Additional Methods for Generating Subject Key Identifiers and Subject Key Identifier Semantics Extension

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Network Working Group                                          S. Turner
Internet-Draft                                                      IECA
Intended Status: Informational                                   S. Kent
Expires: January 4, 2013                                             BBN
                                                              J. Manager
                                                            July 3, 2012

       Additional Methods for Generating Subject Key Identifiers
             and Subject Key Identifier Semantics Extension


   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.

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Turner, Kent, & Manage r  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
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