GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc3820

Network Working Group S. Tuecke Request for Comments: 3820 ANL Category: Standards Track V. Welch

                                                                  NCSA
                                                             D. Engert
                                                                   ANL
                                                           L. Pearlman
                                                               USC/ISI
                                                           M. Thompson
                                                                  LBNL
                                                             June 2004
          Internet X.509 Public Key Infrastructure (PKI)
                     Proxy Certificate Profile

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2004).

Abstract

 This document forms a certificate profile for Proxy Certificates,
 based on X.509 Public Key Infrastructure (PKI) certificates as
 defined in RFC 3280, for use in the Internet.  The term Proxy
 Certificate is used to describe a certificate that is derived from,
 and signed by, a normal X.509 Public Key End Entity Certificate or by
 another Proxy Certificate for the purpose of providing restricted
 proxying and delegation within a PKI based authentication system.

Tuecke, et al. Standards Track [Page 1] RFC 3820 X.509 Proxy Certificate Profile June 2004

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
 2.  Overview of Approach . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Terminology. . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Background . . . . . . . . . . . . . . . . . . . . . . .  5
     2.3.  Motivation for Proxying. . . . . . . . . . . . . . . . .  5
     2.4.  Motivation for Restricted Proxies. . . . . . . . . . . .  7
     2.5.  Motivation for Unique Proxy Name . . . . . . . . . . . .  8
     2.6.  Description Of Approach. . . . . . . . . . . . . . . . .  9
     2.7.  Features Of This Approach. . . . . . . . . . . . . . . . 10
 3.  Certificate and Certificate Extensions Profile . . . . . . . . 12
     3.1.  Issuer . . . . . . . . . . . . . . . . . . . . . . . . . 12
     3.2.  Issuer Alternative Name. . . . . . . . . . . . . . . . . 12
     3.3.  Serial Number. . . . . . . . . . . . . . . . . . . . . . 12
     3.4.  Subject. . . . . . . . . . . . . . . . . . . . . . . . . 13
     3.5.  Subject Alternative Name . . . . . . . . . . . . . . . . 13
     3.6.  Key Usage and Extended Key Usage . . . . . . . . . . . . 13
     3.7.  Basic Constraints. . . . . . . . . . . . . . . . . . . . 14
     3.8.  The ProxyCertInfo Extension. . . . . . . . . . . . . . . 14
 4.  Proxy Certificate Path Validation. . . . . . . . . . . . . . . 17
     4.1.  Basic Proxy Certificate Path Validation. . . . . . . . . 19
     4.2.  Using the Path Validation Algorithm. . . . . . . . . . . 23
 5.  Commentary . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     5.1.  Relationship to Attribute Certificates . . . . . . . . . 24
     5.2.  Kerberos 5 Tickets . . . . . . . . . . . . . . . . . . . 28
     5.3.  Examples of usage of Proxy Restrictions. . . . . . . . . 28
     5.4.  Delegation Tracing . . . . . . . . . . . . . . . . . . . 29
 6.  Security Considerations. . . . . . . . . . . . . . . . . . . . 30
     6.1.  Compromise of a Proxy Certificate. . . . . . . . . . . . 30
     6.2.  Restricting Proxy Certificates . . . . . . . . . . . . . 31
     6.3.  Relying Party Trust of Proxy Certificates. . . . . . . . 31
     6.4.  Protecting Against Denial of Service with Key Generation 32
     6.5.  Use of Proxy Certificates in a Central Repository. . . . 32
 7.  IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 33
 8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
     8.1.  Normative References . . . . . . . . . . . . . . . . . . 33
     8.2.  Informative References . . . . . . . . . . . . . . . . . 33
 9.  Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 34
 Appendix A. 1988 ASN.1 Module. . . . . . . . . . . . . . . . . . . 35
 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 36
 Full Copyright Notice. . . . . . . . . . . . . . . . . . . . . . . 37

Tuecke, et al. Standards Track [Page 2] RFC 3820 X.509 Proxy Certificate Profile June 2004

1. Introduction

 Use of a proxy credential [i7] is a common technique used in security
 systems to allow entity A to grant to another entity B the right for
 B to be authorized with others as if it were A.  In other words,
 entity B is acting as a proxy on behalf of entity A.  This document
 forms a certificate profile for Proxy Certificates, based on the RFC
 3280, "Internet X.509 Public Key Infrastructure Certificate and CRL
 Profile" [n2].
 In addition to simple, unrestricted proxying, this profile defines:
  • A framework for carrying policies in Proxy Certificates that

allows proxying to be limited (perhaps completely disallowed)

    through either restrictions or enumeration of rights.
  • Proxy Certificates with unique names, derived from the name of the

end entity certificate name. This allows the Proxy Certificates

    to be used in conjunction with attribute assertion approaches such
    as Attribute Certificates [i3] and have their own rights
    independent of their issuer.
 Section 2 provides a non-normative overview of the approach.  It
 begins by defining terminology, motivating Proxy Certificates, and
 giving a brief overview of the approach.  It then introduces the
 notion of a Proxy Issuer, as distinct from a Certificate Authority,
 to describe how end entity signing of a Proxy Certificate is
 different from end entity signing of another end entity certificate,
 and therefore why this approach does not violate the end entity
 signing restrictions contained in the X.509 keyCertSign field of the
 keyUsage extension.  It then continues with discussions of how
 subject names are used by this proxying approach, and features of
 this approach.
 Section 3 defines requirements on information content in Proxy
 Certificates.  This profile addresses two fields in the basic
 certificate as well as five certificate extensions.  The certificate
 fields are the subject and issuer fields.  The certificate extensions
 are subject alternative name, issuer alternative name, key usage,
 basic constraints, and extended key usage.  A new certificate
 extension, Proxy Certificate Information, is introduced.
 Section 4 defines path validation rules for Proxy Certificates.
 Section 5 provides non-normative commentary on Proxy Certificates.
 Section 6 discusses security considerations relating to Proxy
 Certificates.

Tuecke, et al. Standards Track [Page 3] RFC 3820 X.509 Proxy Certificate Profile June 2004

 References, listed in Section 8, are sorted into normative and
 information references.  Normative references, listed in Section 8.1,
 are in the form [nXX].  Informative references, listed in Section
 8.2, are in the form [iXX].
 Section 9 contains acknowledgements.
 Following Section 9, contains the Appendix, the contact information
 for the authors, the intellectual property information, and the
 copyright information for this document.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in BCP 14, RFC 2119 [n1].

2. Overview of Approach

 This section provides non-normative commentary on Proxy Certificates.
 The goal of this specification is to develop a X.509 Proxy
 Certificate profile and to facilitate their use within Internet
 applications for those communities wishing to make use of restricted
 proxying and delegation within an X.509 Public Key Infrastructure
 (PKI) authentication based system.
 This section provides relevant background, motivation, an overview of
 the approach, and related work.

2.1. Terminology

 This document uses the following terms:
  • CA: A "Certification Authority", as defined by X.509 [n2]
  • EEC: An "End Entity Certificate", as defined by X.509. That is,

it is an X.509 Public Key Certificate issued to an end entity,

    such as a user or a service, by a CA.
  • PKC: An end entity "Public Key Certificate". This is synonymous

with an EEC.

  • PC: A "Proxy Certificate", the profile of which is defined by this

document.

Tuecke, et al. Standards Track [Page 4] RFC 3820 X.509 Proxy Certificate Profile June 2004

  • PI: A "Proxy Issuer" is an entity with an End Entity Certificate

or Proxy Certificate that issues a Proxy Certificate. The Proxy

    Certificate is signed using the private key associated with the
    public key in the Proxy Issuer's certificate.
  • AC: An "Attribute Certificate", as defined by "An Internet

Attribute Certificate Profile for Authorization" [i3].

  • AA: An "Attribute Authority", as defined in [i3].

2.2. Background

 Computational and Data "Grids" have emerged as a common approach to
 constructing dynamic, inter-domain, distributed computing
 environments.  As explained in [i5], large research and development
 efforts starting around 1995 have focused on the question of what
 protocols, services, and APIs are required for effective, coordinated
 use of resources in these Grid environments.
 In 1997, the Globus Project (www.globus.org) introduced the Grid
 Security Infrastructure (GSI) [i4].  This library provides for public
 key based authentication and message protection, based on standard
 X.509 certificates and public key infrastructure, the SSL/TLS
 protocol [i2], and delegation using proxy certificates similar to
 those profiled in this document.  GSI has been used, in turn, to
 build numerous middleware libraries and applications, which have been
 deployed in large-scale production and experimental Grids [i1].  GSI
 has emerged as the dominant security solution used by Grid efforts
 worldwide.
 This experience with GSI has proven the viability of restricted
 proxying as a basis for authorization within Grids, and has further
 proven the viability of using X.509 Proxy Certificates, as defined in
 this document, as the basis for that proxying.  This document is one
 part of an effort to migrate this experience with GSI into standards,
 and in the process clean up the approach and better reconcile it with
 existing and recent standards.

