponding to Current-directory-name in Trusted-Key-Set,
       shorten Current-directory-name by one RDN (pushing it onto
       the Saved-RDNs stack) and repeat this process until
       Current-directory-name is equal to
       Least-common-ancestor-name  or there is at least one triple
       in Trusted-key-set corresponding to Current-directory-name.
  d) Descending the tree
     While the list Saved-RDNs is not Empty the algorithm moves

Kaufman [Page 84] RFC 1507 DASS September 1993

     down the tree. At each step it does the following operations.
    1) Remove the first RDN from Saved-RDNs and append it to the
       Current-directory-name.
    2) Find all the child directory certificates for the directory
       whose name is in the current-directory-name.
    3) Filter these certificates to find only those which meet the
       following criteria:
      (i)  The current time is within the validity interval.
      (ii) For some PK/UID/Name triple in the Current-key-set for
           the parent directory, the Public Key verifies the
           signature on the certificate and either the UID matches
           the issuer UID of the certificate   or the UID is missing
           from the triple and/or the certificate and the Name in
           the triple matches the issuer name in the certificate.
      (iii)The issuer name in the certificate is a prefix of the
           subject name and the difference between the two names is
           the final RDN of Current-directory-name.
    4) Take the key, UID, and name from each remaining certificate
       and form a new triple corresponding to
       Current-directory-name in Trusted-Key-Set. If this set is
       empty then the algorithm exits with the
       'Incomplete-chain-of-trustworthy-CAs' error condition.
    5) repeat from step (a), appending a new simple name to
       Current-directory-name.
  e) Find public keys:
     If there are no triples in the Trusted-Key-Set for the named
     principal, then the algorithm exits with the `Target-has-no-keys-w
     error condition. Otherwise, the Public Key and UID are
     extracted from each pair, duplicates are eliminated, and this
     set is returned as the Pub_keys.

3.10.10.2 Allowed Variations - Caching

 Some use of caches can be implemented without affecting the semantics
 of the Get_Pub_Keys routine.  For example, a crypto-cache could
 remember the public key that verified a signature in the past and
 could avoid the verification operation if the same key was used to
 verify the same data structure again.  In some cases, however, it is
 impossible (or at least inconvenient) for a cache implementation to

Kaufman [Page 85] RFC 1507 DASS September 1993

 be completely transparent.
 In particular, for good performance it is important that certificates
 not be re-retrieved from the naming service on every authentication.
 This must be balanced against the need to have changes to the
 contents of the naming service be reflected in DASS calls on a timely
 basis.  There are two cases of interest: changes which cause an
 authentication which previously would have succeeded to fail and
 changes which cause an authentication which previously would have
 failed to succeed.  These two cases are subject to different time
 constraints.
 In general, changes that cause authentication to succeed must be
 reflected quite quickly - on the order of minutes.  If a user
 attempts an operation, it fails, the user tracks down a system
 manager and causes the appropriate updates to take place, and the
 user retries the operation, it is unacceptable for the operation to
 continue to fail for an extended period because of stale caches.
 Changes that cause authentication to fail must be reflected reliably
 within a bounded period of time for security reasons.  If a user
 leaves the company, it must be possible to revoke his ability to
 authenticate within a relatively short period of time - say hours.
 These constraints mean that a naming service cache which contains
 arbitrarily old information is unacceptable.  To meet the second
 constraint, naming service cache entries must be timed out within a
 reasonable period of time unless in implementation verifies that the
 certificate is still present (a crypto-cache which lasted longer
 would be legal; rather than deleting a name service cache entry, in
 implementation might instead verify that the entry was still present
 in the naming service.  This would avoid repeating the cryptographic
 "verify").
 In order to assure that information cached for even a few hours not
 deny authentication for that extended period, it must be possible to
 bypass caches when the result would otherwise be a failure.  Since
 the performance of authentication failures is not a serious concern,
 it is acceptable to expect that before an operation fails a retry
 will be made to the naming service to see if there are any new
 relevant certificates (or in certain obscure conditions, to see if
 any relevant certificates have been deleted).
 If on a call to Get_Pub_Keys, the Try_Hard bit is True, then this
 procedure must return results based on the contents of the naming
 service no more than five minutes previous (this would normally be
 accomplished by ignoring name service caches and making all
 operations directly to the naming service).  If the Try_Hard bit is

Kaufman [Page 86] RFC 1507 DASS September 1993

 False, this procedure may return results based on the contents of the
 naming service any time in the previous few hours, in the sense that
 it may ignore any certificate added in the previous few hours and may
 use any certificate deleted in the previous few hours.  Procedures
 which call this routine with Try_Hard set to false must be prepared
 to call it again with Try_Hard True if their operation fails possibly
 from this result.
 The exact timer values for "five minutes" and "a few hours" are
 expected to be implementation constants.
 In the envisioned implementation, the entire "ascending treewalk" is
 retrieved, verified, and its digested contents cached when a
 principal first establishes credentials.  A mechanism should be
 provided to refresh this information periodically for principals
 whose sessions might be long lived, but it would probably be
 acceptable in the unlikely event of a user's ancestor's keys changing
 to require that the user log out and log back in.  This is consistent
 with the observed behavior of existing security mechanisms.
 The descending treewalk, on the other hand, is expected to be
 maintained as a more conventional cache, where entries are kept in a
 fixed amount of memory with a "least recently used" replacement
 policy and a watchdog timer that assures that stale information is
 not kept indefinitely.  A call to Get_Pub_Keys with Try_Hard set
 false would first check that cache for relevant certificates and only
 if none were found there would it go out to the naming service.  If
 there were newer certificates in the naming service, they might not
 be found and an authentication might therefore fail.
 When Try_Hard is false, an implementation may assume that
 certificates not in the cache do not exist so long as that assumption
 does not cause an authentication to falsely succeed.  In that case,
 it may only make that assumption if the certificates have been
 verified to not exist within the revocation time (a few hours).

3.11 DASSlessness Determination Functions

 In order to provide better interoperability with alternative
 authentication mechanisms and to provide backward compatibility with
 older (insecure) authentication mechanisms, it is sometimes important
 to be able to determine in a secure way what the appropriate
 authentication mechanism is for a particular named principal.  For
 some applications, this will be done by a local mechanism, where
 either the person creating access control information must know and
 specify the mechanism for each principal or a system administrator on
 the node must maintain a database mapping names to mechanisms.  Three
 applications come to mind where scaleability makes such mechanisms

Kaufman [Page 87] RFC 1507 DASS September 1993

 implausible:
  a) To transparently secure proxy-based applications (like rlogin)
     in an environment where some hosts have been upgraded to
     support DASS and some have not, a node must be willing to
     accept connections authenticated only by their network
     addresses but only if they can be assured that such nodes do
     not have DASS installed.  Access to a resource becomes secure
     without administrative action when all nodes authorized to
     access it have been upgraded.
     In this scenario, the server node must be able to determine
     whether the client node is DASSless in a secure fashion.
  b) Similarly, in a mixed environment where some servers are
     running DASS and some are not, it may be desirable for clients
     to authenticate servers if they can but it would be
     unacceptable for a client to stop being able to access a
     DASSless server once DASS is installed on the client.  In such
     a situation where server authentication is desirable but not
     essential, the client would like to determine in a secure
     fashion whether the server can accept DASS authentication.
  c) In a DASS/Kerberos interoperability scenario, a server may
     decide that Kerberos authentication is "good enough" for
     principals that do not have DASS credentials without
     introducing trust in on-line authorities when DASS credentials
     are available.  In parallel with case 1, we want it to be true
     that when the last principal with authority to access an
     object is upgraded to DASS, we automatically cease to trust
     PasswdEtc servers without administrative action on the part of
     the object owner.  For this purpose, the authenticator must
     learn in a secure fashion that the principal is incapable of
     DASS authentication.
 Reliably determining DASSlessness is optional for implementations of
 DASS and for applications.  No other capabilities of DASS rely on
 this one.
 The interface to the DASSlessness inquiry function is specified as a
 call independent of all others.  This capability must be exposed to
 the calling application so that a server that receives a request and
 no token can ask whether the named principal should be believed
 without a token.  It might improve performance and usability if in
 real interfaces DASSlessness were returned in addition to a bad
 status on the function that creates a token if the token is targeted
 toward a server incapable or processing it.  An application could
 then decide whether to make the request without a token (and give up

Kaufman [Page 88] RFC 1507 DASS September 1993

 server authentication) or to abort the request.

