RFC 4476

Network Working Group                                         C. Francis
Request for Comments: 4476                                      Raytheon
Category: Standards Track                                      D. Pinkas
                                                                May 2006

             Attribute Certificate (AC) Policies Extension

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document describes one certificate extension that explicitly
   states the Attribute Certificate Policies (ACPs) that apply to a
   given Attribute Certificate (AC).  The goal of this document is to
   allow relying parties to perform an additional test when validating
   an AC, i.e., to assess whether a given AC carrying some attributes
   can be accepted on the basis of references to one or more specific

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1.  Introduction

   When issuing a Public Key Certificate (PKC), a Certificate Authority
   (CA) can perform various levels of verification with regard to the
   subject identity (see [RFC3280]).  A CA makes its verification
   procedures, as well as other operational rules it abides by,
   "visible" through a certificate policy, which may be referenced by a
   certificate policies extension in the PKC.

   The purpose of this document is to define an Attribute Certificate
   (AC) policies extension able to explicitly state the AC policies that
   apply to a given AC, but not the AC policies themselves.  Attribute
   Certificates are defined in [RFC3281].

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  AC Policies Extension Semantics

   An Attribute Certificate Policy is a named set of rules that
   indicates the applicability of an AC to a particular community and/or
   class of applications with common security requirements.  It defines
   rules for the generation, issuance, and revocation of ACs.  It may
   also include additional rules for attributes registration.

   Thus, note that an Attribute Authority (AA) does not necessarily
   support one single ACP.  However, for each AC that is delivered, the
   AA SHALL make sure that the policy applies to all the attributes that
   are contained in it.

   An ACP may be used by an AC user to decide whether or not to trust
   the attributes contained in an AC for a particular purpose.

   When an AC contains an AC policies extension, the extension MAY, at
   the option of the AA, be either critical or non-critical.

   The AC Policies extension MAY be included in an AC.  Like all X.509
   certificate extensions [X.509], the AC policies extension is defined
   using ASN.1 [ASN1].  See Appendix A.

   The definitions are presented in the 1988 Abstract Syntax Notation
   One (ASN.1) rather than the 1997 ASN.1 syntax used in the most recent
   ISO/IEC/ITU-T standards.

   The AC policies extension is identified by id-pe-acPolicies.

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      id-pe-acPolicies OBJECT IDENTIFIER ::= { iso(1)
        identified-organization(3) dod(6) internet(1) security(5)
        mechanisms(5) id-pkix(7) id-pe(1) 15 }

   The AC policies extension includes a list of AC policies recognized
   by the AA that apply to the attributes included in the AC.

   AC Policies may be defined by any organization with a need.  Object
   identifiers used to identify AC Policies are assigned in accordance
   with [X.660|ISO9834-1].

   The AC policies extension in an AC indicates the AC policies for
   which the AC is valid.

   An application that recognizes this extension and its content SHALL
   process the extension regardless of the value of the criticality

   If the extension is both flagged non-critical and not recognized by
   the AC-using application, then the application MAY ignore it.

   If the extension is marked critical or is recognized by the AC-using
   application, it indicates that the attributes contained in the
   attribute certificate SHALL only be used for the purpose, and in
   accordance with the rules associated with one of the indicated AC
   policies.  If none of the ACP identifiers is adequate for the
   application, then the AC MUST be rejected.

   If the extension is marked critical or is recognized by the AC using
   application, the AC-using application MUST use the list of AC
   policies to determine whether it is appropriate to use the attributes
   contained in that AC for a particular transaction.  When the
   appropriate policy is not found, the AC SHALL be rejected.

2.1.  AC Policy Extension Syntax

   The syntax for the AC Policy extension is:

   AcPoliciesSyntax ::= SEQUENCE SIZE (1..MAX) OF PolicyInformation

   PolicyInformation ::= SEQUENCE {
       policyIdentifier      AcPolicyId,
       policyQualifiers      SEQUENCE SIZE (1..MAX) OF
                                      PolicyQualifierInfo OPTIONAL}


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    PolicyQualifierInfo ::= SEQUENCE {
         policyQualifierId  PolicyQualifierId,
         qualifier          ANY DEFINED BY policyQualifierId }