2.3. Motivation for Proxying

 A motivating example will assist in understanding the role proxying
 can play in building Internet based applications.
 Steve is an engineer who wants to use a reliable file transfer
 service to manage the movement of a number of large files around
 between various hosts on his company's Intranet-based Grid.  From his
 laptop he wants to submit a number of transfer requests to the
 service and have the files transferred while he is doing other

Tuecke, et al. Standards Track [Page 5] RFC 3820 X.509 Proxy Certificate Profile June 2004

 things, including being offline.  The transfer service may queue the
 requests for some time (e.g., until after hours or a period of low
 resource usage) before initiating the transfers.  The transfer
 service will then, for each file, connect to each of the source and
 destination hosts, and instruct them to initiate a data connection
 directly from the source to the destination in order to transfer the
 file.  Steve will leave an agent running on his laptop that will
 periodically check on progress of the transfer by contacting the
 transfer service.  Of course, he wants all of this to happen securely
 on his company's resources, which requires that he initiate all of
 this using his PKI smartcard.
 This scenario requires authentication and delegation in a variety of
 places:
  • Steve needs to be able to mutually authenticate with the reliable

file transfer service to submit the transfer request.

  • Since the storage hosts know nothing about the file transfer

service, the file transfer service needs to be delegated the

    rights to mutually authenticate with the various storage hosts
    involved directly in the file transfer, in order to initiate the
    file transfer.
  • The source and destination hosts of a particular transfer must be

able to mutual authenticate with each other, to ensure the file is

    being transferred to and from the proper parties.
  • The agent running on Steve's laptop must mutually authenticate

with the file transfer service in order to check the result of the

    transfers.
 Proxying is a viable approach to solving two (related) problems in
 this scenario:
  • Single sign-on: Steve wants to enter his smartcard password (or

pin) once, and then run a program that will submit all the file

    transfer requests to the transfer service, and then periodically
    check on the status of the transfer.  This program needs to be
    given the rights to be able to perform all of these operations
    securely, without requiring repeated access to the smartcard or
    Steve's password.
  • Delegation: Various remote processes in this scenario need to

perform secure operations on Steve's behalf, and therefore must be

    delegated the necessary rights.  For example, the file transfer

Tuecke, et al. Standards Track [Page 6] RFC 3820 X.509 Proxy Certificate Profile June 2004

    service needs to be able to authenticate on Steve's behalf with
    the source and destination hosts, and must in turn delegate rights
    to those hosts so that they can authenticate with each other.
 Proxying can be used to secure all of these interactions:
  • Proxying allows for the private key stored on the smartcard to be

accessed just once, in order to create the necessary proxy

    credential, which allows the client/agent program to be authorized
    as Steve when submitting the requests to the transfer service.
    Access to the smartcard and Steve's password is not required after
    the initial creation of the proxy credential.
  • The client program on the laptop can delegate to the file transfer

service the right to act on Steve's behalf. This, in turn, allows

    the service to authenticate to the storage hosts and inherit
    Steve's privileges in order to start the file transfers.
  • When the transfer service authenticates to hosts to start the file

transfer, the service can delegate to the hosts the right to act

    on Steve's behalf so that each pair of hosts involved in a file
    transfer can mutually authenticate to ensure the file is securely
    transferred.
  • When the agent on the laptop reconnects to the file transfer

service to check on the status of the transfer, it can perform

    mutual authentication.  The laptop may use a newly generated proxy
    credential, which is just created anew using the smartcard.
 This scenario, and others similar to it, is being built today within
 the Grid community.  The Grid Security Infrastructure's single sign-
 on and delegation capabilities, built on X.509 Proxy Certificates,
 are being employed to provide authentication services to these
 applications.

2.4. Motivation for Restricted Proxies

 One concern that arises is what happens if a machine that has been
 delegated the right to inherit Steve's privileges has been
 compromised?  For example, in the above scenario, what if the machine
 running the file transfer service is compromised, such that the
 attacker can gain access to the credential that Steve delegated to
 that service?  Can the attacker now do everything that Steve is
 allowed to do?
 A solution to this problem is to allow for restrictions to be placed
 on the proxy by means of policies on the proxy certificates. For
 example, the machine running the reliable file transfer service in

Tuecke, et al. Standards Track [Page 7] RFC 3820 X.509 Proxy Certificate Profile June 2004

 the above example might only be given Steve's right for the purpose
 of reading the source files and writing the destination files.
 Therefore, if that file transfer service is compromised, the attacker
 cannot modify source files, cannot create or modify other files to
 which Steve has access, cannot start jobs on behalf of Steve, etc.
 All that an attacker would be able to do is read the specific files
 to which the file transfer service has been delegated read access,
 and write bogus files in place of those that the file transfer
 service has been delegated write access. Further, by limiting the
 lifetime of the credential that is delegated to the file transfer
 service, the effects of a compromise can be further mitigated.
 Other potential uses for restricted proxy credentials are discussed
 in [i7].

2.5. Motivation for Unique Proxy Name

 The dynamic creation of entities (e.g., processes and services) is an
 essential part of Grid computing.  These entities will require rights
 in order to securely perform their function.  While it is possible to
 obtain rights solely through proxying as described in previous
 sections, this has limitations.  For example what if an entity should
 have rights that are granted not just from the proxy issuer but from
 a third party as well?  While it is possible in this case for the
 entity to obtain and hold two proxy certifications, in practice it is
 simpler for subsequent credentials to take the form of attribute
 certificates.
 It is also desirable for these entities to have a unique identity so
 that they can be explicitly discussed in policy statements.  For
 example, a user initiating a third-party FTP transfer could grant
 each FTP server a PC with a unique identity and inform each server of
 the identity of the other, then when the two servers connected they
 could authenticate themselves and know they are connected to the
 proper party.
 In order for a party to have rights of it's own it requires a unique
 identity.  Possible options for obtaining an unique identity are:
 1) Obtain an identity from a traditional Certification Authority
    (CA).
 2) Obtain a new identity independently - for example by using the
    generated public key and a self-signed certificate.
 3) Derive the new identity from an existing identity.

Tuecke, et al. Standards Track [Page 8] RFC 3820 X.509 Proxy Certificate Profile June 2004

 In this document we describe an approach to option #3, because:
  • It is reasonably light-weight, as it can be done without

interacting with a third party. This is important when

       creating identities dynamically.
  • As described in the previous section, a common use for PCs is

for restricted proxying, so deriving their identity from the

       identity of the EEC makes this straightforward.  Nonetheless
       there are circumstances where the creator does not wish to
       delegate all or any of its rights to a new entity.  Since the
       name is unique, this is easily accomplished by #3 as well, by
       allowing the application of a policy to limit proxying.

2.6. Description Of Approach

 This document defines an X.509 "Proxy Certificate" or "PC" as a means
 of providing for restricted proxying within an (extended) X.509 PKI
 based authentication system.
 A Proxy Certificate is an X.509 public key certificate with the
 following properties:
 1) It is signed by either an X.509 End Entity Certificate (EEC), or
    by another PC.  This EEC or PC is referred to as the Proxy Issuer
    (PI).
 2) It can sign only another PC.  It cannot sign an EEC.
 3) It has its own public and private key pair, distinct from any
    other EEC or PC.
 4) It has an identity derived from the identity of the EEC that
    signed the PC.  When a PC is used for authentication, in may
    inherit rights of the EEC that signed the PC, subject to the
    restrictions that are placed on that PC by the EEC.
 5) Although its identity is derived from the EEC's identity, it is
    also unique.  This allows this identity to be used for
    authorization as an independent identity from the identity of the
    issuing EEC, for example in conjunction with attribute assertions
    as defined in [i3].
 6) It contains a new X.509 extension to identify it as a PC and to
    place policies on the use of the PC.  This new extension, along
    with other X.509 fields and extensions, are used to enable proper
    path validation and use of the PC.