3.11.1 Query DASSlessness

 Query_DASSlessness(
                                                    --inputs
                     verifying_credentials Credentials,
                     principal_name        Name,
                                                    --outputs
                     alternate_authentication Set of OIDs)
 This function uses the verifying credentials to search for an
 alternative authentication mechanism certificate for the named
 principal or for any CA on the path between the verifying credentials
 and the named principal.  Such a certificate is identical to an DASS
 X.509 certificate except that it lists a different algorithm
 identifier for the public key of the subject than that expected by
 DASS.
 This function is implemented identically to Get_Pub_Keys except:
  a) If in any set of certificates found, no valid DASS certificate
     is found and one or more certificates are found that would
     otherwise be valid except for an invalid subject public key
     OID, the OID from that certificate or certificates is returned
     and the algorithm terminates.
  b) On initial execution, Try_Hard=False.  If the first execution
     fails to retrieve any valid PK/UID pairs but also fails to
     find any invalid OID certificates, repeat the execution with
     Try_Hard=True.
  c) If the either execution finds PK/UID pairs or if neither finds
     and invalid OID certificates, fail by returning a null set.

4. Certificate and message formats

4.1 ASN.1 encoding

 Some definitions are taken from X.501 and X.509.
 Dass DEFINITIONS ::=
 BEGIN
  1. -CCITT Definitions:
 joint-iso-ccitt      OBJECT IDENTIFIER ::= {2}

Kaufman [Page 89] RFC 1507 DASS September 1993

 ds                   OBJECT IDENTIFIER ::= {joint-iso-ccitt 5}
 algorithm            OBJECT IDENTIFIER ::= {ds 8}
 encryptionAlgorithm  OBJECT IDENTIFIER ::= {algorithm 1}
 hashAlgorithm        OBJECT IDENTIFIER ::= {algorithm 2}
 signatureAlgorithm   OBJECT IDENTIFIER ::= {algorithm 3}
 rsa                  OBJECT IDENTIFIER ::= {encryptionAlgorithm 1}
 iso                  OBJECT IDENTIFIER ::= {1}
 identified-organization OBJECT IDENTIFIER ::= {iso 3}
 ecma               OBJECT IDENTIFIER ::= {identified-organization 12}
 member-company       OBJECT IDENTIFIER ::= {ecma 2}
 digital              OBJECT IDENTIFIER ::= {member-company 1011}
  1. -1989 OSI Implementors Workshop "Stable" Agreements
 oiw                OBJECT IDENTIFIER ::= {identified-organization 14}
 dssig                  OBJECT IDENTIFIER ::= {oiw 7}
 oiwAlgorithm           OBJECT IDENTIFIER ::= {dssig 2}
 oiwEncryptionAlgorithm OBJECT IDENTIFIER ::= {oiwAlgorithm 1}
 oiwHashAlgorithm       OBJECT IDENTIFIER ::= {oiwAlgorithm 2}
 oiwSignatureAlgorithm  OBJECT IDENTIFIER ::= {oiwAlgorithm 3}
 oiwMD2                 OBJECT IDENTIFIER ::= {oiwHashAlgorithm 1}
                                                --null parameter
 oiwMD2withRSA          OBJECT IDENTIFIER ::= {oiwSignatureAlgorithm 1}
                                                --null parameter
  1. -X.501 definitions
 AttributeType ::= OBJECT IDENTIFIER
 AttributeValue ::= ANY
 AttributeValueAssertion ::= SEQUENCE {AttributeType,AttributeValue}
 Name ::= CHOICE {       --only one for now
                 RDNSequence
                     }
 RDNSequence ::= SEQUENCE OF RelativeDistinguishedName
 DistinguishedName ::= RDNSequence
 RelativeDistinguishedName ::= SET OF AttributeValueAssertion
  1. -X.509 definitions (with proposed 1992 extensions presumed)
 ENCRYPTED MACRO ::=
 BEGIN
 TYPE NOTATION   ::= type(ToBeEnciphered)
 VALUE NOTATION  ::= value(VALUE BIT STRING)
 END     -- of ENCRYPTED

Kaufman [Page 90] RFC 1507 DASS September 1993

 SIGNED MACRO    ::=
 BEGIN
 TYPE NOTATION   ::= type (ToBeSigned)
 VALUE NOTATION  ::= value (VALUE
 SEQUENCE{
         ToBeSigned,
         AlgorithmIdentifier,    --of the algorithm used to
                                 --generate the signature
         ENCRYPTED OCTET STRING  --where the octet string is the
                                 --result of the hashing of the
                                 --value of "ToBeSigned"
         }
                         )
 END     -- of SIGNED
 SIGNATURE MACRO ::=
 BEGIN
 TYPE NOTATION   ::= type (OfSignature)
 VALUE NOTATION  ::= value (VALUE
 SEQUENCE {
         AlgorithmIdentifier,    --of the algorithm used to compute
         ENCRYPTED OCTET STRING  -- the signature where the octet
                                 -- string is a function (e.g., a
                                 -- compressed or hashed version)
                                 -- of the value 'OfSignature',
                                 -- which may include the
                                 -- identifier of the algorithm
                                 -- used to compute the signature
         }
                         )
 END     -- of SIGNATURE
 Certificate ::= SIGNED SEQUENCE {
         version [0]             Version DEFAULT v1988,
         serialNumber    CertificateSerialNumber,
         signature               AlgorithmIdentifier,
         issuer          Name,
         valid           Validity,
         subject         Name,
         subjectPublicKey        SubjectPublicKeyInfo,
         issuerUID [1]   IMPLICIT UID OPTIONAL,  -- v1992
         subjectUID [2]  IMPLICIT UID OPTIONAL   -- v1992
         }
  1. -The Algorithm Identifier for both the signature field

Kaufman [Page 91] RFC 1507 DASS September 1993

  1. -and in the signature itself is:
  2. - oiwMD2withRSA (1.3.14.7.2.3.1)
 Version ::= INTEGER {v1988(0), v1992(1)}
 CertificateSerialNumber ::= INTEGER
 Validity ::= SEQUENCE {
         NotBefore       UTCTime,
         NotAfter        UTCTime
         }
 AlgorithmIdentifier ::= SEQUENCE {
         algorithm       OBJECT IDENTIFIER,
         parameter       ANY DEFINED BY algorithm OPTIONAL
         }
  1. -The algorithms we support in one context or another are:
    1. -oiwMD2withRSA (1.3.14.7.2.3.1) with parameter NULL
    2. -rsa (2.5.8.1.1) with parameter keysize INTEGER which is
    3. - the keysize in bits
    4. -decDEA (1.3.12.1001.7.1.2) with optional parameter
    5. - missing
    6. -decDEAMAC (1.3.12.2.1011.7.3.3) with optional parameter
    7. - missing
 SubjectPublicKeyInfo  ::=  SEQUENCE {
         algorithm       AlgorithmIdentifier,     -- rsa (2.5.8.1.1)
         subjectPublicKey        BIT STRING
                 -- the "bits" further decode into a DASS public key
         }
 UID ::= BIT STRING
  1. - the following definitions are for Digital specified Algorithms
 cryptoAlgorithm OBJECT IDENTIFIER ::= {digital 7}
 decEncryptionAlgorithm  OBJECT IDENTIFIER ::= {cryptoAlgorithm 1}
 decHashAlgorithm        OBJECT IDENTIFIER ::= {cryptoAlgorithm 2}
 decSignatureAlgorithm   OBJECT IDENTIFIER ::= {cryptoAlgorithm 3}
 decDASSLessness         OBJECT IDENTIFIER ::= {cryptoAlgorithm 6}
 decMD2withRSA   OBJECT IDENTIFIER ::= {decSignatureAlgorithm 1}
 decMD4withRSA   OBJECT IDENTIFIER ::= {decSignatureAlgorithm 2}
 decDEAMAC       OBJECT IDENTIFIER ::= {decSignatureAlgorithm 3}

