Network Working Group S. Turner
Internet-Draft IECA
Intended Status: Informational S. Kent
Expires: January 4, 2013 BBN
J. Manger
Telstra
July 3, 2012
Additional Methods for Generating Subject Key Identifiers
and Subject Key Identifier Semantics Extension
draft-turner-additional-methods-4kis-07.txt
Abstract
This document specifies additional methods for generating Subject Key
Identifiers (SKI). This document also specifies an extension to
identify the algorithms used to generate the SKI.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Turner, Kent, & Manger Exp. December 29, 2012 [Page 1]
Internet-Draft Additional Methods For Key Identifier July 3, 2012
1. Introduction
[RFC5280] defines the AKI (Authority Key Identifier) and SKI (Subject
Key Identifier) certificate extensions. These extensions allow one
certificate to refer to another certificate via the matching of these
corresponding values. These identifiers enable a relying party to
disambiguate between two CA (Certification Authority) certificates
with the same Subject name, located in the same directory entry.
These identifiers are used during certification path construction in
support of heuristics to reduce relying party workload. These
identifiers are not used during certificate path validation. These
key identifiers are used by PKI-enabled security protocols, such as
CMP (Certificate Management Protocol) [RFC4210] and CMS
(Cryptographic Message Syntax) [RFC5652], to identify the certificate
used to protect a message, a session, etc.
[RFC5280] describes two example mechanisms for generating AKI/SKI
values: a 160-bit SHA-1 (Secure Hash Algorithm) hash of the public
key and a four-bit type field with the value 0100 followed by the
least significant 60 bits of the SHA-1 hash. Both of these
mechanisms were designed to be non-security critical. That is, the
use of a hash algorithm was intended to provide a high probability
(but not a guarantee) of uniqueness. [RFC5280] allows for additional
mechanisms. (This is consistent with the fact that the SKI and AKI
extensions are always marked non-critical.) In addition, some
security protocols (e.g., SMIME [RFC5751]) use key identifiers as a
shorthand way to refer to a cert.
This document defines three additional mechanisms for generating SKI
values, using SHA-256, SHA-384, and SHA-512 [SHS], that are similar
to those examples defined in [RFC5280]. Sample code for SHA-256, SHA-
384, and SHA-512 can be found in [RFC6234]. The motivation for
defining these additional means of generating SKI values is to
accommodate use of additional, standard one-way hash functions that
are becoming more widely used in PKI contexts. Note that these
example methods, like the examples methods from [RFC5280] are
designed to be non-security critical.
With these additional mechanisms, CAs can omit code for algorithms
that are otherwise unwanted or unused. For example, a CA that issues
certificates hashed with SHA-256 and signed with ECDSA on the P-256
curve [RFC5480] might no longer need to implement SHA-1 as part of
their CA application.
This document also defines an additional mechanism for generating an
SKI value that hashes the public key algorithm identifier, in
addition to the actual public key. This method is aligned
with key identifiers defined in [ID.dane-protocol].
Turner, Kent, & Manger Exp. December 29, 2012 [Page 2]
Internet-Draft Additional Methods For Key Identifier July 3, 2012
This document also specifies an extension to identify the algorithm
used to generate the SKI.
1.1. Requirements Terminology
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 [RFC2119].
1.2. ASN.1
The extension is defined using ASN.1 [X.680], [X.681], [X.682], and
[X.683].
2. Additional Methods for Generating Key Identifiers
As specified in [RFC5280], both authority and subject key identifiers
SHOULD be derived from the public key. Four additional mechanisms
CAs can use to identify public keys are as follows:
1) The keyIdentifier is composed of the leftmost 160-bits of the
SHA-256 hash of the value of the BIT STRING subjectPublicKey
(excluding the tag, length, and number of unused bits).
2) The keyIdentifier is composed of the leftmost 160-bits of the
SHA-384 hash of the value of the BIT STRING subjectPublicKey
(excluding the tag, length, and number of unused bits).
3) The keyIdentifier is composed of the leftmost 160-bits of the
SHA-512 hash of the value of the BIT STRING subjectPublicKey
(excluding the tag, length, and number of unused bits).
4) The keyIdentifier is composed of the hash of the DER-encoding of
the SubjectPublicKeyInfo value.