Tuecke, et al. Standards Track [Page 9] RFC 3820 X.509 Proxy Certificate Profile June 2004

 The process of creating a PC is as follows:
 1) A new public and private key pair is generated.
 2) That key pair is used to create a request for a Proxy Certificate
    that conforms to the profile described in this document.
 3) A Proxy Certificate, signed by the private key of the EEC or by
    another PC, is created in response to the request.  During this
    process, the PC request is verified to ensure that the requested
    PC is valid (e.g., it is not an EEC, the PC fields are
    appropriately set, etc).
 When a PC is created as part of a delegation from entity A to entity
 B, this process is modified by performing steps #1 and #2 within
 entity B, then passing the PC request from entity B to entity A over
 an authenticated, integrity checked channel, then entity A performs
 step #3 and passes the PC back to entity B.
 Path validation of a PC is very similar to normal path validation,
 with a few additional checks to ensure, for example, proper PC
 signing constraints.

2.7. Features Of This Approach

 Using Proxy Certificates to perform delegation has several features
 that make it attractive:
  • Ease of integration
    o  Because a PC requires only a minimal change to path validation,
       it is very easy to incorporate support for Proxy Certificates
       into existing X.509 based software.  For example, SSL/TLS
       requires no protocol changes to support authentication using a
       PC.  Further, an SSL/TLS implementation requires only minor
       changes to support PC path validation, and to retrieve the
       authenticated subject of the signing EEC instead of the subject
       of the PC for authorization purposes.
    o  Many existing authorization systems use the X.509 subject name
       as the basis for access control.  Proxy Certificates can be
       used with such authorization systems without modification,
       since such a PC inherits its name and rights from the EEC that
       signed it and the EEC name can be used in place of the PC name
       for authorization decisions.

Tuecke, et al. Standards Track [Page 10] RFC 3820 X.509 Proxy Certificate Profile June 2004

  • Ease of use
    o  Using PC for single sign-on helps make X.509 PKI authentication
       easier to use, by allowing users to "login" once and then
       perform various operations securely.
    o  For many users, properly managing their own EEC private key is
       a nuisance at best, and a security risk at worst.  One option
       easily enabled with a PC is to manage the EEC private keys and
       certificates in a centrally managed repository. When a user
       needs a PKI credential, the user can login to the repository
       using name/password, one time password, etc.  Then the
       repository can delegate a PC to the user with proxy rights, but
       continue to protect the EEC private key in the repository.
  • Protection of private keys
    o  By using the remote delegation approach outlined above, entity
       A can delegate a PC to entity B, without entity B ever seeing
       the private key of entity A, and without entity A ever seeing
       the private key of the newly delegated PC held by entity B.  In
       other words, private keys never need to be shared or
       communicated by the entities participating in a delegation of a
       PC.
    o  When implementing single sign-on, using a PC helps protect the
       private key of the EEC, because it minimizes the exposure and
       use of that private key.  For example, when an EEC private key
       is password protected on disk, the password and unencrypted
       private key need only be available during the creation of the
       PC.  That PC can then be used for the remainder of its valid
       lifetime, without requiring access to the EEC password or
       private key.  Similarly, when the EEC private key lives on a
       smartcard, the smartcard need only be present in the machine
       during the creation of the PC.
  • Limiting consequences of a compromised key
    o  When creating a PC, the PI can limit the validity period of the
       PC, the depth of the PC path that can be created by that PC,
       and key usage of the PC and its descendents.  Further, fine-
       grained policies can be carried by a PC to even further
       restrict the operations that can be performed using the PC.
       These restrictions permit the PI to limit damage that could be
       done by the bearer of the PC, either accidentally or
       maliciously.

Tuecke, et al. Standards Track [Page 11] RFC 3820 X.509 Proxy Certificate Profile June 2004

    o  A compromised PC private key does NOT compromise the EEC
       private key.  This makes a short term, or an otherwise
       restricted PC attractive for day-to-day use, since a
       compromised PC does not require the user to go through the
       usually cumbersome and time consuming process of having the EEC
       with a new private key reissued by the CA.
 See Section 5 below for more discussion on how Proxy Certificates
 relate to Attribute Certificates.

3. Certificate and Certificate Extensions Profile

 This section defines the usage of X.509 certificate fields and
 extensions in Proxy Certificates, and defines one new extension for
 Proxy Certificate Information.
 All Proxy Certificates MUST include the Proxy Certificate Information
 (ProxyCertInfo) extension defined in this section and the extension
 MUST be critical.

3.1. Issuer

 The Proxy Issuer of a Proxy Certificate MUST be either an End Entity
 Certificate, or another Proxy Certificate.
 The Proxy Issuer MUST NOT have an empty subject field.
 The issuer field of a Proxy Certificate MUST contain the subject
 field of its Proxy Issuer.
 If the Proxy Issuer certificate has the KeyUsage extension, the
 Digital Signature bit MUST be asserted.

3.2. Issuer Alternative Name

 The issuerAltName extension MUST NOT be present in a Proxy
 Certificate.

3.3. Serial Number

 The serial number of a Proxy Certificate (PC) SHOULD be unique
 amongst all Proxy Certificates issued by a particular Proxy Issuer.
 However, a Proxy Issuer MAY use an approach to assigning serial
 numbers that merely ensures a high probability of uniqueness.

Tuecke, et al. Standards Track [Page 12] RFC 3820 X.509 Proxy Certificate Profile June 2004

 For example, a Proxy Issuer MAY use a sequentially assigned integer
 or a UUID to assign a unique serial number to a PC it issues.  Or a
 Proxy Issuer MAY use a SHA-1 hash of the PC public key to assign a
 serial number with a high probability of uniqueness.

3.4. Subject

 The subject field of a Proxy Certificate MUST be the issuer field
 (that is the subject of the Proxy Issuer) appended with a single
 Common Name component.
 The value of the Common Name SHOULD be unique to each Proxy
 Certificate bearer amongst all Proxy Certificates with the same
 issuer.
 If a Proxy Issuer issues two proxy certificates to the same bearer,
 the Proxy Issuer MAY choose to use the same Common Name for both.
 Examples of this include Proxy Certificates for different uses (e.g.,
 signing vs encryption) or the re-issuance of an expired Proxy
 Certificate.
 The Proxy Issuer MAY use an approach to assigning Common Name values
 that merely ensures a high probability of uniqueness.  This value MAY
 be the same value used for the serial number.
 The result of this approach is that all subject names of Proxy
 Certificates are derived from the name of the issuing EEC (it will be
 the first part of the subject name appended with one or more CN
 components) and are unique to each bearer.

3.5. Subject Alternative Name

 The subjectAltName extension MUST NOT be present in a Proxy
 Certificate.

3.6. Key Usage and Extended Key Usage

 If the Proxy Issuer certificate has a Key Usage extension, the
 Digital Signature bit MUST be asserted.
 This document places no constraints on the presence or contents of
 the key usage and extended key usage extension.  However, section 4.2
 explains what functions should be allowed a proxy certificate by a
 relying party.

Tuecke, et al. Standards Track [Page 13] RFC 3820 X.509 Proxy Certificate Profile June 2004

3.7. Basic Constraints

 The cA field in the basic constraints extension MUST NOT be TRUE.

3.8. The ProxyCertInfo Extension

 A new extension, ProxyCertInfo, is defined in this subsection.
 Presence of the ProxyCertInfo extension indicates that a certificate
 is a Proxy Certificate and whether or not the issuer of the
 certificate has placed any restrictions on its use.
 id-pkix OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
          dod(6) internet(1) security(5) mechanisms(5) pkix(7) }
 id-pe OBJECT IDENTIFIER ::= { id-pkix 1 }
 id-pe-proxyCertInfo OBJECT IDENTIFIER ::= { id-pe 14 }
 ProxyCertInfo ::= SEQUENCE {
      pCPathLenConstraint   INTEGER (0..MAX) OPTIONAL,
      proxyPolicy           ProxyPolicy }
 ProxyPolicy ::= SEQUENCE {
      policyLanguage        OBJECT IDENTIFIER,
      policy          OCTET STRING OPTIONAL }
 If a certificate is a Proxy Certificate, then the proxyCertInfo
 extension MUST be present, and this extension MUST be marked as
 critical.
 If a certificate is not a Proxy Certificate, then the proxyCertInfo
 extension MUST be absent.
 The ProxyCertInfo extension consists of one required and two optional
 fields, which are described in detail in the following subsections.

3.8.1. pCPathLenConstraint

 The pCPathLenConstraint field, if present, specifies the maximum
 depth of the path of Proxy Certificates that can be signed by this
 Proxy Certificate.  A pCPathLenConstraint of 0 means that this
 certificate MUST NOT be used to sign a Proxy Certificate.  If the
 pCPathLenConstraint field is not present then the maximum proxy path
 length is unlimited.  End entity certificates have unlimited maximum
 proxy path lengths.

