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rfc:rfc1004

Network Working Group D.L. Mills Request for Comments: 1004 University of Delaware

                                                            April 1987
            A Distributed-Protocol Authentication Scheme

Status of this Memo

 The purpose of this RFC is to focus discussion on authentication
 problems in the Internet and possible methods of solution.  The
 proposed solutions this document are not intended as standards for
 the Internet at this time.  Rather, it is hoped that a general
 consensus will emerge as to the appropriate solution to
 authentication problems, leading eventually to the adoption of
 standards.  Distribution of this memo is unlimited.

1. Introduction and Overview

 This document suggests mediated access-control and authentication
 procedures suitable for those cases when an association is to be set
 up between multiple users belonging to different trust environments,
 but running distributed protocols like the existing Exterior Gateway
 Protocol (EGP) [2], proposed Dissimilar Gateway Protocol (DGP) [3]
 and similar protocols. The proposed prcedures are evolved from those
 described by Needham and Shroeder [5], but specialized to the
 distributed, multiple-user model typical of these protocols.
 The trust model and threat environment are identical to that used by
 Kent and others [1]. An association is defined as the end-to-end
 network path between two users, where the users themselves are
 secured, but the path between them is not. The network may drop,
 duplicate or deliver messages with errors. In addition, it is
 possible that a hostile user (host or gateway) might intercept,
 modify and retransmit messages. An association is similar to the
 traditional connection, but without the usual connection requirements
 for error-free delivery.  The users of the association are sometimes
 called associates.
 The proposed procedures require each association to be assigned a
 random session key, which is provided by an authentication server
 called the Cookie Jar. The procedures are designed to permit only
 those associations sanctioned by the Cookie Jar while operating over
 arbitrary network topologies, including non-secured networks and
 broadcast-media networks, and in the presence of hostile attackers.
 However, it is not the intent of these procedures to hide the data

Mills [Page 1] RFC 1004 April 1987

 (except for private keys) transmitted via these networks, but only to
 authenticate messages to avoid spoofing and replay attacks.
 The procedures are intended for distributed systems where each user i
 runs a common protocol automaton using private state variables for
 each of possibly several associations simultaneously, one for each
 user j. An association is initiated by interrogating the Cookie Jar
 for a one-time key K(i,j), which is used to encrypt the checksum
 which authenticates messages exchanged between the users. The
 initiator then communicates the key to its associate as part of a
 connection establishment procedure such as described in [3].
 The information being exchanged in this protocol model is largely
 intended to converge a distributed data base to specified (as far as
 practical) contents, and does not ordinarily require a reliable
 distribution of event occurances, other than to speed the convergence
 process. Thus, the model is intrinsically resistant to message loss
 or duplication. Where important, sequence numbers are used to reduce
 the impact of message reordering. The model assumes that associations
 between peers, once having been sanctioned, are maintained
 indefinitely.  The exception when an association is broken may be due
 to a crash, loss of connectivity or administrative action such as
 reconfiguration or rekeying. Finally, the rate of information
 exchange is specifically designed to be much less than the nominal
 capabilities of the network, in order to keep overheads low.

2. Procedures

 Each user i is assigned a public address A(i) and private key K(i) by
 an out-of-band procedure beyond the scope of this discussion. The
 address can take many forms: an autonomous system identifier [2], an
 Internet address [6] or simply an arbitrary name. However, no matter
 what form it takes, every message is presumed to carry both the
 sender and receiver addresses in its header. Each address and its
 access-control list is presumed available in a public directory
 accessable to all users, but the private key is known only to the
 user and Cookie Jar and is not disclosed in messages exchanged
 between users or between users and the Cookie Jar.
 An association between i and j is identified by the bitstring
 consisting of the catenation of the addresses A(i) and A(j), together
 with a one-time key K(i,j), in the form [A(i),A(j),K(i,j)]. Note that
 the reciprocal association [A(j),A(i),K(j,i)] is distinguished only
 by which associate calls the Cookie Jar first. It is the intent in
 the protocol model that all state variables and keys relevant to a
 previous association be erased when a new association is initiated
 and no caching (as suggested in [5]) is allowed.