Kaufman [Page 92] RFC 1507 DASS September 1993

 decDEA          OBJECT IDENTIFIER ::= {decEncryptionAlgorithm 2}
 decMD2          OBJECT IDENTIFIER ::= {decHashAlgorithm 1}
 decMD4          OBJECT IDENTIFIER ::= {decHashAlgorithm 2}
 ShortPosixTime ::= INTEGER   -- number of seconds since base time
 LongPosixTime ::= SEQUENCE {
         INTEGER,             -- number of seconds since base time
         INTEGER              -- number of nanoseconds since second
         }
 ShortPosixValidity ::=  SEQUENCE {
         notBefore       ShortPosixTime,
         notAfter        ShortPosixTime }
  1. - Note: Annex C of X.509 prescribes the following format for the
  2. - representation of a public key, but does not give the structure
  3. - a name.
 DASSPublicKey ::=  SEQUENCE {
         modulus         INTEGER,
         exponent        INTEGER
         }
 DASSPrivateKey ::= SEQUENCE {
         p       INTEGER ,                      -- prime p
         q [0]   IMPLICIT INTEGER OPTIONAL ,    -- prime q
         mod[1]  IMPLICIT INTEGER OPTIONAL,     -- modulus
         exp [2] IMPLICIT INTEGER OPTIONAL,     -- public exponent
         dp [3]  IMPLICIT INTEGER OPTIONAL ,    -- exponent mod p
         dq [4]  IMPLICIT INTEGER OPTIONAL ,    -- exponent mod q
         cr [5]  IMPLICIT INTEGER OPTIONAL ,    -- Chinese
                                            --remainder coefficient
         uid[6]  IMPLICIT UID OPTIONAL,
         more[7] IMPLICIT BIT STRING OPTIONAL   --Reserved for
                                                --future use
         }
 LocalUserName   ::= OCTET STRING
 ChannelId               ::= OCTET STRING
 VersionNumber           ::= OCTET STRING (SIZE(3))
                         -- first octet is major version
                         -- second octet is minor version
                         -- third octet is ECO rev.
 versionZero  VersionNumber ::= '000000'H

Kaufman [Page 93] RFC 1507 DASS September 1993

 Authenticator ::= SIGNED SEQUENCE {
         type            BIT STRING,
                  -- first bit `delegation required'
                  -- second bit `Mutual Authentication Requested'
         whenSigned      LongPosixTime ,
         channelId  [3]  IMPLICIT ChannelId OPTIONAL
                 -- channel bindings are included when doing the
                 -- signature, but excluded when transmitting the
                 -- Authenticator
         }
                 -- uses decDEAMAC (1.3.12.2.1011.7.3.3)
 EncryptedKey ::= SEQUENCE {
         algorithm               AlgorithmIdentifier,
                         -- uses rsa (2.5.8.1.1)
         encryptedAuthKey        BIT STRING
                         -- as defined in section 4.4.5
         }
 SignatureOnEncryptedKey ::=  SIGNATURE EncryptedKey
              -- uses oiwMD2withRSA (1.3.14.7.2.3.1)
              -- Signature bits computed over EncryptedKey structure
 LoginTicket ::= SIGNED SEQUENCE {
         version [0]         IMPLICIT VersionNumber DEFAULT versionZero,
         validity            ShortPosixValidity ,
         subjectUID          UID ,
         delegatingPublicKey SubjectPublicKeyInfo
         }
         -- uses oiwMD2withRSA (1.3.14.7.2.3.1)
 Delegator ::= SEQUENCE {
         algorithm               AlgorithmIdentifier
                         -- decDEA encryption (1.3.12.1001.7.1.2)
         encryptedPrivKey        ENCRYPTED  DASSPrivateKey,
                         -- (only p is included)
         }
 UserClaimant ::=  SEQUENCE {
         userTicket [0]  IMPLICIT LoginTicket,
         evidence  CHOICE {
                 delegator [1]   IMPLICIT Delegator ,
                              -- encrypted delegation private key
                              -- under DES authenticating key
                              -- present if delegating
                 sharedKeyTicketSignature [2]

Kaufman [Page 94] RFC 1507 DASS September 1993

                         IMPLICIT SignatureOnEncryptedKey
                              -- present if not delegating
                 } ,
         userName [3]    IMPLICIT Name OPTIONAL
                              -- name of user principal
         }
 EncryptedKeyandUserName ::= SEQUENCE {
         encryptedKey    EncryptedKey ,
         username                LocalUserName
         }
 SignatureOnEncryptedKeyandUserName ::=
         SIGNATURE EncryptedKeyandUserName
                 -- uses oiwMD2withRSA (1.3.14.7.2.3.1)
                 -- Signature bits computed over
                 -- EncryptedKeyandUserName structure
                 -- using node private key
         }
 NodeClaimant ::= SEQUENCE {
         nodeTicket Signature[0] IMPLICIT
                 SignatureOnEncryptedKeyandUserName,
         nodeName  [1]   IMPLICIT Name OPTIONAL,
         username  [2]   IMPLICIT LocalUserName OPTIONAL
         }
 AuthenticationToken ::= SEQUENCE {
         version [0]    IMPLICIT VersionNumber DEFAULT versionZero,
         authenticator [1]       IMPLICIT Authenticator ,
         encryptedKey  [2]       IMPLICIT EncryptedKey OPTIONAL ,
                  -- required if initiating token
         userclaimant  [3]       IMPLICIT UserClaimant OPTIONAL ,
                  -- missing if only doing node authentication
                  -- required if not doing node authentication
         nodeclaimant [4]        IMPLICIT NodeClaimant OPTIONAL
                  -- missing if only doing principal authentication
                  -- required if not doing principal authentication
         }
 MutualAuthenticationToken ::= CHOICE {
         v1Response [0] IMPLICIT  OCTET STRING (SIZE(6))
               -- Constructed as follows:  A single DES block
               -- of eight octets is constructed from the two
               -- integers in the timestamp.  First four bytes
               -- are the high order integer encoded MSB
               -- first; Last four bytes are the low order

Kaufman [Page 95] RFC 1507 DASS September 1993

  1. - integer encoded MSB first. The block is
  2. - encrypted using the shared DES key, and
  3. - the first six bytes are the OCTET STRING.
  4. - With the [0] type and 6-byte length, the
  5. - MutualAuthenticationToken has a fixed
  6. - length of eight bytes.

}

 END

4.2 Encoding Rules

 Whenever a structure is to be signed it must always be constructed
 the same way. This is particularly important where a signed structure
 has to be reconstructed by the recipient before the signature is
 verified. The rules listed below are taken from X.509.
  1. the definite form of length encoding shall be used, encoded in

the minimum number of octets;

  1. for string types, the constructed form of encoding shall not

be used;

  1. if the value of a type is its default value, it shall be

absent;

  1. the components of a Set type shall be encoded in ascending

order of their tag value;

  1. the components of a Set-of type shall be encoded in ascending

order of their octet value;

  1. if the value of a Boolean type is true, the encoding shall

have its contents octet set to `FF'16;

  1. each unused bits in the final octet of the encoding of a

BitString value, if there are any, shall be set to zero;

  1. the encoding of a Real type shall be such that bases 8, 10 and

16 shall not be used, and the binary scaling factor shall be

    zero.