Tuecke, et al. Standards Track [Page 14] RFC 3820 X.509 Proxy Certificate Profile June 2004

3.8.2. proxyPolicy

 The proxyPolicy field specifies a policy on the use of this
 certificate for the purposes of authorization.  Within the
 proxyPolicy, the policy field is an expression of policy, and the
 policyLanguage field indicates the language in which the policy is
 expressed.
 The proxyPolicy field in the proxyCertInfo extension does not define
 a policy language to be used for proxy restrictions; rather, it
 places the burden on those parties using that extension to define an
 appropriate language, and to acquire an OID for that language (or to
 select an appropriate previously-defined language/OID).  Because it
 is essential for the PI that issues a certificate with a proxyPolicy
 field and the relying party that interprets that field to agree on
 its meaning, the policy language OID must correspond to a policy
 language (including semantics), not just a policy grammar.
 The policyLanguage field has two values of special importance,
 defined in Appendix A, that MUST be understood by all parties
 accepting Proxy Certificates:
  • id-ppl-inheritAll indicates that this is an unrestricted proxy

that inherits all rights from the issuing PI. An unrestricted

    proxy is a statement that the Proxy Issuer wishes to delegate all
    of its authority to the bearer (i.e., to anyone who has that proxy
    certificate and can prove possession of the associated private
    key).  For purposes of authorization, this an unrestricted proxy
    effectively impersonates the issuing PI.
  • id-ppl-independent indicates that this is an independent proxy

that inherits no rights from the issuing PI. This PC MUST be

    treated as an independent identity by relying parties.  The only
    rights this PC has are those granted explicitly to it.
 For either of the policyLanguage values listed above, the policy
 field MUST NOT be present.
 Other values for the policyLanguage field indicates that this is a
 restricted proxy certification and have some other policy limiting
 its ability to do proxying.  In this case the policy field MAY be
 present and it MUST contain information expressing the policy.  If
 the policy field is not present the policy MUST be implicit in the
 value of the policyLanguage field itself.  Authors of additional
 policy languages are encouraged to publicly document their policy
 language and list it in the IANA registry (see Section 7).

Tuecke, et al. Standards Track [Page 15] RFC 3820 X.509 Proxy Certificate Profile June 2004

 Proxy policies are used to limit the amount of authority delegated,
 for example to assert that the proxy certificate may be used only to
 make requests to a specific server, or only to authorize specific
 operations on specific resources.  This document is agnostic to the
 policies that can be placed in the policy field.
 Proxy policies impose additional requirements on the relying party,
 because only the relying party is in a position to ensure that those
 policies are enforced.  When making an authorization decision based
 on a proxy certificate based on rights that proxy certificate
 inherited from its issuer, it is the relying party's responsibility
 to verify that the requested authority is compatible with all
 policies in the PC's certificate path.  In other words, the relying
 party MUST verify that the following three conditions are all met:
 1) The relying party MUST know how to interpret the proxy policy and
    the request is allowed under that policy.
 2) If the Proxy Issuer is an EEC then the relying party's local
    policies MUST authorize the request for the entity named in the
    EEC.
 3) If the Proxy Issuer is another PC, then one of the following MUST
    be true:
    a. The relying party's local policies authorize the Proxy Issuer
       to perform the request.
    b. The Proxy Issuer inherits the right to perform the request from
       its issuer by means of its proxy policy.  This must be verified
       by verifying these three conditions on the Proxy Issuer in a
       recursive manner.
 If these conditions are not met, the relying party MUST either deny
 authorization, or ignore the PC and the whole certificate chain
 including the EEC entirely when making its authorization decision
 (i.e., make the same decision that it would have made had the PC and
 it's certificate chain never been presented).
 The relying party MAY impose additional restrictions as to which
 proxy certificates it accepts.  For example, a relying party MAY
 choose to reject all proxy certificates, or MAY choose to accept
 proxy certificates only for certain operations, etc.
 Note that since a proxy certificate has a unique identity it MAY also
 have rights granted to it by means other than inheritance from it's
 issuer via its proxy policy.  The rights granted to the bearer of a
 PC are the union of the rights granted to the PC identity and the

Tuecke, et al. Standards Track [Page 16] RFC 3820 X.509 Proxy Certificate Profile June 2004

 inherited rights.  The inherited rights consist of the intersection
 of the rights granted to the PI identity intersected with the proxy
 policy in the PC.
 For example, imagine that Steve is authorized to read and write files
 A and B on a file server, and that he uses his EEC to create a PC
 that includes the policy that it can be used only to read or write
 files A and C.  Then a trusted attribute authority grants an
 Attribute Certificate granting the PC the right to read file D. This
 would make the rights of the PC equal to the union of the rights
 granted to the PC identity (right to read file D) with the
 intersection of the rights granted to Steve, the PI, (right to read
 files A and B) with the policy in the PC (can only read files A and
 C).  This would mean the PC would have the following rights:
  • Right to read file A: Steve has this right and he issued the PC

and his policy grants this right to the PC.

  • Right to read file D: This right is granted explicitly to the PC

by a trusted authority.

 The PC would NOT have the following rights:
  • Right to read file B: Although Steve has this right, it is

excluded by his policy on the PC.

  • Right to read file C: Although Steve's policy grants this right,

he does not have this right himself.

 In many cases, the relying party will not have enough information to
 evaluate the above criteria at the time that the certificate path is
 validated.  For example, if a certificate is used to authenticate a
 connection to some server, that certificate is typically validated
 during that authentication step, before any requests have been made
 of the server.  In that case, the relying party MUST either have some
 authorization mechanism in place that will check the proxy policies,
 or reject any certificate that contains proxy policies (or that has a
 parent certificate that contains proxy policies).

4. Proxy Certificate Path Validation

 Proxy Certification path processing verifies the binding between the
 proxy certificate distinguished name and proxy certificate public
 key.  The binding is limited by constraints which are specified in
 the certificates which comprise the path and inputs which are
 specified by the relying party.

Tuecke, et al. Standards Track [Page 17] RFC 3820 X.509 Proxy Certificate Profile June 2004

 This section describes an algorithm for validating proxy
 certification paths.  Conforming implementations of this
 specification are not required to implement this algorithm, but MUST
 provide functionality equivalent to the external behavior resulting
 from this procedure.  Any algorithm may be used by a particular
 implementation so long as it derives the correct result.
 The algorithm presented in this section validates the proxy
 certificate with respect to the current date and time.  A conformant
 implementation MAY also support validation with respect to some point
 in the past.  Note that mechanisms are not available for validating a
 proxy certificate with respect to a time outside the certificate
 validity period.
 Valid paths begin with the end entity certificate (EEC) that has
 already been validated by public key certificate validation
 procedures in RFC 3280 [n2].  The algorithm requires the public key
 of the EEC and the EEC's subject distinguished name.
 To meet the goal of verifying the proxy certificate, the proxy
 certificate path validation process verifies, among other things,
 that a prospective certification path (a sequence of n certificates)
 satisfies the following conditions:
 (a) for all x in {1, ..., n-1}, the subject of certificate x is the
     issuer of proxy certificate x+1 and the subject distinguished
     name of certificate x+1 is a legal subject distinguished name to
     have been issued by certificate x;
 (b) certificate 1 is valid proxy certificate issued by the end entity
     certificate whose information is given as input to the proxy
     certificate path validation process;
 (c) certificate n is the proxy certificate to be validated;
 (d) for all x in {1, ..., n}, the certificate was valid at the time
     in question; and
 (e) for all certificates in the path with a pCPathLenConstraint
     field, the number of certificates in the path following that
     certificate does not exceed the length specified in that field.
 At this point there is no mechanism defined for revoking proxy
 certificates.

Tuecke, et al. Standards Track [Page 18] RFC 3820 X.509 Proxy Certificate Profile June 2004

4.1. Basic Proxy Certificate Path Validation

 This section presents the algorithm in four basic steps to mirror the
 description of public key certificate path validation in RFC 3280:
 (1) initialization, (2) basic proxy certificate processing, (3)
 preparation for the next proxy certificate, and (4) wrap-up. Steps
 (1) and (4) are performed exactly once.  Step (2) is performed for
 all proxy certificates in the path.  Step (3) is performed for all
 proxy certificates in the path except the final proxy certificate.
 Certificate path validation as described in RFC 3280 MUST have been
 done prior to using this algorithm to validate the end entity
 certificate.  This algorithm then processes the proxy certificate
 chain using the end entity certificate information produced by RFC
 3280 path validation.