3. Subject Key Identifier Semantics Extension
The SKI semantics extension indicates the hash algorithm, the hash
algorithm input used to compute the SKI, and any semantics the issuer
chooses to communicate via the SKI. This allows the CA to embed
additional semantics in to the SKI, allowing it to be used for
purposes beyond certificate path building. This extension MAY, at
the option of the certificate issuer, be either critical or non-
critical. This extension is identified by id-pe-skiSemantics.
ext-skiSemantics EXTENSION ::= {
SYNTAX AlgorithmIdentifier { KI-ALGORITHM, {KIAlgs} }
IDENTIFIED BY id-pe-skiSemantics }
Turner, Kent, & Manger Exp. December 29, 2012 [Page 3]
Internet-Draft Additional Methods For Key Identifier July 3, 2012
id-pe-skiSemantics OBJECT IDENTIFIER ::= { id-pe TBD }
KIAlgs KI-ALGORITHM ::= {
kialg-keyHash |
kialg-4BitKeyHash |
kialg-keyInfoHash,
... }
KI-ALGORITHM ::= CLASS {
&id OBJECT IDENTIFIER UNIQUE,
&Params OPTIONAL
} WITH SYNTAX {
IDENTIFIER &id
[ PARAMS TYPE &Params ]
}
DigestAlgs DIGEST-ALGORITHM ::= {
mda-sha1 |
mda-sha256 |
mda-sha384 |
mda-sha512,
... }
kialg-keyHash KI-ALGORITHM ::= {
IDENTIFIER id-kialg-keyHash
PARAMS TYPE AlgorithmIdentifier {DIGEST-ALGORITHM, {DigestAlgs}}
}
id-kialg OBJECT IDENTIFIER ::= { id-pkix TBD }
id-kialg-keyHash OBJECT IDENTIFIER ::= { id-kialg TBD }
kialg-4BitKeyHash KI-ALGORITHM ::= {
IDENTIFIER id-kialg-4BitKeyHash
PARAMS TYPE AlgorithmIdentifier {DIGEST-ALGORITHM, {DigestAlgs}}
}
id-kialg-4BitKeyHash OBJECT IDENTIFIER ::= { id-kialg TBD }
kialg-keyInfoHash KI-ALGORITHM ::= {
IDENTIFIER id-kialg-keyInfoHash
PARAMS TYPE AlgorithmIdentifier {DIGEST-ALGORITHM, {DigestAlgs}}
}
id-kialg-keyInfoHash OBJECT IDENTIFIER ::= { id-kialg TBD }
This document defines the following set of algorithms:
Turner, Kent, & Manger Exp. December 29, 2012 [Page 4]
Internet-Draft Additional Methods For Key Identifier July 3, 2012
o kialg-keyHash indicates that the key id is the, possibly
truncated, hash of the value of the BIT STRING subjectPublicKey
(excluding the tag, length, and number of unused bits). The
amount of truncation can be determined from the length of the
actual key identifier. Truncation keeps the least significant
bits of the hash. The required parameter of this algorithm
identifies the hash algorithm that is used.
o kialg-4BitKeyHash indicates that the key id is a four-bit type
field with the value 0100 followed the, possibly truncated, hash
of the value of the BIT STRING subjectPublicKey (excluding the
tag, length, and number of unused bits). The amount of truncation
can be determined from the length of the actual key identifier
minus the first 4 bits. Truncation keeps the least significant
bits of the hash. The required parameter of this algorithm
identifies the hash algorithm that is used.
o kialg-keyInfoHash differs from kialg-keyHash in that the hash
covers the key algorithm id in addition to the actual public key.
kialg-keyInfoHash indicates that the key id is the, possibly
truncated, hash of the subjectPublicKeyInfo field. The amount of
truncation can be determined from the actual key identifier
value. Truncation keeps the least significant bits of the hash.
The required parameter of this algorithm identifies the hash
algorithm that is used.
NOTE: If a CA wishes to indicate that it used the 2 example methods
from [RFC5280], then kialg-keyHash and kialg-4BitKeyHash would use
SHA-1 and in the latter case truncated the SHA-1 output to 60-bits.
Turner, Kent, & Manger Exp. December 29, 2012 [Page 5]
Internet-Draft Additional Methods For Key Identifier July 3, 2012
4. Examples
This section provides some examples. The keys and SKIs are presented
in hexadecimal (two hex digits per byte).
Given the following DER-encoded SubjectPublicKeyInfo value holding an
P-256 ECDSA key:
30 59
30 13
06 07 2A8648CE3D0201 -- id-ecPublicKey
06 08 2A8648CE3D030107 -- secp256r1
03 42 00
04 7F7F35A79794C950060B8029FC8F363A
28F11159692D9D34E6AC948190434735
F833B1A66652DC514337AFF7F5C9C75D
670C019D95A5D639B72744C64A9128BB
The SHA-256 hash of the 65 bytes 047F7F...BB is:
BF37B3E5808FD46D54B28E846311BCCE1CAD2E1A62AA9092EF3EFB3F11451F44
The SHA-1 hash of these 65 bytes is:
6FEF9162C0A3F2E7608956D41C37DA0C8E87F0AE
The SHA-256 hash of the 91 bytes 305930...BB is:
6D20896AB8BD833B6B66554BD59B20225D8A75A296088148399D7BF763D57405
EDITOR'S NOTE: The XXs in the following will be replaced with actual
values once the OIDs have been assigned. We've assumed the extension
OID will come from the PKIX id-pe arc and the method OIDs will come
from the id-pkix arc.
