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Network Working Group                                        R. Housley
Request for Comments: 2528                                       SPYRUS
Category: Informational                                         W. Polk
                                                                   NIST
                                                             March 1999


                Internet X.509 Public Key Infrastructure

         Representation of Key Exchange Algorithm (KEA) Keys in
         Internet X.509 Public Key Infrastructure Certificates

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Table of Contents

   Abstract ........................................................ 2
   1.  Executive Summary ........................................... 2
   2.  Requirements and Assumptions ................................ 2
   2.1.  Communication and Topology ................................ 2
   2.2.  Acceptability Criteria .................................... 2
   2.3.  User Expectations ......................................... 3
   2.4.  Administrator Expectations ................................ 3
   3.  KEA Algorithm Support ....................................... 3
   3.1.  Subject Public Key Info ................................... 3
   3.1.1.  Algorithm Identifier and Parameters ..................... 4
   3.1.2.  Encoding of KEA Public Keys ............................. 5
   3.2.  Key Usage Extension in KEA certificates ................... 5
   4. ASN.1 Modules ................................................ 5
   4.1 1988 Syntax ................................................. 5
   4.2 1993 Syntax ................................................. 6
   5. References ................................................... 6
   6. Security Considerations ...................................... 7
   7. Authors' Addresses ........................................... 8
   8. Full Copyright Statement ..................................... 9








Housley & Polk               Informational                      [Page 1]

RFC 2528                        PKIX KEA                      March 1999


Abstract

   The Key Exchange Algorithm (KEA) is a classified algorithm for
   exchanging keys.  This specification profiles the format and
   semantics of fields in X.509 V3 certificates containing KEA keys. The
   specification addresses the subjectPublicKeyInfo field and the
   keyUsage extension.

1.  Executive Summary

   This specification contains guidance on the use of the Internet
   Public Key Infrastructure certificates to convey Key Exchange
   Algorithm (KEA) keys. This specification is an addendum to RFC 2459,
   "Internet X.509 Public Key Infrastructure: Certificate and CRL
   Profile".  Implementations of this specification must also conform to
   RFC 2459.  Implementations of this specification are not required to
   conform to other parts from that series.

2.  Requirements and Assumptions

   The goal is to augment the X.509 certificate profile presented in
   Part 1 to facilitate the management of KEA keys for those communities
   which use this algorithm.

2.1.  Communication and Topology

   This profile, as presented in [RFC 2459] and augmented by this
   specification, supports users without high bandwidth, real-time IP
   connectivity, or high connection availability.  In addition, the
   profile allows for the presence of firewall or other filtered
   communication.

   This profile does not assume the deployment of an X.500 Directory
   system.  The profile does not prohibit the use of an X.500 Directory,
   but other means of distributing certificates and certificate
   revocation lists (CRLs) are supported.

2.2.  Acceptability Criteria

   The goal of the Internet Public Key Infrastructure (PKI) is to meet
   the needs of deterministic, automated identification, authentication,
   access control, and authorization functions. Support for these
   services determines the attributes contained in the certificate as
   well as the ancillary control information in the certificate such as
   policy data and certification path constraints.






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RFC 2528                        PKIX KEA                      March 1999


   The goal of this document is to profile KEA certificates, specifying
   the contents and semantics of attributes which were not fully
   specified by [RFC 2459].  If not specifically addressed by this
   document, the contents and semantics of the fields and extensions
   must be as described in [RFC 2459].

2.3.  User Expectations

   Users of the Internet PKI are people and processes who use client
   software and are the subjects named in certificates.  These uses
   include readers and writers of electronic mail, the clients for WWW
   browsers, WWW servers, and the key manager for IPSEC within a router.
   This profile recognizes the limitations of the platforms these users
   employ and the sophistication/attentiveness of the users themselves.
   This manifests itself in minimal user configuration responsibility
   (e.g., root keys, rules), explicit platform usage constraints within
   the certificate, certification path constraints which shield the user
   from many malicious actions, and applications which sensibly automate
   validation functions.

2.4.  Administrator Expectations

   As with users, the Internet PKI profile is structured to support the
   individuals who generally operate Certification Authorities (CAs).
   Providing administrators with unbounded choices increases the chances
   that a subtle CA administrator mistake will result in broad
   compromise or unnecessarily limit interoperability.  This profile
   defines the object identifiers and data formats that must be
   supported to interpret KEA public keys.

3.  KEA Algorithm Support

   This section describes object identifiers and data formats which may
   be used with [RFC 2459] to describe X.509 certificates containing a
   KEA public key.  Conforming CAs are required to use the object
   identifiers and data formats when issuing KEA certificates.
   Conforming applications shall recognize the object identifiers and
   process the data formats when processing such certificates.