4.1.1. Inputs

 This algorithm assumes the following inputs are provided to the path
 processing logic:
 (a) information about the entity certificate already verified using
     RFC 3280 path validation.  This information includes:
    (1) the end entity name,
    (2) the working_public_key output from RFC 3280 path validation,
    (3) the working_public_key_algorithm output from RFC 3280,
    (4) and the working_public_key_parameters output from RFC 3280
        path validation.
 (b) prospective proxy certificate path of length n.
 (c) acceptable-pc-policy-language-set: A set of proxy certificate
     policy languages understood by the policy evaluation code.  The
     acceptable-pc-policy-language-set MAY contain the special value
     id-ppl-anyLanguage (as defined in Appendix A) if the path
     validation code should not check the proxy certificate policy
     languages (typically because the set of known policy languages is
     not known yet and will be checked later in the authorization
     process).
 (d) the current date and time.

Tuecke, et al. Standards Track [Page 19] RFC 3820 X.509 Proxy Certificate Profile June 2004

4.1.2. Initialization

 This initialization phase establishes the following state variables
 based upon the inputs:
 (a) working_public_key_algorithm: the digital signature algorithm
     used to verify the signature of a proxy certificate. The
     working_public_key_algorithm is initialized from the input
     information provided from RFC 3280 path validation.
 (b) working_public_key: the public key used to verify the signature
     of a proxy certificate.  The working_public_key is initialized
     from the input information provided from RFC 3280 path
     validation.
 (c) working_public_key_parameters: parameters associated with the
     current public key, that may be required to verify a signature
     (depending upon the algorithm).  The
     proxy_issuer_public_key_parameters variable is initialized from
     the input information provided from RFC 3280 path validation.
 (d) working_issuer_name: the issuer distinguished name expected in
     the next proxy certificate in the chain.  The working_issuer_name
     is initialized to the distinguished name in the end entity
     certificate validated by RFC 3280 path validation.
 (e) max_path_length: this integer is initialized to n, is decremented
     for each proxy certificate in the path.  This value may also be
     reduced by the pcPathLenConstraint value of any proxy certificate
     in the chain.
 (f) proxy_policy_list: this list is empty to start and will be filled
     in with the key usage extensions, extended key usage extensions
     and proxy policies in the chain.
 Upon completion of the initialization steps, perform the basic
 certificate processing steps specified in 4.1.3.

4.1.3. Basic Proxy Certificate Processing

 The basic path processing actions to be performed for proxy
 certificate i (for all i in [1..n]) are listed below.
 (a) Verify the basic certificate information.  The certificate MUST
     satisfy each of the following:

Tuecke, et al. Standards Track [Page 20] RFC 3820 X.509 Proxy Certificate Profile June 2004

    (1) The certificate was signed with the
        working_public_key_algorithm using the working_public_key and
        the working_public_key_parameters.
    (2) The certificate validity period includes the current time.
    (3) The certificate issuer name is the working_issuer_name.
    (4) The certificate subject name is the working_issuer_name with a
        CN component appended.
 (b) The proxy certificate MUST have a ProxyCertInfo extension.
     Process the extension as follows:
    (1) If the pCPathLenConstraint field is present in the
        ProxyCertInfo field and the value it contains is less than
        max_path_length, set max_path_length to its value.
    (2) If acceptable-pc-policy-language-set is not id-ppl-
        anyLanguage, the OID in the policyLanguage field MUST be
        present in acceptable-pc-policy-language-set.
 (c) The tuple containing the certificate subject name, policyPolicy,
     key usage extension (if present) and extended key usage extension
     (if present) must be appended to proxy_policy_list.
 (d) Process other certificate extensions, as described in [n2]:
    (1) Recognize and process any other critical extensions present in
        the proxy certificate.
    (2) Process any recognized non-critical extension present in the
        proxy certificate.
 If either step (a), (b) or (d) fails, the procedure terminates,
 returning a failure indication and an appropriate reason.
 If i is not equal to n, continue by performing the preparatory steps
 listed in 4.1.4.  If i is equal to n, perform the wrap-up steps
 listed in 4.1.5.

4.1.4. Preparation for next Proxy Certificate

 (a) Verify max_path_length is greater than zero and decrement
     max_path_length.
 (b) Assign the certificate subject name to working_issuer_name.

Tuecke, et al. Standards Track [Page 21] RFC 3820 X.509 Proxy Certificate Profile June 2004

 (c) Assign the certificate subjectPublicKey to working_public_key.
 (d) If the subjectPublicKeyInfo field of the certificate contains an
     algorithm field with non-null parameters, assign the parameters
     to the working_public_key_parameters variable.
     If the subjectPublicKeyInfo field of the certificate contains an
     algorithm field with null parameters or parameters are omitted,
     compare the certificate subjectPublicKey algorithm to the
     working_public_key_algorithm.  If the certificate
     subjectPublicKey algorithm and the working_public_key_algorithm
     are different, set the working_public_key_parameters to null.
 (e) Assign the certificate subjectPublicKey algorithm to the
     working_public_key_algorithm variable.
 (f) If a key usage extension is present, verify that the
     digitalSignature bit is set.
 If either check (a) or (f) fails, the procedure terminates, returning
 a failure indication and an appropriate reason.
 If (a) and (f) complete successfully, increment i and perform the
 basic certificate processing specified in 4.1.3.

4.1.5. Wrap-up Procedures

 (a) Assign the certificate subject name to working_issuer_name.
 (b) Assign the certificate subjectPublicKey to working_public_key.
 (c) If the subjectPublicKeyInfo field of the certificate contains an
     algorithm field with non-null parameters, assign the parameters
     to the proxy_issuer_public_key_parameters variable.
     If the subjectPublicKeyInfo field of the certificate contains an
     algorithm field with null parameters or parameters are omitted,
     compare the certificate subjectPublicKey algorithm to the
     proxy_issuer_public_key_algorithm.  If the certificate
     subjectPublicKey algorithm and the
     proxy_issuer_public_key_algorithm are different, set the
     proxy_issuer_public_key_parameters to null.
 (d) Assign the certificate subjectPublicKey algorithm to the
     proxy_issuer_public_key_algorithm variable.

Tuecke, et al. Standards Track [Page 22] RFC 3820 X.509 Proxy Certificate Profile June 2004

4.1.6. Outputs

 If path processing succeeds, the procedure terminates, returning a
 success indication together with final value of the
 working_public_key, the working_public_key_algorithm, the
 working_public_key_parameters, and the proxy_policy_list.

4.2. Using the Path Validation Algorithm

 Each Proxy Certificate contains a ProxyCertInfo extension, which
 always contains a policy language OID, and may also contain a policy
 OCTET STRING.  These policies serve to indicate the desire of each
 issuer in the proxy certificate chain, starting with the EEC, to
 delegate some subset of their rights to the issued proxy certificate.
 This chain of policies is returned by the algorithm to the
 application.
 The application MAY make authorization decisions based on the subject
 distinguished name of the proxy certificate or on one of the proxy
 certificates in it's issuing chain or on the EEC that serves as the
 root of the chain.  If an application chooses to use the subject
 distinguished name of a proxy certificate in the issuing chain or the
 EEC it MUST use the returned policies to restrict the rights it
 grants to the proxy certificate.  If the application does not know
 how to parse any policy in the policy chain it MUST not use, for the
 purposes of making authorization decisions, the subject distinguished
 name of any certificate in the chain prior to the certificate in
 which the unrecognized policy appears.
 Application making authorization decisions based on the contents of
 the proxy certificate key usage or extended key usage extensions MUST
 examine the list of key usage, extended key usage and proxy policies
 resulting from proxy certificate path validation and determine the
 effective key usage functions of the proxy certificate as follows:
  • If a certificate is a proxy certificate with a proxy policy of

id-ppl-independent or an end entity certificate, the effective key

    usage functions of that certificate is as defined by the key usage
    and extended key usage extensions in that certificate.  The key
    usage functionality of the issuer has no bearing on the effective
    key usage functionality.
  • If a certificate is a proxy certificate with a policy other than

id-ppl-independent, the effective key usage and extended key usage

    functionality of the proxy certificate is the intersection of the
    functionality of those extensions in the proxy certificate and the
    effective key usage functionality of the proxy issuer.

Tuecke, et al. Standards Track [Page 23] RFC 3820 X.509 Proxy Certificate Profile June 2004

5. Commentary

 This section provides non-normative commentary on Proxy Certificates.