Mills [Page 2] RFC 1004 April 1987

 The one-time key K(i,j) is generated by the Cookie Jar upon receipt
 of a request from user i to associate with user j. The Cookie Jar has
 access to a private table of entries in the form [A(i),K(i)], where i
 ranges over the set of sanctioned users. The public directory
 includes for each A(i) a list L(i) = {j1, j2, ...} of permitted
 associates for i, which can be updated only by the Cookie Jar. The
 Cookie Jar first checks that the requested user j is in L(i), then
 rolls a random number for K(i,j) and returns this to the requestor,
 which saves it and passes it along to its associate during the
 connection establishment procedure.
 In the diagrams that follow all fields not specifically mentioned are
 unencrypted. While the natural implementation would include the
 address fields of the message header in the checksum, this raises
 significant difficulties, since they may be necessary to determine
 the route through the network itself. As will be evident below, even
 if a perpetrator could successfully tamper with the address fields in
 order to cause messages to be misdelivered, the result would not be a
 useful association.
 The checksum field is computed by a algorithm using all the bits in
 the message including the address fields in the message header, then
 is ordinarily encrypted along with the sequence-number field by an
 appropriate algorithm using the specified key, so that the intended
 receiver is assured only the intended sender could have generated it.
 In the Internet system, the natural choice for checksum is the 16-
 bit, ones-complement algorithm [6], while the natural choice for
 encryption is the DES algorithm [4] (see the discussion following for
 further consideration on these points). The detailed procedures are
 as follows:
    1. The requestor i rolls a random message ID I and sends it and
    the association specifier [A(i),A(j)] as a request to the Cookie
    Jar. The message header includes the addresses [A(i),A(C)], where
    A(C) is the address of the Cookie Jar. The following schematic
    illustrates the result:
    +-----------+-----------+
    |   A(i)    |   A(C)    |       message header
    +-----------+-----------+
    |     I     | checksum  |       message ID
    +-----------+-----------+
    |   A(i)    |   A(j)    |       assoc specifier
    +-----------+-----------+
    2. The Cookie Jar checks the access list to determine if the
    association [A(i),A(j)] is valid. If so, it rolls a random number
    K(i,j) and constructs the reply below. It checksums the message,

Mills [Page 3] RFC 1004 April 1987

    encrypts the j cookie field with K(j), then encrypts it and the
    other fields indicated with K(i) and finally sends the reply:
    +-----------+-----------+
    |   A(C)    |   A(i)    |       message header
    +-----------+-----------+
    |     I     | checksum  |       message ID (encrypt K(i))
    +-----------+-----------+
    |   K(i,j)  |                   i cookie (encrypt K(i))
    +-----------+
    |   K(i,j)  |                   j cookie (encrypt K(j)K(i))
    +-----------+
    3. Upon receipt of the reply the requestor i decrypts the
    indicated fields, saves the (encrypted) j cookie field and copies
    the i cookie field to the j cookie field, so that both cookie
    fields are now the original K(i,j) provided by the Cookie Jar.
    Then it verifies the checksum and matches the message ID with its
    list of outstanding requests, retaining K(i,j) for its own use. It
    then rolls a random number X for the j cookie field (to confuse
    wiretappers) and another I' for the (initial) message ID, then
    recomputes the checksum.  Finally, it inserts the saved j cookie
    field in the i cookie field, encrypts the message ID fields with
    K(i,j) and sends the following message to its associate:
    +-----------+-----------+
    |   A(i)    |   A(j)    |       message header
    +-----------+-----------+
    |     I'    | checksum  |       message ID (encrypt K(i,j))
    +-----------+-----------+
    |  K(i,j)   |                   i cookie (encrypt K(j))
    +-----------+
    |     X     |                   j cookie (noise)
    +-----------+
    4. Upon receipt of the above message the associate j decrypts the
    i cookie field, uses it to decrypt the message ID fields and
    verifies the checksum, retaining the I' and K(i,j) for later use.
    Finally, it rolls a random number J' as its own initial message
    ID, inserts it and the checksum in the confirm message, encrypts
    the message ID fields with K(i,j) and sends the message:
    +-----------+-----------+
    |   A(j)    |   A(i)    |       message header
    +-----------+-----------+
    |     J'    | checksum  |       message ID (encrypt K(i,j))
    +-----------+-----------+

Mills [Page 4] RFC 1004 April 1987

    5. Subsequent messages are all coded in the same way. As new data
    are generated the message ID is incremented, a new checksum
    computed and the message ID fields encrypted with K(i,j). The
    receiver decrypts the message ID fields with K(i,j) and discards
    the message in case of incorrect checksum or sequence number.