4.3 Version numbers and forward compatibility

 The LoginTicket and AuthenticationToken structures contain a
 three octet version identifier which is intended to ease
 transition to future revisions of this architecture.  The default
 value, and the value which should always be supplied by
 implementations of this version of the architecture is 0.0.0

Kaufman [Page 96] RFC 1507 DASS September 1993

 (three zero octets).  The first octet is the major version.  An
 implementation of this version of the architecture should refuse
 to process data structures where it is other than zero, because
 changing it indicates that the interpretation of some subsidiary
 data structure has changed.  The second octet is the minor
 version.  An implementation of this version of the architecture
 should ignore the value of this octet.  Some future version of
 the architecture may set a value other than zero and may specify
 some different processing of the remainder of the structure based
 on that different value.  Such a change would be backward compatible
 and interoperable.  The third octet is the ECO revision.  No
 implementation should make any processing decisions based on the
 value of that octet.  It may be logged, however, to help in
 debugging interoperability problems.
 In the CDC protocol, there is also a three octet version
 numbering scheme, where versions 1.0.0 and 2.0.0 have been
 defined.  Implementations should follow the same rules above and
 reject major version numbers greater than 2.
 ASN.1 is inherently extensible because it allows new fields to be
 added "onto the end" of existing data structures in an
 unambiguous way.  Implementations of DASS are encouraged to
 ignore any such additional fields in order to enhance backwards
 compatibility with future versions of the architecture.
 Unfortunately, commonly available ASN.1 compilers lack this
 capability, so this behavior cannot reasonably be required and
 may limit options for future extensions.

4.4 Cryptographic Encoding

 Some of the substructures listed in the previous sections are
 specified as ENCRYPTED OCTET STRINGs containing encrypted
 information.  DASS uses the DES, RSA, and MD2 cryptosystems  Each
 of those cryptosystems specifies a function from octet string
 into another in the presence of a key (except MD2, which is
 keyless).  This section describes how to form the octet strings
 on which the DES and RSA operations are performed.

4.4.1 Algorithm Independence vs. Key Parity

 All of the defined encodings for DASS for secret key encryption
 are based on DES.  It is intended, however, that other
 cryptosystems could be substituted without any other changes for
 formats or algorithms.  The required "form factor" for such a
 cryptosystem is that it have a 64 bit key and operate on 64 bit
 blocks (this appears to be a common form factor for a
 cryptosystem).  For this reason, DES keys are in all places

Kaufman [Page 97] RFC 1507 DASS September 1993

 treated as though they were 64 bits long rather than 56.  Only in
 the operation of the algorithm itself are eight bits of the key
 dropped and key parity bits substituted. Choosing a key always
 involves picking a 64 bit random number.

4.4.2 Password Hashing

 Encrypted credentials are encrypted using DES as described in the
 next section.  The key for that encryption is derived from the
 user's password and name by the following algorithm:
  a) Put the rightmost RDN of the user's name in canonical form
     according to BER and the X.509 encoding rules.  For any string
     types that are case insensitive, map to upper case, and where
     matching is independent of number of spaces collapse all
     multiple spaces to a single space and delete leading and
     trailing spaces.
     Note:  the RDN is used to add "salt" to the hash calculation
     so that someone can't precompute the hash of all the words in
     a dictionary and then apply them against all names.  Deriving
     the salt from the last RDN of the name is a compromise.  If it
     were derived from the whole name, all encrypted keys would be
     obsoleted when a branch of the namespace was renamed.  If it
     were independent of name, interaction with a login agent would
     take two extra messages to retrieve the salt.  With this
     scheme, encrypted keys are obsoleted by a change in the last
     RDN and if a final RDN is common to a large number of users,
     dictionary attacks against them are easier; but the common
     case works as desired.
  b) Compute TEMP as the MD2 message digest of the concatenation of
     the password and the RDN computed above.
  c) Repeat the following 40 times:  Use the first 64 bits of TEMP
     as a DES key to encrypt the second 64 bits;  XOR the result
     with the first 64 bits of TEMP; and compute a new TEMP as MD2
     of the 128 bit result.
  d) Use the final 64 bits of the result (called hash1) as the key
     to decrypt the encrypted credentials.  Use the first 64 bits
     (called hash2) as the proof of knowledge of the password for
     presentation to a login agent (if any).

Kaufman [Page 98] RFC 1507 DASS September 1993

4.4.3 Digital DEA encryption

 DES encryption is used in the following places:
  1. In the encryption of the encrypted credentials structure
  1. To encrypt the delegator in authentication tokens
  1. To encrypt the time in the mutual authenticator
 In the first two cases, a varying length block of information
 coded in ASN.1 is encrypted.  This is done by dividing the block
 of information into 8 octet blocks, padding the last block with
 zero bytes if necessary, and encrypting the result using the CBC
 mode of DES.  A zero IV is used.
 In the third case, a fixed length (8 byte) quantity (a timestamp)
 is encrypted.  The timestamp is mapped to a byte string using
 "big endian" order and the block is encrypted using the ECB mode
 of DES.

4.4.4 Digital MAC Signing

 DES signing is used in the Authenticator.  Here, the signature is
 computed over an ASN.1 structure.  The signature is the CBC residue
 of the structure padded to a multiple of eight bytes with zeros.  The
 CBC is computed with an IV of zero.

4.4.5 RSA Encryption

 RSA encryption is used in the Encrypted Shared Key.  RSA encryption
 is best thought of as operating on blocks which are integers rather
 than octet strings and the results are also integers.  Because an RSA
 encryption permutes the integers between zero and (modulus-1), it is
 generally thought of as acting on a block of size (keysizeinbits-1)
 and producing a block of size (keysizeinbits) where keysizeinbits is
 the smallest number of bits in which the modulus can be represented.
 DASS only supports key sizes which are a multiple of eight bits (This
 restriction is only required to support interoperation with certain
 existing implementations.  If the key size is not a multiple of eight
 bits, the high order byte may not be able to hold values as large as
 the mandated '64'.  This is not a problem so long as the two high
 order bytes together are non-zero, but certain early implementations
 check for the value '64' and will not interoperate with
 implementations that use some other value.).
 The encrypted shared key structure is laid out as follows:

Kaufman [Page 99] RFC 1507 DASS September 1993

  1. The DES key to be shared is placed in the last eight bytes
  1. The POSIX format creation time encoded in four bytes using big

endian byte order is placed in the next four (from the end)

    bytes
  1. The POSIX format expiration time encoded in four bytes using

big endian byte order is placed in the next four (from the

    end) bytes
  1. Four zero bytes are placed in the next four (from the end)

bytes

  1. The first byte contains the constant '64' (decimal)
  1. All remaining bytes are filled with random bytes (the security

of the system does not depend on the cryptographic randomness

    of these bytes, but they should not be a frequently repeating
    or predictable value.  Repeating the DES key from the last
    bytes would be good).
 The RSA algorithm is applied to the integer formed by treating the
 bytes above as an integer in big endian order and the resulting
 integer is converted to a BIT STRING by laying out the integer in
 'big endian' order.
 On decryption, the process is reversed; the decryptor should verify
 the four explicitly zero bytes but should not verify the contents of
 the high order byte or the random bytes.

4.4.6 oiwMD2withRSA Signatures

 RSA-MD2 signatures are used on certificates, login tickets, shared
 key tickets, and node tickets.  In all cases, a signature is computed
 on an ASN.1 encoded string using an RSA private key.  This is done as
 follows:
  1. The MD2 algorithm is applied to the ASN.1 encoded string to

produce a 128 bit message digest

  1. The message digest is placed in the low order bytes of the RSA

block (big endian)

  1. The next two lowest order bytes are the ASN.1 'T' and 'L' for

an OCTET STRING.