5.1. Relationship to Attribute Certificates

 An Attribute Certificate [i3] can be used to grant to one identity,
 the holder, some attribute such as a role, clearance level, or
 alternative identity such as "charging identity" or "audit identity".
 This is accomplished by way of a trusted Attribute Authority (AA),
 which issues signed Attribute Certificates (AC), each of which binds
 an identity to a particular set of attributes. Authorization
 decisions can then be made by combining information from the
 authenticated End Entity Certificate providing the identity, with the
 signed Attribute Certificates providing binding of that identity to
 attributes.
 There is clearly some overlap between the capabilities provided by
 Proxy Certificates and Attribute Certificates.  However, the
 combination of the two approaches together provides a broader
 spectrum of solutions to authorization in X.509 based systems, than
 either solution alone.  This section seeks to clarify some of the
 overlaps, differences, and synergies between Proxy Certificate and
 Attribute Certificates.

5.1.1. Types of Attribute Authorities

 For the purposes of this discussion, Attribute Authorities, and the
 uses of the Attribute Certificates that they produce, can be broken
 down into two broad classes:
 1) End entity AA: An End Entity Certificate may be used to sign an
    AC.  This can be used, for example, to allow an end entity to
    delegate some of its privileges to another entity.
 2) Third party AA: A separate entity, aside from the end entity
    involved in an authenticated interaction, may sign ACs in order to
    bind the authenticated identity with additional attributes, such
    as role, group, etc.  For example, when a client authenticates
    with a server, the third party AA may provide an AC that binds the
    client identity to a particular group, which the server then uses
    for authorization purposes.
 This second type of Attribute Authority, the third party AA, works
 equally well with an EEC or a PC.  For example, unrestricted Proxy
 Certificates can be used to delegate the EEC's identity to various
 other parties.  Then when one of those other parties uses the PC to
 authenticate with a service, that service will receive the EEC's

Tuecke, et al. Standards Track [Page 24] RFC 3820 X.509 Proxy Certificate Profile June 2004

 identity via the PC, and can apply any ACs that bind that identity to
 attributes in order to determine authorization rights. Additionally
 PC with policies could be used to selectively deny the binding of ACs
 to a particular proxy.  An AC could also be bound to a particular PC
 using the subject or issuer and serial number of the proxy
 certificate.  There would appear to be great synergies between the
 use of Proxy Certificates and Attribute Certificates produced by
 third party Attribute Authorities.
 However, the uses of Attribute Certificates that are granted by the
 first type of Attribute Authority, the end entity AA, overlap
 considerably with the uses of Proxy Certificates as described in the
 previous sections.  Such Attribute Certificates are generally used
 for delegation of rights from one end entity to others, which clearly
 overlaps with the stated purpose of Proxy Certificates, namely single
 sign-on and delegation.

5.1.2. Delegation Using Attribute Certificates

 In the motivating example in Section 2, PCs are used to delegate
 Steve's identity to the various other jobs and entities that need to
 act on Steve's behalf.  This allows those other entities to
 authenticate as if they were Steve, for example to the mass storage
 system.
 A solution to this example could also be cast using Attribute
 Certificates that are signed by Steve's EEC, which grant to the other
 entities in this example the right to perform various operations on
 Steve's behalf.  In this example, the reliable file transfer service
 and all the hosts involved in file transfers, the starter program,
 the agent, the simulation jobs, and the post-processing job would
 each have their own EECs.  Steve's EEC would therefore issue ACs to
 bind each of those other EEC identities to attributes that grant the
 necessary privileges allow them to, for example, access the mass
 storage system.
 However, this AC based solution to delegation has some disadvantages
 as compared to the PC based solution:
  • All protocols, authentication code, and identity based

authorization services must be modified to understand ACs. With

    the PC solution, protocols (e.g., TLS) likely need no
    modification, authentication code needs minimal modification
    (e.g., to perform PC aware path validation), and identity based
    authorization services need minimal modification (e.g., possibly
    to find the EEC name and to check for any proxy policies).

Tuecke, et al. Standards Track [Page 25] RFC 3820 X.509 Proxy Certificate Profile June 2004

  • ACs need to be created by Steve's EEC, which bind attributes to

each of the other identities involved in the distributed

    application (i.e., the agent, simulation jobs, and post-processing
    job the file transfer service, the hosts transferring files).
    This implies that Steve must know in advance which other
    identities may be involved in this distributed application, in
    order to generate the appropriate ACs which are signed by Steve's
    ECC.  On the other hand, the PC solution allows for much more
    flexibility, since parties can further delegate a PC without a
    priori knowledge by the originating EEC.
 There are many unexplored tradeoffs and implications in this
 discussion of delegation.  However, reasonable arguments can be made
 in favor of either an AC based solution to delegation or a PC based
 solution to delegation.  The choice of which approach should be taken
 in a given instance may depend on factors such as the software that
 it needs to be integrated into, the type of delegation required, and
 other factors.

5.1.3. Propagation of Authorization Information

 One possible use of Proxy Certificates is to carry authorization
 information associated with a particular identity.
 The merits of placing authorization information into End Entity
 Certificates (also called a Public Key Certificate or PKC) have been
 widely debated.  For example, Section 1 of "An Internet Attribute
 Certificate Profile for Authorization" [i3] states:
    "Authorization information may be placed in a PKC extension or
    placed in a separate attribute certificate (AC).  The placement of
    authorization information in PKCs is usually undesirable for two
    reasons.  First, authorization information often does not have the
    same lifetime as the binding of the identity and the public key.
    When authorization information is placed in a PKC extension, the
    general result is the shortening of the PKC useful lifetime.
    Second, the PKC issuer is not usually authoritative for the
    authorization information.  This results in additional steps for
    the PKC issuer to obtain authorization information from the
    authoritative source.
    For these reasons, it is often better to separate authorization
    information from the PKC.  Yet, authorization information also
    needs to be bound to an identity.  An AC provides this binding; it
    is simply a digitally signed (or certified) identity and set of
    attributes."

Tuecke, et al. Standards Track [Page 26] RFC 3820 X.509 Proxy Certificate Profile June 2004

 Placing authorization information in a PC mitigates the first
 undesirable property cited above.  Since a PC has a lifetime that is
 mostly independent of (always shorter than) its signing EEC, a PC
 becomes a viable approach for carrying authorization information for
 the purpose of delegation.
 The second undesirable property cited above is true.  If a third
 party AA is authoritative, then using ACs issued by that third party
 AA is a natural approach to disseminating authorization information.
 However, this is true whether the identity being bound by these ACs
 comes from an EEC (PKC), or from a PC.
 There is one case, however, that the above text does not consider.
 When performing delegation, it is usually the EEC itself that is
 authoritative (not the EEC issuer, or any third party AA).  That is,
 it is up to the EEC to decide what authorization rights it is willing
 to grant to another party.  In this situation, including such
 authorization information into PCs that are generated by the EEC
 seems a reasonable approach to disseminating such information.

5.1.4. Proxy Certificate as Attribute Certificate Holder

 In a system that employs both PCs and ACs, one can imagine the
 utility of allowing a PC to be the holder of an AC.  This would allow
 for a particular delegated instance of an identity to be given an
 attribute, rather than all delegated instances of that identity being
 given the attribute.
 However, the issue of how to specify a PC as the holder of an AC
 remains open.  An AC could be bound to a particular instance of a PC
 using the unique subject name of the PC, or it's issuer and serial
 number combination.
 Unrestricted PCs issued by that PC would then inherit those ACs and
 independent PCs would not.  PCs issued with a policy would depend on
 the policy as to whether or not they inherit the issuing PC's ACs
 (and potentially which ACs they inherit).
 While an AC can be bound to one PC by the AA, how can the AA restrict
 that PC from passing it on to a subsequently delegated PC? One
 possible solution would be to define an extension to attribute
 certificates that allows the attribute authority to state whether an
 issued AC is to apply only to the particular entity to which it is
 bound, or if it may apply to PCs issued by that entity.
 One issue that an AA in this circumstance would need to be aware of
 is that the PI of the PC that the AA bound the AC to, could issue
 another PC with the same name as the original PC to a different

Tuecke, et al. Standards Track [Page 27] RFC 3820 X.509 Proxy Certificate Profile June 2004

 entity, effectively stealing the AC.  This implies that an AA issuing
 an AC to a PC need to not only trust the entity holding the PC, but
 the entity holding the PC's issuer as well.

