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IETF RFC 2528
Internet X.509 Public Key Infrastructure Representation of Key Exchange Algorithm (KEA) Keys in Internet X.509 Public Key Infrastructure Certificates
Last modified on Friday, March 5th, 1999
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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 © 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.
Housley & Polk Informational PAGE 2
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.
Housley & Polk Informational PAGE 3
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 --
Housley & Polk Informational PAGE 5
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.
Housley & Polk Informational PAGE 6
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
Housley & Polk Informational PAGE 7
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 © 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.
Housley & Polk Informational PAGE 9
Internet X.509 Public Key Infrastructure Representation of Key Exchange Algorithm (KEA) Keys in Internet X.509 Public Key Infrastructure Certificates
RFC TOTAL SIZE: 18273 bytes
PUBLICATION DATE: Friday, March 5th, 1999
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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