  1. The remainder of the RSA block is filled with zeros

Kaufman [Page 100] RFC 1507 DASS September 1993

  1. The RSA operation is performed, and the resulting integer is

converted to an octet string by laying out the bytes in big

    endian order.
 On verification, a value like the above  or one where the message
 digest is present but the 'T' and 'L' are missing (zero) should be
 accepted for backwards compatibility with an earlier definition of
 this crypto algorithm.

4.4.7 decMD2withRSA Signatures

 This algorithm is the same as the oiwMD2withRSA algorithm as defined
 above.  We allocated an algorithm object identifier from the Digital
 space in case the definition of that OID should change.  It will not
 be used unless the meaning of oiwMD2withRSA becomes unstable.

Annex A

Typical Usage

 This annex describes one way a system could use DASS services (as
 described in section 3) to provide security services.  While this
 example provided motivation for some of the properties of DASS, it is
 not intended to represent the only way that DASS may be used.  This
 goes through the steps that would be needed to install DASS "from
 scratch".

A.1 Creating a CA

 A CA is created by initializing its state. Each CA can sign
 certificates that will be placed in some directory in the name
 service. Before these certificates will be believed in a wider
 context than the sub-tree of the name space which is headed by that
 directory, the CA must be certified by a CA for the parent directory.
 The procedure below accomplishes this. For most secure operation, the
 CA should run on an off-line system and communicate with the rest of
 the network by interchanging files using a simple specialized
 mechanism such as an RS232 line or a floppy disk. It is assumed that
 access to the CA is controlled and that the CA will accept
 instructions from an operator.
  1. Call Install_CA to create the CA State.

This state is saved within the CA system and is never

    disclosed.
  1. If this is the first CA in the namespace and the CA is

intended to certify only members of a single directory, we are

    done.  Otherwise, the new CA must be linked into the CA

Kaufman [Page 101] RFC 1507 DASS September 1993

    hierarchy by cross-certifying the parent and children of this
    CA.  There is no requirement that CA hierarchies be created
    from the root down, but to simplify exposition, only this case
    will be described.  The newly created CA must learn its name,
    its UID, the UID of its parent directory, and the public key
    of the parent directory CA by some out of band reliable means.
    Most likely, this would be done by looking up the information
    in the naming service and asking the CA operator to verify it.
    The CA then forms this information into a   parent certificate
    and signs it using the Create_certificate function.  It
    communicates the certificate to the network and posts it in
    the naming service.
  1. This name, UID, and public key of the new CA are taken to the

CA of the parent directory, which verifies it (again by some

    unspecified out-of-band mechanism) and calls
    Create_Certificate to create a child  certificate using its own
    Name and UID in the issuer fields. This certificate is also
    placed in the naming service.
 A CA can sign certificates for more than one directory. In this case
 it is possible that a single CA will take the role of both CAs in the
 example above. The above procedure can be simplified in this case, as
 no interchange of information is required.

A.2 Creating a User Principal

 A system manager may create a new user principal by invoking the
 Create_principal function supplying the principal's name, UID, and
 the public key/UID of the parent CA.  The public key and UID must be
 obtained in a reliable out of band manner.  This is probably by
 having knowledge of that information "wired into" the utility which
 creates new principals.  At account creation time, the system manager
 must supply what will become the user's password.  This might be done
 by having the user present and directly enter a password or by having
 the password selected by some random generator.
 The trusted authority certificate and corresponding user public key
 generated by the Create_principal function are sent to the CA which
 verifies its contents (again by an out-of-band mechanism) and signs a
 corresponding certificate.  The encrypted credentials, CA signed
 certificate, and trusted authority certificates are all placed in the
 naming service.  The process by which the password is made known to
 the user must be protected by some out-of-band mechanism.
 In some cases the principal may wish to generate its own key, and not
 use the Encrypted_Credentials. (e.g., if the Principal is represented
 by a Smart Card). This may be done using a procedure similar to the

Kaufman [Page 102] RFC 1507 DASS September 1993

 one for creating a new CA.

A.3 Creating a Server Principal

 A server also has a public/private key pair. Conceptually, the same
 procedure used to create a user principal can be used to create a
 server.  In practice, the most important difference  is likely to be
 how the password is protected when installing it on a server compared
 to giving it to a user.
 A server may wish to retrieve (and store) its Encrypted Credentials
 directly and never have them placed in the naming service. In this
 case some other mechanism can be used (e.g., passing the floppy disk
 containing the encrypted credentials to the server). This would
 require a variant of the Initialize_Server routine which does not
 fetch the Encrypted Credentials from the naming service.

A.4 Booting a Server Principal

 When the server first boots it needs its name (unreliably) and
 password (reliably). It then calls Initialize_Server to obtain its
 credentials and trusted authority certificates (which it will later
 need in order to authenticate users).  These credentials never time
 out, and are expected to be saved for a long time.  In particular the
 associated Incoming Timestamp List must be preserved while there are
 any timestamps on it. It is desirable to preserve the Cached Incoming
 Contexts as long as there are any contexts likely to be reused.
 If a server wants to initiate associations on its own behalf then it
 must call Generate_Server_Ticket.  It must repeat this at intervals
 if the expiration period expires.
 A node that wishes to do node authentication (or which acts as a
 server under its own name) must be created as a server.

A.5 A user logs on to the network

 The system that the user logs onto finds the user's name and
 password. It then calls Network_Login to obtain credentials for the
 user. These credentials are saved until the user wants to make a
 network connection. The credentials have a time limit, so the user
 will have to obtain new credentials in order to make connections
 after the time limit. The credentials are then checked by calling
 Verify_Principal_Name, in order to check that the key specified in
 the encrypted credentials has been certified by the CA.
 If the system does source node authentication it will call
 Combine_credentials, once the local username has been found.  (This

Kaufman [Page 103] RFC 1507 DASS September 1993

 can either be found by looking the principal's global name up in a
 file, or the user can be asked to give the local name directly.
 Alternatively the user can be asked to give his local username, which
 the system looks up to find the global name).

A.6 An Rlogin (TCP/IP) connection is made

 When the user calls a modified version of the rlogin utility, it
 calls Create_token in order to create the Initial Authentication
 Token, which is passed to the other system as part of the rlogin
 protocol.  The rlogind utility at the destination node calls
 Accept_token to verify it.  It then looks up in a local rhosts-like
 database to determine whether this global user is allowed access to
 the requested destination account.  It calls Verify_principal_name
 and/or Verify_node_name to confirm the identity of the requester.  If
 access is allowed, the connection is accepted and the Mutual
 Authentication Token is returned in the response message.
 The source receives the returned Mutual Authentication Token and uses
 it to confirm it communicating with the correct destination node.
 Rlogind then calls Combine_credentials to combine its node/account
 information with the global user identification in the received
 credentials in case the user accesses any network resources from the
 destination system.

A.7 A Transport-Independent Connection

 As an alternative to the description in A.6, an application wishing
 to be portable between different underlying transports may call
 create_token to create an authentication token which it then sends to
 its peer.  The peer can then call accept_token and
 verify_principal_name and learn the identity of the requester.