5.2. Kerberos 5 Tickets

 The Kerberos Network Authentication Protocol (RFC 1510 [i6]) is a
 widely used authentication system based on conventional (shared
 secret key) cryptography.  It provides support for single sign-on via
 creation of "Ticket Granting Tickets" or "TGT", and support for
 delegation of rights via "forwardable tickets".
 Kerberos 5 tickets have informed many of the ideas surrounding X.509
 Proxy Certificates.  For example, the local creation of a short-lived
 PC can be used to provide single sign-on in an X.509 PKI based
 system, just as creation of short-lived TGT allows for single sign-on
 in a Kerberos based system.  And just as a TGT can be forwarded
 (i.e., delegated) to another entity to allow for proxying in a
 Kerberos based system, so can a PC can be delegated to allow for
 proxying in an X.509 PKI based system.
 A major difference between a Kerberos TGT and an X.509 PC is that
 while creation and delegation of a TGT requires the involvement of a
 third party (Key Distribution Center), a PC can be unilaterally
 created without the active involvement of a third party.  That is, a
 user can directly create a PC from an EEC for single sign-on
 capability, without requiring communication with a third party.  And
 an entity with a PC can delegate the PC to another entity (i.e., by
 creating a new PC, signed by the first) without requiring
 communication with a third party.
 The method used by Kerberos implementations to protect a TGT can also
 be used to protect the private key of a PC.  For example, some Unix
 implementations of Kerberos use standard Unix file system security to
 protect a user's TGT from compromise.  Similarly, the Globus
 Toolkit's Grid Security Infrastructure implementation of Proxy
 Certificates protects a user's PC private key using this same
 approach.

5.3. Examples of usage of Proxy Restrictions

 This section gives some examples of Proxy Certificate usage and some
 examples of how the Proxy policy can be used to restrict Proxy
 Certificates.

Tuecke, et al. Standards Track [Page 28] RFC 3820 X.509 Proxy Certificate Profile June 2004

5.3.1. Example use of proxies without Restrictions

 Steve wishes to perform a third-party FTP transfer between two FTP
 servers.  Steve would use an existing PC to authenticate to both
 servers and delegate a PC to both hosts.  He would inform each host
 of the unique subject name of the PC given to the other host.  When
 the servers establish the data channel connection to each other, they
 use these delegated credentials to perform authentication and verify
 they are talking to the correct entity by checking the result of the
 authentication matches the name as provided by Steve.

5.3.2. Example use of proxies with Restrictions

 Steve wishes to delegate to a process the right to perform a transfer
 of a file from host H1 to host H2 on his behalf.  Steve would
 delegate a PC to the process and he would use Proxy Policy to
 restrict the delegated PC to two rights - the right to read file F1
 on host H1 and the right to write file F2 on host H2.
 The process then uses this restricted PC to authenticate to servers
 H1 and H2.  The process would also delegate a PC to both servers.
 Note that these delegated PCs would inherit the restrictions of their
 parents, though this is not relevant to this example.  As in the
 example in the previous Section, each host would be provided with the
 unique name of the PC given to the other server.
 Now when the process issues the command to transfer the file F1 on H1
 and to F2 on H2, these two servers perform an authorization check
 based on the restrictions in the PC that the process used to
 authenticate with them (in addition to any local policy they have).
 Namely H1 checks that the PC gives the user the right to read F1 and
 H2 checks that the PC gives the user the right to write F2. When
 setting up the data channel the servers would again verify the names
 resulting from the authentication match the names provided by Steve
 as in the example in the previous Section.
 The extra security provided by these restrictions is that now if the
 PC delegated to the process by Steve is stolen, its use is greatly
 limited.

5.4. Delegation Tracing

 A relying party accepting a Proxy Certificate may have an interest in
 knowing which parties issued earlier Proxy Certificates in the
 certificate chain and to whom they delegated them.  For example it
 may know that a particular service or resource is known to have been

Tuecke, et al. Standards Track [Page 29] RFC 3820 X.509 Proxy Certificate Profile June 2004

 compromised and if any part of a Proxy Certificate's chain was issued
 to the compromised service a relying party may wish to disregard the
 chain.
 A delegation tracing mechanism was considered by the authors as
 additional information to be carried in the ProxyCertInfo extension.
 However at this time agreement has not been reached as to what this
 information should include so it was left out of this document, and
 will instead be considered in future revisions.  The debate mainly
 centers on whether the tracing information should simply contain the
 identity of the issuer and receiver or it should also contain all the
 details of the delegated proxy and a signed statement from the
 receiver that the proxy was actually acceptable to it.

5.4.1. Site Information in Delegation Tracing

 In some cases, it may be desirable to know the hosts involved in a
 delegation transaction (for example, a relying party may wish to
 reject proxy certificates that were created on a specific host or
 domain).  An extension could be modified to include the PA's and
 Acceptor's IP addresses; however, IP addresses are typically easy to
 spoof, and in some cases the two parties to a transaction may not
 agree on the IP addresses being used (e.g., if the Acceptor is on a
 host that uses NAT, the Acceptor and the PA may disagree about the
 Acceptor's IP address).
 Another suggestion was, in those cases where domain information is
 needed, to require that the subject names of all End Entities
 involved (the Acceptor(s) and the End Entity that appears in a PC's
 certificate path) include domain information.

6. Security Considerations

 In this Section we discuss security considerations related to the use
 of Proxy Certificates.

6.1. Compromise of a Proxy Certificate

 A Proxy Certificate is generally less secure than the EEC that issued
 it.  This is due to the fact that the private key of a PC is
 generally not protected as rigorously as that of the EEC.  For
 example, the private key of a PC is often protected using only file
 system security, in order to allow that PC to be used for single
 sign-on purposes.  This makes the PC more susceptible to compromise.
 However, the risk of a compromised PC is only the misuse of a single
 user's privileges.  Due to the PC path validation checks, a PC cannot
 be used to sign an EEC or PC for another user.

Tuecke, et al. Standards Track [Page 30] RFC 3820 X.509 Proxy Certificate Profile June 2004

 Further, a compromised PC can only be misused for the lifetime of the
 PC, and within the bound of the restriction policy carried by the PC.
 Therefore, one common way to limit the misuse of a compromised PC is
 to limit its validity period to no longer than is needed, and/or to
 include a restriction policy in the PC that limits the use of the
 (compromised) PC.
 In addition, if a PC is compromised, it does NOT compromise the EEC
 that created the PC.  This property is of great utility in protecting
 the highly valuable, and hard to replace, public key of the EEC.  In
 other words, the use of Proxy Certificates to provide single sign-on
 capabilities in an X.509 PKI environment can actually increase the
 security of the end entity certificates, because creation and use of
 the PCs for user authentication limits the exposure of the EEC
 private key to only the creation of the first level PC.

6.2. Restricting Proxy Certificates

 The pCPathLenConstraint field of the proxyCertInfo extension can be
 used by an EEC to limit subsequent delegation of the PC.  A service
 may choose to only authorize a request if a valid PC can be delegated
 to it.  An example of such as service is a job starter, which may
 choose to reject a job start request if a valid PC cannot be
 delegated to it.  By limiting the pCPathLenConstraint, an EEC can
 ensure that a compromised PC of one job cannot be used to start
 additional jobs elsewhere.
 An EEC or PC can limit what a new PC can be used for by turning off
 bits in the Key Usage and Extended Key Usage extensions.  Once a key
 usage or extended key usage has been removed, the path validation
 algorithm ensures that it cannot be added back in a subsequent PC.
 In other words, key usage can only be decreased in PC chains.
 The EEC could use the CRL Distribution Points extension and/or OCSP
 to take on the responsibility of revoking PCs that it had issued, if
 it felt that they were being misused.

6.3. Relying Party Trust of Proxy Certificates

 The relying party that is going to authorize some actions on the
 basis of a PC will be aware that it has been presented with a PC, and
 can determine the depth of the delegation and the time that the
 delegation took place.  It may want to use this information in
 addition to the information from the signing EEC.  Thus a highly
 secure resource might refuse to accept a PC at all, or maybe only a
 single level of delegation, etc.

Tuecke, et al. Standards Track [Page 31] RFC 3820 X.509 Proxy Certificate Profile June 2004

 The relying party should also be aware that since the policy
 restricting the rights of a PC is the intersection of the policy of
 all the PCs in it's certificate chain, this means any change in the
 certificate chain can effect the policy of the PC.  Since there is no
 mechanism in place to enforce unique subject names of PCs, if an
 issuer were to issue two PCs with identical names and keys, but
 different rights, this could allow the two PCs to be substituted for
 each other in path validation and effect the rights of a PC down the
 chain.  Ultimately, this means the relying party places trust in the
 entities that are acting as Proxy Issuers in the chain to behave
 properly.

