Independent Submission                                         S. Turner
Request for Comments: 7093                                          IECA
Category: Informational                                          S. Kent
ISSN: 2070-1721                                                      BBN
                                                               J. Manger
                                                                 Telstra
                                                           December 2013


        Additional Methods for Generating Key Identifiers Values

Abstract

   This document specifies additional example methods for generating Key
   Identifier values for use in the AKI (Authority Key Identifier) and
   SKI (Subject Key Identifier) certificate extensions.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7093.

Copyright Notice

   Copyright (c) 2013 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.

1.  Introduction

   [RFC5280] defines the AKI (Authority Key Identifier) and SKI (Subject
   Key Identifier) certificate extensions.  [RFC5280] describes two
   example mechanisms for generating AKI and 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 not be
   critical to security.  This document defines three additional
   mechanisms for generating Key Identifier values using SHA-256,
   SHA-384, and SHA-512 [SHS] that are similar to those examples defined
   in [RFC5280] as well as one based on hashing the certificate's
   Subject Public Key Info field.

2.  Additional Methods for Generating Key Identifiers

   [RFC5280] specifies two examples for generating key identifiers from
   public keys.  Four additional mechanisms 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.  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 (Elliptic Curve Digital Signature Algorithm) 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

   Using method 1 from Section 2, the subjectKeyIdentifier would be:

     30 1D
        06 03 551D0E -- id-ce-subjectKeyIdentifier
        04 16
           04 14 BF37B3E5808FD46D54B28E846311BCCE1CAD2E1A

   Using method 4 from Section 2 with SHA-256 and no truncation, the
   subjectKeyIdentifier extensions would be:

     30 29
        06 03 551D0E -- id-ce-subjectKeyIdentifier
        04 22
           04 20 6D20896AB8BD833B6B66554BD59B2022
                 5D8A75A296088148399D7BF763D57405

4.  Security Considerations

   The security considerations of [RFC5280] apply to certificates.  The
   security considerations of [RFC5758] apply to the hash algorithms.

   While hash algorithms provide preimage resistance, second-preimage
   resistance, and collision resistance, none of these properties are
   needed for key identifiers.

5.  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.

6.  Normative References

   [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.

   [SHS]      National Institute of Standards and Technology (NIST),
              FIPS Publication 180-3: Secure Hash Standard, October
              2008.

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
   USA

   EMail: kent@bbn.com


   James Manger
   Telstra
   6 / 150 Lonsdale Street
   Melbourne, Victoria 3000
   Australia

   EMail: james.h.manger@team.telstra.com