Using method 1 from section 2, the subjectKeyIdentifier and ext-
skiSemantics extensions would be:
30 1D
06 03 551D0E -- id-ce-subjectKeyIdentifier
04 16
04 14 BF37B3E5808FD46D54B28E846311BCCE1CAD2E1A
30 23
06 08 2B060105050701XX -- id-pe-skiSemantics
04 17
30 15
06 08 2B060105050717XX -- id-kialg-keyHash
06 09 608648016503040201 -- id-sha256
Turner, Kent, & Manger Exp. December 29, 2012 [Page 6]
Internet-Draft Additional Methods For Key Identifier July 3, 2012
Using the 1st method in [RFC5280], the two extensions would be:
30 1D
06 03 551D0E -- id-ce-subjectKeyIdentifier
04 16
04 14 6FEF9162C0A3F2E7608956D41C37DA0C8E87F0AE
30 1F
06 08 2B060105050701XX -- id-pe-skiSemantics
04 13
30 11
06 08 2B060105050717XX -- id-kialg-keyHash
06 05 2B0E03021A -- id-sha1
Using the 2nd method in [RFC5280], the two extensions would be:
30 11
06 03 551D0E -- id-ce-subjectKeyIdentifier
04 0A
04 08 46FEF9162C0A3F2E
30 1F
06 08 2B060105050701XX -- id-pe-skiSemantics
04 13
30 11
06 08 2B060105050717XX -- id-kialg-keyHash
06 05 2B0E03021A -- id-sha1
Using method 4 from section 2 with SHA-256 and no truncation,
the two extensions would be:
30 26
06 03 551D0E -- id-ce-subjectKeyIdentifier
04 22
04 20 6D20896AB8BD833B6B66554BD59B2022
5D8A75A296088148399D7BF763D57405
30 23
06 08 2B060105050701XX -- id-pe-skiSemantics
04 17
30 15
06 08 2B060105050717XX -- id-kialg-keyInfoHash
06 09 608648016503040201 -- id-sha256
Turner, Kent, & Manger Exp. December 29, 2012 [Page 7]
Internet-Draft Additional Methods For Key Identifier July 3, 2012
5. Security Considerations
The security considerations of [RFC5280] apply to certificates. The
security considerations of [RFC5758] apply to the hash algorithms.
The security considerations of [RFC5912] apply to the ASN.1.
While hash algorithms provide preimage resistance, second-preimage
resistance, and collision resistance, none of these properties are
needed for key identifiers.
6. IANA Considerations
None.
NOTE there are some OIDs that need to be registered in the PKIX Arc.
This will be completed later in the process.
7. Acknowledgements
The authors wish to thank Santosh Chokhani, Stephen Farrell, Tom
Gindin, Peter Gutmann, Henry Holtz, David Kemp,
Timothy Miller, Michael StJohns, Stefan Santesson, Jim Schaad, Rene
Struik, Koichi Sugimoto, and Carl Wallace for taking the time to
participate in the discussions about this document. The discussions
resulted in numerous editorial and technical changes to the document.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC5758] Dang, Q., Santesson, S., Moriarty, K., Brown, D., and T.
Polk, "Internet X.509 Public Key Infrastructure: Additional
Algorithms and Identifiers for DSA and ECDSA", RFC 5758,
January 2010.
Turner, Kent, & Manger Exp. December 29, 2012 [Page 8]
Internet-Draft Additional Methods For Key Identifier July 3, 2012
[RFC5912] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
June 2010.
[SHS] National Institute of Standards and Technology (NIST), FIPS
Publication 180-3: Secure Hash Standard, October 2008.
[X.680] ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002,
Information technology - Abstract Syntax Notation One
(ASN.1): Specification of basic notation.
[X.681] ITU-T Recommendation X.681 (2002) | ISO/IEC 8824-2:2002,
Information Technology - Abstract Syntax Notation One:
Information Object Specification.
[X.682] ITU-T Recommendation X.682 (2002) | ISO/IEC 8824-3:2002,
Information Technology - Abstract Syntax Notation One:
Constraint Specification.