6.4. Protecting Against Denial of Service with Key Generation

 As discussed in Section 2.3, one of the motivations for Proxy
 Certificates is to allow for dynamic delegation between parties. This
 delegation potentially requires, by the party receiving the
 delegation, the generation of a new key pair which is a potentially
 computationally expensive operation.  Care should be taken by such
 parties to prevent another entity from performing a denial of service
 attack by causing them to consume large amount of resource doing key
 generation.
 A general guideline would always to perform authentication of the
 delegating party to prevent such attacks from being performed
 anonymously.  Another guideline would be to maintain some state to
 detect and prevent such attacks.

6.5. Use of Proxy Certificates with a Central Repository

 As discussed in Section 2.7, one potential use of Proxy Certificates
 is to ease certificate management for end users by storing the EEC
 private keys and certificates in a centrally managed repository.
 When a user needs a PKI credential, the user can login to the
 repository using name/password, one time password, etc. and the
 repository would then delegate a PC to the user with proxy rights,
 but continue to protect the EEC private key in the repository.
 Care must be taken with this approach since compromise of the
 repository will potentially give the attacker access to the long-term
 private keys stored in the repository.  It is strongly suggested that
 some form of hardware module be used to store the long-term private
 keys, which will serve to help prevent their direct threat though it
 may still allow a successful attacker to use the keys while the
 repository is compromised to sign arbitrary objects (including Proxy
 Certificates).

Tuecke, et al. Standards Track [Page 32] RFC 3820 X.509 Proxy Certificate Profile June 2004

7. IANA Considerations

 IANA has established a registry for policy languages.  Registration
 under IETF space is by IETF standards action as described in [i8].
 Private policy languages should be under organizational OIDs; policy
 language authors are encouraged to list such languages in the IANA
 registry, along with a pointer to a specification.
 OID                      Description
 ---                      -----------
 1.3.6.1.5.5.7.21.1       id-ppl-inheritALL
 1.3.6.1.5.5.7.21.2       id-ppl-independent

8. References

8.1. Normative References

 [n1]    Bradner, S., "Key words for use in RFCs to Indicate
         Requirement Levels", BCP 14, RFC 2119, March 1997.
 [n2]    Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509
         Public Key Infrastructure Certificate and Certificate
         Revocation List (CRL) Profile", RFC 3280, April 2002.

8.2. Informative References

 [i1]    Butler, R., Engert, D., Foster, I., Kesselman, C., and S.
         Tuecke, "A National-Scale Authentication Infrastructure",
         IEEE Computer, vol. 33, pp. 60-66, 2000.
 [i2]    Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
         2246, January 1999.
 [i3]    Farrell, S. and R. Housley, "An Internet Attribute
         Certificate Profile for Authorization", RFC 3281, April 2002.
 [i4]    Foster, I., Kesselman, C., Tsudik, G., and S. Tuecke, "A
         Security Architecture for Computational Grids", presented at
         Proceedings of the 5th ACM Conference on Computer and
         Communications Security, 1998.
 [i5]    Foster, I., Kesselman, C., and S. Tuecke, "The Anatomy of the
         Grid: Enabling Scalable Virtual Organizations", International
         Journal of Supercomputer Applications, 2001.
 [i6]    Kohl, J. and C. Neuman, "The Kerberos Network Authentication
         Service (V5)", RFC 1510, September 1993.

Tuecke, et al. Standards Track [Page 33] RFC 3820 X.509 Proxy Certificate Profile June 2004

 [i7]    Neuman, B. Clifford, "Proxy-Based Authorization and
         Accounting for Distributed Systems", In Proceedings of the
         13th International Conference on Distributed Computing
         Systems, pages 283-291, May 1993.
 [i8]    Narten, T. and H. Alvestrand. "Guidelines for Writing an IANA
         Considerations Section in RFC", RFC 2434, October 1998.

9. Acknowledgments

 We are pleased to acknowledge significant contributions to this
 document by David Chadwick, Ian Foster, Jarek Gawor, Carl Kesselman,
 Sam Meder, Jim Schaad, and Frank Siebenlist.
 We are grateful to numerous colleagues for discussions on the topics
 covered in this paper, in particular (in alphabetical order, with
 apologies to anybody we've missed): Carlisle Adams, Joe Bester, Randy
 Butler, Keith Jackson, Steve Hanna, Russ Housley, Stephen Kent, Bill
 Johnston, Marty Humphrey, Sam Lang, Ellen McDermott, Clifford Neuman,
 Gene Tsudik.
 We are also grateful to members of the Global Grid Forum (GGF) Grid
 Security Infrastructure working group (GSI-WG), and the Internet
 Engineering Task Force (IETF) Public-Key Infrastructure (X.509)
 working group (PKIX) for feedback on this document.
 This work was supported in part by the Mathematical, Information, and
 Computational Sciences Division subprogram of the Office of Advanced
 Scientific Computing Research, U.S. Department of Energy, under
 Contract W-31-109-Eng-38 and DE-AC03-76SF0098; by the Defense
 Advanced Research Projects Agency under contract N66001-96-C-8523; by
 the National Science Foundation; and by the NASA Information Power
 Grid project.

Tuecke, et al. Standards Track [Page 34] RFC 3820 X.509 Proxy Certificate Profile June 2004

Appendix A. 1988 ASN.1 Module

 PKIXproxy88 { iso(1) identified-organization(3) dod(6)
     internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
     proxy-cert-extns(25) }
 DEFINITIONS EXPLICIT TAGS ::=
 BEGIN
  1. - EXPORTS ALL –
  1. - IMPORTS NONE –
  1. - PKIX specific OIDs
 id-pkix OBJECT IDENTIFIER ::=
         { iso(1) identified-organization(3)
              dod(6) internet(1) security(5) mechanisms(5) pkix(7) }
  1. - private certificate extensions

id-pe OBJECT IDENTIFIER ::= { id-pkix 1 }

  1. - Locally defined OIDs
  1. - The proxy certificate extension

id-pe-proxyCertInfo OBJECT IDENTIFIER ::= { id-pe 14 }

  1. - Proxy certificate policy languages

id-ppl OBJECT IDENTIFIER ::= { id-pkix 21 }

  1. - Proxy certificate policies languages defined in

id-ppl-anyLanguage OBJECT IDENTIFIER ::= { id-ppl 0 }

 id-ppl-inheritAll      OBJECT IDENTIFIER ::= { id-ppl 1 }
 id-ppl-independent     OBJECT IDENTIFIER ::= { id-ppl 2 }
  1. - The ProxyCertInfo Extension

ProxyCertInfoExtension ::= SEQUENCE {

       pCPathLenConstraint     ProxyCertPathLengthConstraint
                                     OPTIONAL,
       proxyPolicy             ProxyPolicy }
 ProxyCertPathLengthConstraint  ::= INTEGER
 ProxyPolicy  ::= SEQUENCE {
       policyLanguage          OBJECT IDENTIFIER,
       policy                  OCTET STRING OPTIONAL }
 END

Tuecke, et al. Standards Track [Page 35] RFC 3820 X.509 Proxy Certificate Profile June 2004

Authors' Addresses

 Steven Tuecke
 Distributed Systems Laboratory
 Mathematics and Computer Science Division
 Argonne National Laboratory
 Argonne, IL 60439
 Phone: 630-252-8711
 EMail: tuecke@mcs.anl.gov
 Von Welch
 National Center for Supercomputing Applications
 University of Illinois
 EMail: vwelch@ncsa.uiuc.edu
 Doug Engert
 Argonne National Laboratory
 EMail: deengert@anl.gov
 Laura Pearlman
 University of Southern California, Information Sciences Institute
 EMail: laura@isi.edu
 Mary Thompson
 Lawrence Berkeley National Laboratory
 EMail: mrthompson@lbl.gov

Tuecke, et al. Standards Track [Page 36] RFC 3820 X.509 Proxy Certificate Profile June 2004

Full Copyright Statement

 Copyright (C) The Internet Society (2004).  This document is subject
 to the rights, licenses and restrictions contained in BCP 78, and
 except as set forth therein, the authors retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
 assurances of licenses to be made available, or the result of an
 attempt made to obtain a general license or permission for the use of
 such proprietary rights by implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at ietf-
 ipr@ietf.org.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Tuecke, et al. Standards Track [Page 37]

/data/webs/external/dokuwiki/data/pages/rfc/rfc3820.txt · Last modified: 2004/06/14 18:08 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki