Additional Methods for Generating Key Identifiers

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Last updated 2012-04-23
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Network Working Group                                          S. Turner
Internet Draft                                                      IECA
Intended Status: Informational                                   S. Kent
Expires: October 25, 2012                                            BBN
                                                          April 23, 2012

           Additional Methods for Generating Key Identifiers


   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

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Turner & Kent           Expires October 25, 2012                [Page 1]
Internet-Draft       Additional Methods For AKI/SKI       April 23, 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.)

   This document defines four additional mechanisms for generating key
   identifier values, using SHA-224, SHA-256, SHA-384, and SHA-512
   [SHS]. Sample code for SHA-224, SHA-256, SHA-384, 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.

   The additional key identifier generation mechanisms described in this
   document maintain the 160-bit value size, to avoid adversely
   affecting relying party code.  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 algorithms
   used to generate the key identifiers.  The extension also identifies
   the hash algorithm input used to generate to the key identifier. 


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