Network Working Group S. Turner Internet-Draft IECA Intended Status: Informational S. Kent Expires: December 1, 2012 BBN May 30, 2012 Additional Methods for Generating Key Identifiers and Key Identifier Semantic Extension draft-turner-additional-methods-4kis-05.txt Abstract This document specifies additional methods for generating key identifiers from a public key. This document also specifies an extension to identify the algorithms used to generate the key identifiers. 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 Expires December 1, 2012 [Page 1] Internet-Draft Additional Methods For Key Identifiers May 30, 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 secruity protocols (e.g., SMIME [RFC5751]) as a shorthand way to refer to a cert. This document defines two additional mechanisms for generating key identifier values, using SHA-256 and SHA-512 [SHS]. Sample code for SHA-256 and SHA-512 can be found in [RFC6234]. The motivation for defining these additional means of generating AKI/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 specifies an extension to identify the algorithm used to generate the SKI. The extension also identifies the hash algorithm input used to generate to the key identifier. Turner & Kent Expires December 1, 2012 [Page 2] Internet-Draft Additional Methods For Key Identifiers May 30, 2012 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. Two additional mechanisms CAs can use to identify public keys are as follows: 1) The keyIdentifier is composed of the least significant 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 least significant 160-bits of the SHA-512 hash of the value of the BIT STRING subjectPublicKey (excluding the tag, length, and number of unused bits). 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 SKISemantics IDENTIFIED BY id-pe-skiSemantics } id-pe-skiSemantics OBJECT IDENTIFIER ::= { id-pe TBD } Turner & Kent Expires December 1, 2012 [Page 3] Internet-Draft Additional Methods For Key Identifiers May 30, 2012 SKISemantics ::= SEQUENCE { skiAlgorithm AlgorithmIdentifier { DIGEST-ALGORITHM, { SKIHashAlgs } }, skiInput OBJECT IDENTIFIER ( SKIInputs, ... ) OPTIONAL, skiOutput OBJECT IDENTIFIER ( SKIOutputs, ... ) OPTIONAL } SKIHashAlgs DIGEST-ALGORITHM ::= { mda-sha256 | mda-sha512, ... } SKIInputs OBJECT IDENTIFIER ::= { id-subjectPublicKeyInfo, ... } id-subjectPublicKeyInfo OBJECT IDENTIFIER ::= { id-tbd } SKIOutputs OBJECT IDENTIFIER ::= { ... } SKISemantics has three fields: o akiAlgorithm indicates the algorithm used to generate the key identifier. For example, if the CA wanted to indicate that one of the algorithms listed in Section 2 was used, then it would include OIDs (Object Identifiers) from [RFC5758]. o akiInput indicates the semantics for the input to the hash algorithm. If this field is absent, then only the public key is hashed. This document defines the OID id-subjectPublicKeyInfo to be used when the input to the hash algorithm is the certificate's SubjectPublicKeyInfo field [RFC5280]. o skiOutput indicates the key identifier's semantics. If this field is absent, then key identifier is the hash (algorithm identified in kiAlgorithm) of the value of the BIT STRING subjectPublicKey (excluding the tag, length, and number of unused bits). This document defines the OID id-subjectPublicKeyInfo to be used when the input to the hash algorithm is the certificate's SubjectPublicKeyInfo field [RFC5280]. 4. 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 collision resistance, this property is not needed for key identifiers. Turner & Kent Expires December 1, 2012 [Page 4] Internet-Draft Additional Methods For Key Identifiers May 30, 2012 5. 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. 6. Acknowledgements The authors wish to thank Santosh Chokhani, Tom Gindin, Peter Gutmann, Henry Holtz, David Kemp, James Manager, 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. 7. References 7.1. 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. [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. Turner & Kent Expires December 1, 2012 [Page 5] Internet-Draft Additional Methods For Key Identifiers May 30, 2012 [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. 7.2. 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. Turner & Kent Expires December 1, 2012 [Page 6] Internet-Draft Additional Methods For Key Identifiers May 30, 2012 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 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-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) } ; ext-skiSemantics EXTENSION ::= { SYNTAX SKISemantics IDENTIFIED BY id-pe-skiSemantics } id-pe-skiSemantics OBJECT IDENTIFIER ::= { id-pe TBD } SKISemantics ::= SEQUENCE { skiAlgorithm AlgorithmIdentifier { DIGEST-ALGORITHM, { SKIHashAlgs } }, skiInput OBJECT IDENTIFIER ( SKIInputs, ... ) OPTIONAL, skiOutput OBJECT IDENTIFIER ( SKIOutputs, ... ) OPTIONAL } Turner & Kent Expires December 1, 2012 [Page 7] Internet-Draft Additional Methods For Key Identifiers May 30, 2012 SKIHashAlgs DIGEST-ALGORITHM ::= { mda-sha256 | mda-sha512, ... } SKIInputs OBJECT IDENTIFIER ::= { id-subjectPublicKeyInfo, ... } id-subjectPublicKeyInfo OBJECT IDENTIFIER ::= { id-tbd } SKIOutputs OBJECT IDENTIFIER ::= { ... } 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 Turner & Kent Expires December 1, 2012 [Page 8]