   -- policyQualifierIds for Internet policy qualifiers

    id-qt            OBJECT IDENTIFIER ::=  { id-pkix 2 }
    id-qt-acps       OBJECT IDENTIFIER ::=  { id-qt 4 }
    id-qt-acunotice  OBJECT IDENTIFIER ::=  { id-qt 5 }

    id-qt-acps OBJECT IDENTIFIER ::= { iso(1)
      identified-organization(3) dod(6) internet(1) security(5)
      mechanisms(5) id-pkix(7) id-qt(2) 4 }

    id-qt-acunotice OBJECT IDENTIFIER ::= { iso(1)
      identified-organization(3) dod(6) internet(1) security(5)
      mechanisms(5) id-pkix(7) id-qt(2) 5 }

    PolicyQualifierId ::=
         OBJECT IDENTIFIER ( id-qt-acps | id-qt-acunotice )

   -- ACPS pointer qualifier

   ACPSuri ::= IA5String
   -- ACP statement user notice qualifier

   ACUserNotice ::= UserNotice
   -- UserNotice is defined in [RFC3280]

   To promote interoperability, this document RECOMMENDS that policy
   information terms consist of only an object identifier (OID).  When
   more than one policy is used, the policy requirements have to be
   non-conflicting, e.g., one policy may refine the general requirements
   mandated by another policy.

   The extension defined in this specification supports two policy
   qualifier types for use by ACP writers and AAs.  The qualifier types
   are the ACPS Pointer and the AC User.

2.1.1.  Notice Qualifiers

   The ACPS Pointer qualifier contains a pointer to an Attribute
   Certification Practice Statement (ACPS) published by the AA.  The
   pointer is in the form of a URI.  Processing requirements for this
   qualifier are a local matter.

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   The AC User Notice is intended for display to a relying party when an
   attribute certificate is used.  The application software SHOULD
   display the AC User Notice of the AC.  The AC User Notice is defined
   in [RFC3280].  It has two optional fields: the noticeRef field and
   the explicitText field.

      The noticeRef field, if used, names an organization and
      identifies, by number, a particular textual statement prepared by
      that organization.  For example, it might identify the
      organization's name and notice number 1.  In a typical
      implementation, the application software will have a notice file
      containing the current set of notices for the AA; the application
      will extract the notice text from the file and display it.
      Messages MAY be multilingual, allowing the software to select the
      particular language message for its own environment.

      An explicitText field includes the textual statement directly in
      the certificate.  The explicitText field is a string with a
      maximum size of 200 characters.

   If both the noticeRef and explicitText options are included in the
   one qualifier, and if the application software can locate the notice
   text indicated by the noticeRef option, then that text SHOULD be
   displayed; otherwise, the explicitText string SHOULD be displayed.

2.2.  Attribute Certificate Policies

   The scope of this document is not the definition of the detailed
   content of ACPs themselves; therefore, specific policies are not
   defined in this document.

3.  Security Considerations

   The ACP defined in this document applies for all the attributes that
   are included in one AC.  AAs SHALL ensure that the ACP applies to all
   the attributes that are included in the ACs they issue.

   Attributes may be dynamically grouped in several ACs.  It should be
   observed that since an AC may be issued under more than one ACP, the
   attributes included in a given AC MUST be compliant with all the ACPs
   from that AC.

   When verifying an AC, a relying party MUST determine that the AC was
   issued by a trusted AA and then that it has the appropriate policy.

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   Failure of AAs to protect their private keys will permit an attacker
   to masquerade as them, potentially generating false ACs or revocation
   status.  Existence of bogus ACs and revocation status will undermine
   confidence in the system.  If the compromise is detected, then the
   certificate of the AA MUST be revoked.

   Rebuilding after such a compromise will be problematic, so AAs are
   advised to implement a combination of strong technical measures
   (e.g., tamper-resistant cryptographic modules) and appropriate
   management procedures (e.g., separation of duties) to avoid such an

   Loss of an AA's private signing key may also be problematic.  The AA
   would not be able to produce revocation status or perform AC renewal
   (i.e., the issue of a new AC with the same set of attributes with the
   same values, for the same holder, from the same AA but with a
   different validity period).  AC issuers are advised to maintain
   secure backup for signing keys.  The security of the key backup
   procedures is a critical factor in avoiding key compromise.