Annex B

Support of the GSSAPI

 In order to support applications which need to be portable across a
 variety of underlying security mechanisms, a "Generic Security
 Service API" (or GSSAPI) was designed which gives access to a common
 core of security services expected to be provided by several
 mechanisms.  The GSSAPI was designed with DASS, Kerberos V4, and
 Kerberos V5 in mind, and could be written as a front end to any or
 all of those systems.  It is hoped that it could serve as an
 interface to other security systems as well.
 Application portability requires that the security services supported

Kaufman [Page 104] RFC 1507 DASS September 1993

 be comparable.  Applications using the GSSAPI will not be able to
 access all of the features of the underlying security mechanisms.
 For example, the GSSAPI does not allow access to the "node
 authentication" features of DASS.  To the extent the underlying
 security mechanisms do not support all the features of GSSAPI,
 applications using those features will not be portable to those
 security mechanisms.  For example, Kerberos V4 does not support
 delegation, so applications using that feature of the GSSAPI will not
 be portable to Kerberos V4.
 This annex explains how the GSSAPI can be implemented using the
 primitive services provided by DASS.

B.1 Summary of GSSAPI

 The latest draft of the GSSAPI specification is available as an
 internet draft.  The following is a brief summary of that evolving
 document and should not be taken as definitive.  Included here are
 only those aspects of GSSAPI whose implementation would be DASS
 specific.
 The GSSAPI provides four classes of functions: Credential Management,
 Context-Level Calls, Per-message calls, and Support Calls; two types
 of objects: Credentials and Contexts; and two kinds of data
 structures to be transmitted as opaque byte strings: Tokens and
 Messages. Credentials hold keys and support information used in
 creating tokens.  Contexts hold keys and support information used in
 signing and encrypting messages.
 The Credential Management functions of GSSAPI are "incomplete" in the
 sense that one could not build a useful security implementation using
 only GSSAPI.  Functions which create credentials based on passwords
 or smart cards are needed but not provided by GSSAPI.  It is
 envisioned that such functions would be invoked by security mechanism
 specific functions at user login or via some separate utility rather
 than from within applications intended to be portable.  The
 Credential Management functions available to portable applications
 are:
  1. GSS_Acquire_cred: get a handle to an existing credential

structure based on a name or process default.

  1. GSS_Release_cred: release credentials after use.
 The Context-Level Calls use credentials to establish contexts.
 Contexts are like connections: they are created in pairs and are
 generally used at the two ends of a connection to process messages
 associated with that connection.  The Context-Level Calls of interest

Kaufman [Page 105] RFC 1507 DASS September 1993

 are:
  1. GSS_Init_sec_context: given credentials and the name of a

destination, create a new context and a token which will

    permit the destination to create a corresponding context.
  1. GSS_Accept_sec_context: given credentials and an incoming

token, create a context corresponding to the one at the

    initiating end and provide information identifying the
    initiator.
  1. GSS_Delete_sec_context: delete a context after use.
 The Per-Message Calls use contexts to sign, verify, encrypt, and
 decrypt messages between the holders of matching contexts.  The Per-
 Message Calls are:
  1. GSS_Sign: Given a context and a message, produces a string of

bytes which constitute a signature on a provided message.

  1. GSS_Verify: Given a context, a message, and the bytes

returned by GSS_Sign, verifies the message to be authentic

    (unaltered since it was signed by the corresponding context).
  1. GSS_Seal: Given a context and a message, produces a string of

bytes which include the message and a signature; the message

    may optionally be encrypted.
  1. GSS_Unseal: Given a context and the string of bytes from

GSS_Seal, returns the original message and a status indicating

    its authenticity.
 The Support Calls provide utilities like translating names and status
 codes into printable strings.

B.2 Implementation of GSSAPI over DASS

B.2.1 Data Structures

 The objects and data structures of the GSSAPI do not map neatly into
 the objects and data structures of the DASS architecture.
 This section describes how those data structures can be implemented
 using the DASS data structures and primitives
 Credential handles correspond to the credentials structures in DASS,
 where the portable API assumes that the credential structures
 themselves are kept from applications and handles are passed to and

Kaufman [Page 106] RFC 1507 DASS September 1993

 from the various subroutines.
 Context initialization tokens correspond to the tokens of DASS.  The
 GSSAPI prescribes a particular ASN.1 encoded form for tokens which
 includes a mechanism specific bit string within it.  An
 implementation of GSSAPI should enclose the DASS token within the
 GSSAPI "wrapper".
 Context handles have no corresponding structure in DASS. The
 Create_token and Accept_token calls of DASS return a shared key and
 instance identifier. An implementation of the GSSAPI must take those
 values along with some other status information and package it as a
 "context" opaque structure.  These data structures must be allocated
 and freed with the appropriate calls.
 Per-message tokens and sealed messages have no corresponding data
 structure within DASS.  To fully support the GSSAPI functionality,
 DASS must be extended to include this functionality.  These data
 structures are created by cryptographic routines given the keys and
 status information in context structures and the messages passed to
 them.  While not properly part of the DASS architecture, the formats
 of these data structures are included in section C.3.

B.2.2 Procedures

 This section explains how the functions of the GSSAPI can be provided
 in terms of the Services Provided by DASS.  Not all of the DASS
 features are accessible through the GSSAPI.

B.2.2.1 GSS_Acquire_cred

 The GSSAPI does not provide a mechanism for logging in users or
 establishing server credentials. It assumes that some system specific
 mechanism created those credentials and that applications need some
 mechanism for getting at them. A model implementation might save all
 credentials in a node-global pool indexed by some sort of credential
 name. The credentials in the pool would be access controlled by some
 local policy which is not concern of portable applications. Those
 applications would simply call GSS_Acquire_cred and if they passed
 the access control check, they would get a handle to the credentials
 which could be used in subsequent calls.

B.2.2.2 GSS_Release_cred

 This call corresponds to the "delete_credentials" call of DASS.

Kaufman [Page 107] RFC 1507 DASS September 1993

B.2.2.3 GSS_Init_sec_context

 In the course of a normal mutual authentication, this routine will be
 called twice. The procedure can determine whether this is the first
 or second call by seeing whether the "input_context_handle" is zero
 (it will be on the first call).  On the first call, it will use the
 DASS Create_token service to create a token and it will also allocate
 and populate a "context" structure. That structure will hold the key,
 instance identifier, and mutual authentication token returned by
 Create_token and will in addition hold the flags which were passed
 into the Init_sec_context call. The token returned by
 Init_sec_context will be the DASS token included in the GSSAPI token
 "wrapper".  The DASS token will include the optional principal name.
 If mutual authentication is not requested in the GSSAPI call, the
 mutual authentication token returned by DASS will be ignored and the
 initial call will return a COMPLETE status. If mutual authentication
 is requested, the mutual authentication token will be stored in the
 context information and a CONTINUE_NEEDED status returned.
 On the second call to GSS_Init_sec_context (with input_context_handle
 non-zero), the returned token will be compared to the one in the
 context information using the Compare_mutual_token procedure and a
 COMPLETE status will be returned if they match.

B.2.2.4 GSS_Accept_sec_context

 This routine in GSSAPI accepts an incoming token and creates a
 context.  It combines the effects of a series of DASS functions.  It
 could be implemented as follows:
  1. Remove the GSSAPI "wrapper" from the incoming token and pass

the rest and the credentials to "Accept_token". Accept_token

    produces a mutual authentication token and a new credentials
    structure.  If delegation was requested, the new credentials
    structure will be an output of GSS_Accept_sec_context.  In any
    case, it will be used in the subsequent steps of this
    procedure.
  1. Use the DASS Get_principal_name function to extract the

principal name from the credentials produced by Accept_token.

    This name is one of the outputs of "GSS_Accept_sec_context.
  1. Apply the DASS Verify_principal_name to the new credentials

and the retrieved name to authenticate the token as having

    come from the named principal.
  1. Create and populate a context structure with the key and

Kaufman [Page 108] RFC 1507 DASS September 1993

    timestamp returned by Accept_token and a status of COMPLETE.
    Return a handle to that context.
  1. If delegation was requested, return the new credentials from

GSS_Accept_sec_context. Otherwise, call Delete_credentials.

  1. If mutual authentication was requested, wrap the mutual

authentication token from Accept_token in a GSSAPI "wrapper"

    and return it.  Otherwise return a null string.

