PKIX Working Group R. Housley (SPYRUS)
Internet Draft W. Polk (NIST)
expires in six months August 5, 1998
Internet X.509 Public Key Infrastructure
Representation of Key Exchange Algorithm (KEA) Keys in
Internet X.509 Public Key Infrastructure Certificates
<draft-ietf-pkix-ipki-kea-02.txt>
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Copyright (C) The Internet Society (1998). All Rights Reserved.
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Representation of Key Exchange Algorithm (KEA) Keys in
Internet X.509 Public Key Infrastructure Certificates
Table of Contents
Abstract .......................................................... 3
1. Executive Summary ............................................. 3
2. Requirements and Assumptions .................................. 3
2.1. Communication and Topology .................................. 3
2.2. Acceptability Criteria ...................................... 4
2.3. User Expectations ........................................... 4
2.4. Administrator Expectations .................................. 4
3. KEA Algorithm Support ......................................... 5
3.1. Subject Public Key Info ..................................... 5
3.1.1. Algorithm Identifier and Parameters ....................... 5
3.1.2. Encoding of KEA Public Keys ............................... 6
3.2. Key Usage Extension in KEA certificates ..................... 6
4. ASN.1 Modules .................................................. 7
4.1 1988 Syntax ................................................... 7
4.2 1993 Syntax ................................................... 7
5. References ..................................................... 8
6. Patent Statements .............................................. 8
7. Security Considerations ........................................ 8
8. Author Addresses ............................................... 9
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Abstract
This is the third draft of a profile for specification of Key
Exchange Algorithm (KEA) keys in Internet Public Key Infrastructure
X.509 certificates. There are only minor changes in content from the
second draft. Several modifications are required now that KEA has
been declassified. A patent statement is required, and a published
reference for the KEA algorithm is required. After these modifica-
tions, the document will be complete. Please send comments on this
document to the ietf-pkix@imc.org mail list.
1. Executive Summary
This specification contains guidance on the use of the Internet Pub-
lic Key Infrastructure certificates to convey Key Exchange Algorithm
(KEA) keys. This specification is an addendum to RFC xxxx, "Internet
X.509 Public Key Infrastructure: Certificate and CRL Profile".
Implementations of this specification must also conform to RFC xxxx.
Implementations of this specification are not required to conform to
other parts from that series.
The Key Exchange Algorithm (KEA) is a classified algorithm for
exchanging keys. This specification profiles the format and seman-
tics of fields in X.509 V3 certificates containing KEA keys. The
specification addresses the subjectPublicKeyInfo field and the
keyUsage extension.
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 Part 1 and augmented by this specifica-
tion, supports users without high bandwidth, real-time IP connec-
tivity, or high connection availablity. 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,
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but other means of distributing certificates and certificate revoca-
tion 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 ser-
vices determines the attributes contained in the certificate as well
as the ancillary control information in the certificate such as pol-
icy data and certification path constraints.
The goal of this document is to profile KEA certificates, specifying
the contants and semantics of attributes which were not fully speci-
fied by Part 1. If not specifically addressed by this document, the
contents and semantics of the fields and extensions must be as
described in Part 1.
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 comprom-
ise or unnecessarily limit interoperability. This profile defines
the object identifiers and data formats that must be supported to
intepret KEA public keys.
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3. KEA Algorithm Support
This section describes object identifiers and data formats which may
be used with PKIX certicate profile 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 param-
eters, and specifies the KEA public key in the subjectPublicKeyInfo
field. The subjectPublicKeyInfo field is a SEQUENCE of an algorithm
identifier and the subjectPublicKey field.
The certificate indicates the algorithm through an algorithm identif-
ier. 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 subjectPub-
licKey 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 algo-
rithm 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 a classified 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
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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.
The Dss-Parms is provided in [RFC xxx] and reproduced below for com-
pleteness.
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:
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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.
4. ASN.1 Modules
4.1 1988 Syntax
PKIXkea88 {iso(1) identified-organization(3) dod(6) inter-
net(1) security(5) mechanisms(5) pkix(7) id-mod(0) to be
assigned(?) } BEGIN ::=
-- EXPORTS ALL --
-- IMPORTS NONE --
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) inter-
net(1) security(5) mechanisms(5) pkix(7) id-mod(0) to be
assigned(?) }
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 ALGORTHM-ID ::= { OID id-keyExchangeAlgorithm
PARMS KEA-Parms-Id }
id-keyExchangeAlgorithm OBJECT IDENTIFIER ::=
{ 2 16 840 1 101 2 1 1 22 }
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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 xxxx] R. Housley, W. Ford, W. Polk and D. Solo "Internet X.509
Public Key Infrastructure: X.509 Certificate and CRL
Profile", July 28, 1998.
6. Patent Statements
To be added.
7. 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 sec-
tion highlights critical issues that should be considered by imple-
mentors, 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 main-
taining security. Failure of users to protect their KEA private keys
will permit an attacker to masquerade as them, or decrypt their per-
sonal information.
The availability and freshness of revocation information will affect
the degree of assurance that should be placed in a certificate.
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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. Simi-
larly, 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 xxxx] 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 certifi-
cate. The authenticated distribution of trusted CA public keys (usu-
ally in the form of a "self-signed" certificate) is a security criti-
cal out of band process that is beyond the scope of this specifica-
tion.
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
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 algo-
rithm described in section 6 relies upon the integrity of the trusted
CA information, and especially the integrity of the public keys asso-
ciated 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.
8. Author Addresses:
Russell Housley
SPYRUS
PO Box 1198
Herndon, VA 20172
USA
housley@spyrus.com
Tim Polk
NIST
Building 820, Room 426
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Gaithersburg, MD 20899
USA
wpolk@nist.gov
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