3.1.  Subject Public Key Info

   The certificate identifies the KEA algorithm, conveys optional
   parameters, and specifies the KEA public key in the
   subjectPublicKeyInfo field. The subjectPublicKeyInfo field is a
   SEQUENCE of an algorithm identifier and the subjectPublicKey field.






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RFC 2528                        PKIX KEA                      March 1999


   The certificate indicates the algorithm through an algorithm
   identifier.  This algorithm identifier consists of an object
   identifier (OID) and optional associated parameters.  Section 3.1.1
   identifies the preferred OID and parameters for the KEA algorithm.
   Conforming CAs shall use the identified OID when issuing certificates
   containing public keys for the KEA algorithm. Conforming applications
   supporting the KEA algorithm shall, at a minimum, recognize the OID
   identified in section 3.1.1.

   The certificate conveys the KEA public key through the
   subjectPublicKey field.  This subjectPublicKey field is a BIT STRING.
   Section 3.1.2 specifies the method for encoding a KEA public key as a
   BIT STRING.  Conforming CAs shall encode the KEA public key as
   described in Section 3.1.2 when issuing certificates containing
   public keys for the KEA algorithm. Conforming applications supporting
   the KEA algorithm shall decode the subjectPublicKey as described in
   section 3.1.2 when the algorithm identifier is the one presented in
   3.1.1.

3.1.1.  Algorithm Identifier and Parameters

   The Key Exchange Algorithm (KEA) is an algorithm for exchanging keys.
   A KEA "pairwise key" may be generated between two users if their KEA
   public keys were generated with the same KEA parameters.  The KEA
   parameters are not included in a certificate; instead a "domain
   identifier" is supplied in the parameters field.

   When the subjectPublicKeyInfo field contains a KEA key, the algorithm
   identifier and parameters shall be as defined in [sdn.701r]:

      id-keyExchangeAlgorithm  OBJECT IDENTIFIER   ::=
             { 2 16 840 1 101 2 1 1 22 }

      KEA-Parms-Id     ::= OCTET STRING

   CAs shall populate the parameters field of the AlgorithmIdentifier
   within the subjectPublicKeyInfo field of each certificate containing
   a KEA public key with an 80-bit parameter identifier (OCTET STRING),
   also known as the domain identifier. The domain identifier will be
   computed in three steps: (1) the KEA parameters are DER encoded using
   the Dss-Parms structure; (2) a 160-bit SHA-1 hash is generated from
   the parameters; and (3) the 160-bit hash is reduced to 80-bits by
   performing an "exclusive or" of the 80 high order bits with the 80
   low order bits.  The resulting value is encoded such that the most
   significant byte of the 80-bit value is the first octet in the octet
   string.





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RFC 2528                        PKIX KEA                      March 1999


   The Dss-Parms is provided in [RFC 2459] and reproduced below for
   completeness.

        Dss-Parms  ::=  SEQUENCE  {
            p             INTEGER,
            q             INTEGER,
            g             INTEGER  }

3.1.2.  Encoding of KEA Public Keys

   A KEA public key, y, is conveyed in the subjectPublicKey BIT STRING
   such that the most significant bit (MSB) of y becomes the MSB of the
   BIT STRING value field and the least significant bit (LSB) of y
   becomes the LSB of the BIT STRING value field.  This results in the
   following encoding: BIT STRING tag, BIT STRING length, 0 (indicating
   that there are zero unused bits in the final octet of y), BIT STRING
   value field including y.

3.2.  Key Usage Extension in KEA certificates

   The key usage extension may optionally appear in a KEA certificate.
   If a KEA certificate includes the keyUsage extension, only the
   following values may be asserted:

      keyAgreement;
      encipherOnly; and
      decipherOnly.

   The encipherOnly and decipherOnly values may only be asserted if the
   keyAgreement value is also asserted.  At most one of encipherOnly and
   decipherOnly shall be asserted in keyUsage extension.  Generally, the
   keyAgreement value is asserted without either the encipherOnly or
   decipherOnly value being asserted.