3. Discussion

 Since the access lists are considered public read-only, there is no
 need to validate Cookie Jar requests. A perpetrator might intercept,
 modify and replay portions of Cookie Jar replies or subsequent
 messages exchanged between the associates. However, presuming the
 perpetrator does not know the keys involved, tampered messages would
 fail the checksum test and be discarded.
 The "natural" selection of Internet checksum algorithm and DES
 encryption should be reconsidered. The Internet checksum has several
 well-known vulnerabilities, including invariance to word order and
 byte swap. In addition, the checksum field itself is only sixteen
 bits wide, so a determined perpetrator might be able to discover the
 key simply by examining all possible permutations of the checksum
 field. However, the procedures proposed herein are not intended to
 compensate for weaknesses of the checksum algorithm, since this
 vulnerability exists whether authentication is used or not. Also note
 that the encrypted fields include the sequence number as well as the
 checksum. In EGP and the proposed DGP the sequence number is a
 sixteen-bit quantity and increments for each successive message,
 which makes tampering even more difficult.
 In the intended application to EGP, DGP and similar protocols
 associations may have an indefinite lifetime, although messages may
 be sent between associates on a relatively infrequent basis.
 Therefore, every association should be rekeyed occasionally, which
 can be done by either associate simply by sending a new request to
 the Cookie Jar and following the above procedure. To protect against
 stockpiling private user keys, each user should be rekeyed
 occasionally, which is equivalent to changing passwords. The
 mechanisms for doing this are beyond the scope of this proposal.
 It is a feature of this scheme that the private user keys are not
 disclosed, except to the Cookie Jar. This is why two cookies are
 used, one for i, which only it can decrypt, and the other for j,
 decrypted first by i and then by j, which only then is valid. An
 interceptor posing as i and playing back the Cookie Jar reply to j
 will be caught, since the message will fail the checksum test. A
 perpetrator with access to both the Cookie Jar reply to i and the
 subsequent message to j will see essentially a random permutation of

Mills [Page 5] RFC 1004 April 1987

 all fields. In all except the first message to the Cookie Jar, the
 checksum field is encrypted, which makes it difficult to recover the
 original contents of the cookie fields before encryption by
 exploiting the properties of the checksum algorithm itself.
 The fact that the addresses in the message headers are included in
 the checksum protects against playbacks with modified addresses.
 However, it may still be possible to destabilize an association by
 playing back unmodified messages from prior associations. There are
 several possibilities:
    1. Replays of the Cookie Jar messages 1 and 2 are unlikely to
    cause damage, since the requestor matches both the addresses and
    once-only sequence number with its list of pending requests.
    2. Replays of message 3 may cause user j to falsely assume a new
    association. User j will return message 4 encrypted with the
    assumed session key, which might be an old one or even a currently
    valid one, but with invalid sequence number. Either way, user i
    will ignore message 4.
    3. Replays of message 4 or subsequent messages are unlikely to
    cause damage, since the sequence check will eliminate them.
 The second point above represents an issue of legitimate concern,
 since a determined attacker may stockpile message 3 interceptions and
 replay them at speed. While the attack is unlikely to succeed in
 establishing a working association, it might produce frequent
 timeouts and result in denial of service. In the Needham-Shroeder
 scheme this problem is avoided by requiring an additional challenge
 involving a message sent by user j and a reply sent by user i, which
 amounts to a three-way handshake.
 However, even if a three-way handshake were used, the additional
 protocol overhead induced by a determined attacker may still result
 in denial of service; moreover, the protocol model is inherently
 resistant to poor network performance, which has the same performance
 signature as the attacker. The conclusion is that the additional
 expense and overhead of a three-way handshake is unjustified.