B.2.2.5 GSS_Delete_sec_context

 This routine simply deletes the context state.  No calls to DASS are
 required.

B.2.2.6 GSS_Sign

 This routine takes as input a context handle and a message. It
 creates a per_msg_token by computing a digital signature on the
 message using the key and timestamp in the context block.  No DASS
 services are required. If additional cryptographic services were
 requested (replay detection or sequencing), a timestamp or sequence
 number must be prepended to the message and sent with the signature.
 The syntax for this message is listed in section C.3.

B.2.2.7 GSS_Verify

 This routine repeats the calculation of the sign routine and verifies
 the signature provided. If replay detection or sequencing services
 are provided, the context must maintain as part of its state
 information containing the sequence numbers or timestamps of messages
 already received and this one must be checked for acceptability.

B.2.2.8 GSS_Seal

 This routine performs the same functions as Sign but also optionally
 encrypts the message for privacy using the shared key and
 encapsulates the whole thing in a GSSAPI specified ASN.1 wrapper.

B.2.2.9 GSS_Unseal

 This routine performs the same functions as GSS_Verify but also
 parses the data structure including the signature and message and
 decrypts the message if necessary.

Kaufman [Page 109] RFC 1507 DASS September 1993

B.3 Syntax

 The GSSAPI specification recommends the following ASN.1 encoding for
 the tokens and messages generated through the GSSAPI:
  1. -optional top-level token definitions to frame
  2. - different mechanisms
      GSSAPI DEFINITIONS ::=
      BEGIN
      MechType ::= OBJECT IDENTIFIER
      -- data structure definitions
      ContextToken ::=
      -- option indication (delegation, etc.) indicated
      -- within mechanism-specific token
      [APPLICATION 0] IMPLICIT SEQUENCE {
           thisMech MechType,
           responseExpected BOOLEAN,
           innerContextToken ANY DEFINED BY MechType
             -- contents mechanism-specific
           }
      PerMsgToken ::=
      -- as emitted by GSS_Sign and processed by
      -- GSS_Verify
      [APPLICATION 1] IMPLICIT SEQUENCE {
           thisMech MechType,
           innerMsgToken ANY DEFINED BY MechType
             -- contents mechanism-specific
           }
      SealedMessage ::=
      -- as emitted by GSS_Seal and processed by
      -- GSS_Unseal
      [APPLICATION 2] IMPLICIT SEQUENCE {
           sealingToken PERMSGTOKEN,
           confFlag BOOLEAN,
           userData OCTET STRING
             -- encrypted if confFlag TRUE
           }
 The object identifier for the DASS MechType is 1.3.12.2.1011.7.5.
 The innerContextToken of a token is a DASS token or mutual
 authentication token.
 The innerMsgToken is a null string in the case where the message is
 encrypted and the token is included as part of a SealedMessage.

Kaufman [Page 110] RFC 1507 DASS September 1993

 Otherwise, it is an eight octet sequence computed as the CBC residue
 computed using a key and string of bytes defined as follows:
  1. Pad the message provided by the application with 1-8 bytes of

pad to produce a string whose length is a multiple of 8

    octets.  Each pad byte has a value equal to the number of pad
    bytes.
  1. Compute the key by taking the timestamp of the association

(two four byte integers laid out in big endian order with the

    most significant integer first), complementing the high order
    bit (to avoid aliasing with mutual authenticators), and
    encrypting the block in ECB mode with the shared key of the
    association.
 The userData field of a SealedMessage is exactly the application
 provided byte string if confFlag=FALSE.  Otherwise, it is the
 application supplied message encrypted as follows:
  1. Pad the message provided by the application with 1-8 bytes of

pad to produce a string whose length = 4 (mod 8). Each pad

    byte has a value equal to the number of pad bytes.
  1. Append a four byte CRC32 computed over the message + pad.
  1. Compute a key by taking the timestamp of the association (two

four byte integers laid out in big endian order with the most

    significant integer first), complementing the high order bit
    (to avoid aliasing with mutual authenticators), and encrypting
    the block in ECB mode with the shared key of the association.
  1. Encrypt the message + pad + CRC32 using CBC and the key

computed in the previous step.

 A note of the logic behind the above:
  1. Because the shared key of an association may be reused by many

associations between the same pair of principals, it is

    necessary to bind the association timestamp into the messages
    somehow to prevent messages from a previous association being
    replayed into a new sequence.  The technique above of
    generating an association key accomplishes this and has a side
    benefit.  An implementation may with to keep the long term
    keys out of the hands of applications for purposes of
    confinement but may wish to put the encryption associated with
    an association in process context for reasons of performance.
    Defining an association key makes that possible.

Kaufman [Page 111] RFC 1507 DASS September 1993

  1. The reason that the association specific key is not specified

as the output of Create_token and Accept_token is that the DCE

    RPC security implementation requires that a series of
    associations between two principals always have the same key
    and we did not want to have to support a different interface
    in that application.
  1. The CRC32 after pad constitutes a cheap integrity check when

data is encrypted.

  1. The fact that padding is done differently for encrypted and

signed messages means that there are no threats related to

    sending the same message encrypted and unencrypted and using
    the last block of the encrypted message as a signature on the
    unencrypted one.

Annex C

Imported ASN.1 definitions

 This annex contains extracts from the ASN.1 description of X.509 and
 X.500 definitions referenced by the DASS ASN.1 definitions.
 CCITT DEFINITIONS ::=
 BEGIN joint-iso-ccitt          OBJECT IDENTIFIER ::= {2} ds
 OBJECT IDENTIFIER ::= {joint-iso-ccitt 5} algorithm
 OBJECT IDENTIFIER ::= {ds 8}
 iso                      OBJECT IDENTIFIER ::= {1} identified-
 organization  OBJECT IDENTIFIER ::= {iso 3} ecma            OBJECT
 IDENTIFIER ::= {identified-organization 12} digital
 OBJECT IDENTIFIER ::= { ecma 1011 }
  1. - X.501 definitions
 AttributeType ::= OBJECT IDENTIFIER AttributeValue ::= ANY
         -- useful ones are
                 --      OCTET STRING ,
                 --      PrintableString ,
                 --      NumericString ,
                 --      T61String ,
                 --      VisibleString
 AttributeValueAssertion ::= SEQUENCE {AttributeType,
                                               AttributeValue}
 Name ::= CHOICE {-- only one possibility for now --
                 RDNSequence}

Kaufman [Page 112] RFC 1507 DASS September 1993

 RDNSequence ::= SEQUENCE OF RelativeDistinguishedName
 DistinguishedName ::= RDNSequence
 RelativeDistinguishedName ::= SET OF AttributeValueAssertion
  1. - X.509 definitions
 Certificate ::= SIGNED SEQUENCE {
                 version [0]             Version DEFAULT 1988 ,
                 serialNumber            SerialNumber ,
                 signature               AlgorithmIdentifier ,
                 issuer                  Name,
                 valid                   Validity,
                 subject                 Name,
                 subjectPublicKey        SubjectPublicKeyInfo }
 Version ::=      INTEGER { 1988(0)} SerialNumber ::= INTEGER Validity
 ::=     SEQUENCE{
         notBefore               UTCTime,
         notAfter                UTCTime}
 SubjectPublicKeyInfo  ::=  SEQUENCE {
         algorithm               AlgorithmIdentifier ,
         subjectPublicKey        BIT STRING
         }
 AlgorithmIdentifier ::= SEQUENCE {
         algorithm       OBJECT IDENTIFIER ,
                     parameters ANY DEFINED BY algorithm OPTIONAL}
 ALGORITHM MACRO BEGIN TYPE NOTATION   ::= "PARAMETER" type VALUE
 NOTATION  ::= value (VALUE OBJECT IDENTIFIER) END -- of ALGORITHM
 ENCRYPTED MACRO BEGIN TYPE NOTATION   ::=type(ToBeEnciphered) VALUE
 NOTATION  ::= value(VALUE BIT STRING)
         -- the value of the bit string is generated by
         -- taking the octets which form the complete
         -- encoding (using the ASN.1 Basic Encoding Rules)
         -- of the value of the ToBeEnciphered type and
         -- applying an encipherment procedure to those octets-- END
 SIGNED MACRO    ::= BEGIN TYPE NOTATION   ::= type (ToBeSigned) VALUE
 NOTATION  ::= value(VALUE SEQUENCE{
         ToBeSigned,
         AlgorithIdentifier, -- of the algorithm used to generate
                             -- the signature
         ENCRYPTED OCTET STRING
         -- where the octet string is the result