4. ASN.1 Modules

4.1 1988 Syntax

   PKIXkea88 {iso(1) identified-organization(3) dod(6)
            internet(1) security(5) mechanisms(5) pkix(7)
            id-mod(0) id-mod-kea-profile-88(7) }


   BEGIN ::=

   -- EXPORTS ALL --

   -- IMPORTS NONE --



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RFC 2528                        PKIX KEA                      March 1999


      id-keyExchangeAlgorithm  OBJECT IDENTIFIER   ::=
             { 2 16 840 1 101 2 1 1 22 }

      KEA-Parms-Id     ::= OCTET STRING

   END

4.2 1993 Syntax

      PKIXkea93 {iso(1) identified-organization(3) dod(6)
            internet(1) security(5) mechanisms(5) pkix(7)
            id-mod(0) id-mod-kea-profile-93(8) }


      BEGIN ::=

   -- EXPORTS ALL --

   IMPORTS         ALGORITHM-ID
           FROM PKIX1Explicit93 {iso(1) identified-organization(3)
           dod(6) internet(1) security(5) mechanisms(5) pkix(7)
           id-mod(0) id-pkix1-explicit-93(3) }

     KeaPublicKey ALGORITHM-ID ::=  { OID id-keyExchangeAlgorithm
                                     PARMS KEA-Parms-Id }

      id-keyExchangeAlgorithm  OBJECT IDENTIFIER   ::=
             { 2 16 840 1 101 2 1 1 22 }

      KEA-Parms-Id     ::= OCTET STRING

   END

5. References

   [KEA]      "Skipjack and KEA Algorithm Specification", Version 2.0,
              29 May 1998. available from
              http://csrc.nist.gov/encryption/skipjack-kea.htm

   [SDN.701R] SDN.701, "Message Security Protocol", Revision 4.0
              1996-06-07 with "Corrections to Message Security Protocol,
              SDN.701, Rev 4.0, 96-06-07." August 30, 1996.

   [RFC 2459] Housley, R., Ford, W., Polk, W. and D. Solo "Internet
              X.509 Public Key Infrastructure: X.509 Certificate and CRL
              Profile", RFC 2459, January 1999.





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RFC 2528                        PKIX KEA                      March 1999


6. Security Considerations

   This specification is devoted to the format and encoding of KEA keys
   in X.509 certificates.  Since certificates are digitally signed, no
   additional integrity service is necessary. Certificates need not be
   kept secret, and unrestricted and anonymous access to certificates
   and CRLs has no security implications.

   However, security factors outside the scope of this specification
   will affect the assurance provided to certificate users.  This
   section highlights critical issues that should be considered by
   implementors, administrators, and users.

   The procedures performed by CAs and RAs to validate the binding of
   the subject's identity of their public key greatly affect the
   assurance that should be placed in the certificate.  Relying parties
   may wish to review the CA's certificate practice statement.

   The protection afforded private keys is a critical factor in
   maintaining security.  Failure of users to protect their KEA private
   keys will permit an attacker to masquerade as them, or decrypt their
   personal information.

   The availability and freshness of revocation information will affect
   the degree of assurance that should be placed in a certificate.

   While certificates expire naturally, events may occur during its
   natural lifetime which negate the binding between the subject and
   public key.  If revocation information is untimely or unavailable,
   the assurance associated with the binding is clearly reduced.
   Similarly, implementations of the Path Validation mechanism described
   in section 6 that omit revocation checking provide less assurance
   than those that support it.

   The path validation algorithm specified in [RFC 2459] depends on the
   certain knowledge of the public keys (and other information) about
   one or more trusted CAs. The decision to trust a CA is an important
   decision as it ultimately determines the trust afforded a
   certificate.  The authenticated distribution of trusted CA public
   keys (usually in the form of a "self-signed" certificate) is a
   security critical out of band process that is beyond the scope of
   this specification.

   In addition, where a key compromise or CA failure occurs for a
   trusted CA, the user will need to modify the information provided to
   the path validation routine.  Selection of too many trusted CAs will
   make the trusted CA information difficult to maintain.  On the other
   hand, selection of only one trusted CA may limit users to a closed



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RFC 2528                        PKIX KEA                      March 1999


   community of users until a global PKI emerges.

   The quality of implementations that process certificates may also
   affect the degree of assurance provided.  The path validation
   algorithm described in section 6 relies upon the integrity of the
   trusted CA information, and especially the integrity of the public
   keys associated with the trusted CAs.  By substituting public keys
   for which an attacker has the private key, an attacker could trick
   the user into accepting false certificates.

   The binding between a key and certificate subject cannot be stronger
   than the cryptographic module implementation and algorithms used to
   generate the signature.

7. Authors' Addresses

   Russell Housley
   SPYRUS
   381 Elden Street
   Suite 1120
   Herndon, VA 20170
   USA

   EMail: housley@spyrus.com


   Tim Polk
   NIST
   Building 820, Room 426
   Gaithersburg, MD 20899
   USA

   EMail: wpolk@nist.gov


















Housley & Polk               Informational                      [Page 8]

RFC 2528                        PKIX KEA                      March 1999


8.  Full Copyright Statement

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS 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.
























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