4. Application to EGP and DGP

 This scheme can be incorporated in the Exterior Gateway Protocol
 (EGP) [2] and Dissimilar Gateway Protocol (DGP) [3] models by adding
 the fields above to the Request and Confirm messages in a
 straightforward way. An example of how this might be done is given in
 [3]. In order to retain the correctness of the state machine, it is

Mills [Page 6] RFC 1004 April 1987

 convenient to treat the Cookie Jar reply as a Start event, with the
 understanding that the Cookie Jar request represents an extrinsic
 event which evokes that response.
 The neighbor-acquisition strategy intended in the Dissimilar Gateway
 Protocol DGP follows the strategy in EGP. The stability of the EGP
 state machine, used with minor modifications by DGP, was verified by
 state simulation and discussed in an appendix to [2]. Either
 associate can send a Request command at any time, which causes both
 the sender and the receiver to reinitialize all state information and
 send a Confirm response. In DGP the Request operation involves the
 Cookie Jar transaction (messages 1 and 2) and then the Request
 command itself (message 3). In DGP the keys are reinitialized as well
 and each retransmission of a Request command is separately
 authenticated.
 In DGP the Request command (message 3) and all subsequent message
 exchanges assume the keys provided by the Cookie Jar. Use of any
 other keys results in checksum discrepancies and discarded messages.
 Thus the sender knows its command has been effected, at least at the
 time the response was sent. If either associate lost its state
 variables after that time, it would ignore subsequent messages and it
 (or its associate) would eventually time out and reinitiate the whole
 procedure.
 If both associates attempt to authenticate at the same time, they may
 wind up with the authentication sequences crossing in the network.
 Note that the Request message is self-authenticating, so that if a
 Request command is received by an associate before the Confirm
 response to an earlier Request command sent by that associate, the
 keys would be reset.  Thus when the subsequent Confirm response does
 arrive, it will be disregarded and the Request command resent
 following timeout. The race that results can only be broken when, due
 to staggered timeouts, the sequences do not cross in the network.
 This is a little more complicated than EGP and does imply that more
 attention must be paid to the timeouts.
 A reliable dis-association is a slippery concept, as example TCP and
 its closing sequences. However, the protocol model here is much less
 demanding. The usual way an EGP association is dissolved is when one
 associate sends a Cease command to the other, which then sends a
 Cease-ack response; however, this is specifically assumed a non-
 reliable transaction, with timeouts specified to break retry loops.
 In any case, a new Request command will erase all history and result
 in a new association as described above.
 Other than the above, the only way to reliably dis-associate is by
 timeout. In this protocol model the associates engage in a

Mills [Page 7] RFC 1004 April 1987

 reachability protocol, which requires each to send a message to the
 other from time to time. Each associate individually times out after
 a period when no messages are heard from the other.

5. Acknowledgments

 Dan Nessett and Phil Karn both provided valuable ideas and comments
 on early drafts of this report. Steve Kent and Dennis Perry both
 provided valuable advice on its review strategy.

6. References

 [1]  Kent, S.T., "Encryption-Based Protection for Interactive
      User/Computer Communication", Proc. Fifth Data Communications
      Symposium, September 1977.
 [2]  Mills, D.L., "Exterior Gateway Protocol Formal Specification",
      DARPA Network Working Group Report RFC-904, M/A-COM Linkabit,
      April 1984.
 [3]  Mills, D.L., "Dissimilar Gateway Protocol Draft Specification",
      in preparation, University of Delaware.
 [4]  National Bureau of Standards, "Data Encryption Standard",
      Federal Information Processing Standards Publication 46, January
      1977.
 [5]  Needham, R.M., and M.D. Schroeder, "Using Encryption for
      Authentication in Large Networks of Computers", Communications
      of the ACM, Vol. 21, No. 12, pp. 993-999, December 1978.
 [6]  Postel, J., "Internet Protocol", DARPA Network Working Group
      Report RFC-791, USC Information Sciences Institute, September
      1981.

Mills [Page 8]

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