Kaufman [Page 113] RFC 1507 DASS September 1993

  1. - of the hashing of the value of "ToBeSigned" END – of

SIGNED

 SIGNATURE MACRO ::= BEGIN TYPE NOTATION   ::= type(OfSignature) VALUE
 NOTATION  ::= value(VALUE
         SEQUENCE{
                 AlgorithmIdentifier,
                 -- of the algorithm used to compute the signature
                 ENCRYPTED OCTET STRING
                 -- where the octet string is a function (e.g., a
                 -- compressed or hashed version) of the value
                 -- "OfSignature", which may include the identifier
                 -- of the algorithm used to compute
                 -- the signature--}
                         ) END -- of SIGNATURE
  1. - X.509 Annex H (not part of the standard)
 encryptionAlgorithm OBJECT IDENTIFIER ::= {algorithm 1} rsa ALGORITHM
         PARAMETER KeySize
         ::= {encryptionAlgorithm 1}
 KeySize ::= INTEGER
 END

Glossary

 authentication
      The process of determining the identity
      (usually the name) of the other party in some communication
      exchange.
 authentication context
      Cached information used during a particular instance of
      authentication and including a shared symmetric (DES) key as
      well as components of the authentication token conveyed
      during establishment of this context.
 authentication token
      Information conveyed during a strong authentication exchange
      that can be used to authenticate its sender. An
      authentication token can, but is not necessarily limited to,
      include the claimant identity and ticket, as well as signed
      and encrypted secret key exchange messages conveying a
      secret key to be used in future cryptographic operations. An

Kaufman [Page 114] RFC 1507 DASS September 1993

      authentication token names a particular protocol data
      structure component.
 authorization
      The process of determining the rights
      associated with a particular principal.
 certificate
      The public key of a particular principal, together
      with some other information relating to the names of the
      principal and the certifying authority, rendered unforgeable
      by encipherment with the private key of the certification
      authority that issued it.
 certification authority
      An authority trusted by one or more principals to create and
      assign certificates.
 claimant
      The party that initiates the authentication process.
      In the DASS architecture, claimants possess credentials
      which include their identity, authenticating private key and
      a ticket certifying their authenticating public key.
 credentials
      Information "state" required by principals in order
      to for them to authenticate.   Credentials may contain
      information used to initiate the authentication process
      (claimant information), information used to respond to an
      authentication request (verifier information), and cached
      information useful in improving performance.
 cryptographic checksum
      Information which is derived by performing a cryptographic
      transformation on the data unit. This information can be
      used by the receiver to verify the authenticity of data
      passed in cleartext
 decipher
      To reverse the effects of encipherment and render a
      message comprehensible by use of a cryptographic key.
 delegation
      The granting of temporary credentials that allow a
      process to act on behalf of a principal.

Kaufman [Page 115] RFC 1507 DASS September 1993

 delegation key
      A short term public/private key pair used by a claimant
      to act on behalf of a principal for a bounded period. The
      delegation public key appears in the ticket, whereas the
      delegation private key is used to sign secret key exchange
      messages.
 DES
      Data Encryption Standard: a symmetric (secret key)
      encryption algorithm used by DASS. An alternate encryption
      algorithm could be substituted with little or no disruption
      to the architecture.
 DES key
      A 56-bit secret quantity used as a parameter to the
      DES encryption algorithm.
 digital signature
      A value computed from a block of data
      and a key which could only be computed by someone knowing
      the key. A digital signature computed with a secret key can
      only be verified by someone knowing that secret key.  A
      digital signature computed with a private key can be
      verified by anyone knowing the corresponding public key.
 encipher
      To render incomprehensible except to the holder of a
      particular key. If you encipher with a secret key, only the
      holder of the same secret can decipher the message. If you
      encipher with a public key, only the holder of the
      corresponding private key can decipher it.
 initial trust certificate
      A certificate signed by a principal for its own use which
      states the name and public key of a trusted authority.
 global user name
      A hierarchical name for a user which is
      unique within the entire domain of discussion (typically the
      network).
 local user name
      A simple (non-hierarchical) name by
      which a user is known within a limited context such as on a
      single computer.

Kaufman [Page 116] RFC 1507 DASS September 1993

 principal
      Abstract entity which can be authenticated by name.
      In DASS there are user principals and server principals.
 private key
      Cryptographic key used in asymmetric (public key)
      cryptography to decrypt and/or sign messages. In asymmetric
      cryptography, knowing the encryption key is independent of
      knowing the decryption key. The decryption (or signing)
      private key cannot be derived from the encrypting (or
      verifying) public key.
 proxy
      A mapping from an external name to a local account
      name for purposes of establishing a set of local access
      rights. Note that this differs from the definition in ECMA
      TR/46.
 public key
      Cryptographic key used in asymmetric cryptography to
      encrypt messages and/or verify signatures.
 RSA
      The Rivest-Shamir-Adelman public key cryptosystem
      based on modular exponentiation where the modulus is the
      product of two large primes.  When the term RSA key is used,
      it should be clear from context whether the public key, the
      private key, or the public/private pair is intended.
 secret key
      Cryptographic key used in symmetric cryptography to
      encrypt, sign, decrypt and verify messages. In symmetric
      cryptography, knowledge of the decryption key implies
      knowledge of the encryption key, and vice-versa.
 sign
      A process which takes a piece of data and a key and
      produces a digital signature which can only be calculated by
      someone with the key. The holder of a corresponding key can
      verify the signature.
 source
      The initiator of an authentication exchange.
 strong authentication
      Authentication by means of cryptographically derived
      authentication tokens and credentials. The actual working
      definition is closer to that of "zero knowledge" proof:

Kaufman [Page 117] RFC 1507 DASS September 1993

      authentication so as to not reveal any information usable by
      either the verifier, or by an eavesdropping third party, to
      further their potential ability to impersonate the claimant.
 target
      The intended second party (other than the source) to
      an authentication exchange.
 ticket
      A data structure certifying an authenticating
      (public) key by virtue of being signed by a user principal
      using their (long term) private key. The ticket also
      includes the UID of the principal.
 trusted authority
      The public key, name and UID of a
      certification authority trusted in some context to certify
      the public keys of other principals.
 UID
      A 128 bit unique identifier produced according to OSF
      standard specifications.
 user key
      A "long term" RSA key whose private portion
      authenticates its holder as having the access rights of a
      particular person.
 verify
      To cryptographically process a piece of data and a
      digital signature to determine that the holder of a
      particular key signed the data.
 verifier
      The party who will perform the operations necessary
      to verify the claimed identity of a claimant.

Kaufman [Page 118] RFC 1507 DASS September 1993

Security Considerations

 Security issues are discussed throughout this memo.

Author's Address

 Charles Kaufman
 Digital Equipment Corporation
 ZKO3-3/U14
 110 Spit Brook Road
 Nashua, NH 03062
 Phone: (603) 881-1495
 Email: kaufman@zk3.dec.com
 General comments on this document should be sent to cat-ietf@mit.edu.
 Minor corrections should be sent to the author.

Kaufman [Page 119]

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