[X.683] ITU-T Recommendation X.683 (2002) | ISO/IEC 8824-4:2002,
Information Technology - Abstract Syntax Notation One:
Parameterization of ASN.1 Specifications.
8.2. Informative References
[RFC4210] Adams, C., Farrell, S., Kause, T., and T. Mononen,
"Internet X.509 Public Key Infrastructure Certificate
Management Protocol (CMP)", RFC 4210, September 2005.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
[RFC5480] Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
"Elliptic Curve Cryptography Subject Public Key
Information", RFC 5480, March 2009.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011.
[ID.dane-protocol] Hoffman, P., and J. Schlyter, "The DNS-Based
Authentication of Named Entities (DANE) Transport Layer
Security (TLS) Protocol: TLSA", draft-ietf-dane-protocol-
Turner, Kent, & Manger Exp. December 29, 2012 [Page 9]
Internet-Draft Additional Methods For Key Identifier July 3, 2012
23, work-in-progress.
Appendix A ASN.1 Module
KISemantics-2012
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-kiSemantics(TBD) }
DEFINITIONS EXPLICIT TAGS ::=
BEGIN
IMPORTS
-- Imports are all from [RFC5912]
EXTENSION
FROM PKIX-CommonTypes-2009
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkixCommon-02(57) }
id-pe, id-pkix
FROM PKIX1Explicit-2009
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkix1-explicit-02(51) }
AlgorithmIdentifier{}, DIGEST-ALGORITHM
FROM AlgorithmInformation-2009
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-algorithmInformation-02(58) }
mda-sha1
FROM PKIXAlgs-2009
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkix1-algorithms2008-02(56)}
mda-sha224, mda-sha256, mda-sha384, mda-sha512
FROM PKIX1-PSS-OAEP-Algorithms-2009
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkix1-rsa-pkalgs-02(54) } ;
Turner, Kent, & Manger Exp. December 29, 2012 [Page 10]
Internet-Draft Additional Methods For Key Identifier July 3, 2012
ext-skiSemantics EXTENSION ::= {
SYNTAX AlgorithmIdentifier { KI-ALGORITHM, {KIAlgs} }
IDENTIFIED BY id-pe-skiSemantics }
id-pe-skiSemantics OBJECT IDENTIFIER ::= { id-pe TBD }
KIAlgs KI-ALGORITHM ::= {
kialg-keyHash |
kialg-4BitKeyHash |
kialg-keyInfoHash,
... }
KI-ALGORITHM ::= CLASS {
&id OBJECT IDENTIFIER UNIQUE,
&Params OPTIONAL
} WITH SYNTAX {
IDENTIFIER &id
[ PARAMS TYPE &Params ]
}
DigestAlgs DIGEST-ALGORITHM ::= {
mda-sha1 |
mda-sha256 |
mda-sha384 |
mda-sha512,
... }
kialg-keyHash KI-ALGORITHM ::= {
IDENTIFIER id-kialg-keyHash
PARAMS TYPE AlgorithmIdentifier {DIGEST-ALGORITHM, {DigestAlgs}}
}
id-kialg OBJECT IDENTIFIER ::= { id-pkix TBD }
id-kialg-keyHash OBJECT IDENTIFIER ::= { id-kialg TBD }
kialg-4BitKeyHash KI-ALGORITHM ::= {
IDENTIFIER id-kialg-4BitKeyHash
PARAMS TYPE AlgorithmIdentifier {DIGEST-ALGORITHM, {DigestAlgs}}
}
id-kialg-4BitKeyHash OBJECT IDENTIFIER ::= { id-kialg TBD }
kialg-keyInfoHash KI-ALGORITHM ::= {
IDENTIFIER id-kialg-keyInfoHash
PARAMS TYPE AlgorithmIdentifier {DIGEST-ALGORITHM, {DigestAlgs}}
}
Turner, Kent, & Manger Exp. December 29, 2012 [Page 11]
Internet-Draft Additional Methods For Key Identifier July 3, 2012
id-kialg-keyInfoHash OBJECT IDENTIFIER ::= { id-kialg TBD }
END
Authors' Addresses
Sean Turner
IECA, Inc.
3057 Nutley Street, Suite 106
Fairfax, VA 22031
USA
EMail: turners@ieca.com
Stephen Kent
BBN Technologies
10 Moulton St.
Cambridge, MA 02138
EMail: kent@bbn.com
James Manger
Telstra
3 / 35 Collins Street
Melbourne, Victoria 3000
Australia
Email: james.h.manger@team.telstra.com
Turner, Kent, & Manger Exp. December 29, 2012 [Page 12]