   The availability and freshness of revocation status will affect the
   degree of assurance that should be placed in a long-lived AC.  While
   long-lived ACs expire naturally, events may occur during an AC's
   natural lifetime that negate the binding between the AC holder and
   the attributes.  If revocation status is untimely or unavailable, the
   assurance associated with the binding is clearly reduced.

   The binding between an AC holder and attributes cannot be stronger
   than the cryptographic module implementation and algorithms used to
   generate the signature.  Short key lengths or weak hash algorithms
   will limit the utility of an AC.  AAs are encouraged to note advances
   in cryptology so they can employ strong cryptographic techniques.

   If an AC is tied to the holder's PKC using the baseCertificateID
   component of the Holder field and the PKI in use includes a rogue CA
   with the same issuer name specified in the baseCertificateID
   component, this rogue CA could issue a PKC to a malicious party,
   using the same issuer name and serial number as the proper holder's
   PKC.  Then the malicious party could use this PKC in conjunction with
   the AC.  This scenario SHOULD be avoided by properly managing and
   configuring the PKI so that there cannot be two CAs with the same
   name.  Another alternative is to tie ACs to PKCs using the
   publicKeyCert type in the ObjectDigestInfo field.  Failing this, AC
   verifiers have to establish (using other means) that the potential
   collisions cannot actually occur; for example, the Certificate Policy
   Statements (CPSs) of the CAs involved may make it clear that no such
   name collisions can occur.

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   Implementers MUST ensure that following validation of an AC, only
   attributes that the issuer is trusted to issue are used in
   authorization decisions.  Other attributes, which MAY be present,
   MUST be ignored.  AC verifiers SHALL support means of being provided
   with this information.  The AA controls PKC extension (see [RFC3281])
   is one possibility, but it is optional to implement.  Configuration
   information is a likely alternative means, while out-of-band means is
   another.  This becomes very important if an AC verification
   application trusts more than one AC issuer.

4.  IANA Considerations

   The AC policies extension is identified by an object identifier
   (OID).  The OID for the AC policies extension defined in this
   document was assigned from an arc delegated by the IANA to the PKIX
   Working Group.

   No further action by the IANA is necessary for this document.

5.  References

5.1.  Normative References

   [X.660|ISO9834-1] ITU-T Recommendation X.660 (1992) | ISO/IEC 9834-1:
                     1993, Information technology - Open Systems
                     Interconnection Procedures for the operation of OSI
                     Registration Authorities: General procedures.

   [RFC2119]         Bradner, S., "Key words for use in RFCs to Indicate
                     Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3280]         Housley, R., Polk, W., Ford, W., and D. Solo,
                     "Internet X.509 Public Key Infrastructure
                     Certificate and Certificate Revocation List (CRL)
                     Profile", RFC 3280, April 2002.

   [RFC3281]         Farrell, S. and R. Housley, "An Internet Attribute
                     Certificate Profile for Authorization", RFC 3281,
                     April 2002.

   [ASN1]            X.680 - X.693 | ISO/IEC 8824: 1-4 Abstract Syntax
                     Notation One (ASN.1).

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5.2.  Informative Reference

   [X.509]           ITU-T Recommendation X.509 (2000): Information
                     Technology Open Systems Interconnections - The
                     Directory:  Public-key and Attribute Frameworks,
                     March 2000.

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Appendix A.  ASN.1 Definitions

   This appendix is normative.

ASN.1 Module

AcPolicies { iso(1) identified-organization(3) dod(6)
     internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
     id-mod-ac-policies(26) }





-- Imports from RFC 3280 [RFC3280], Appendix A

          FROM PKIX1Implicit88 { iso(1) identified-organization(3)
          dod(6) internet(1) security(5) mechanisms(5) pkix(7)
          id-mod(0) 19 }

       id-pkix, id-pe
          FROM PKIX1Explicit88 { iso(1) identified-organization(3)
          dod(6) internet(1) security(5) mechanisms(5) pkix(7)
          id-mod(0) 18 };

-- Locally defined OIDs

    -- policyQualifierIds for Internet policy qualifiers

   id-qt                    OBJECT IDENTIFIER ::=  { id-pkix 2 }
   id-qt-acps               OBJECT IDENTIFIER ::=  { id-qt 4 }
   id-qt-acunotice          OBJECT IDENTIFIER ::=  { id-qt 5 }

-- Attributes

   id-pe-acPolicies         OBJECT IDENTIFIER ::= { id-pe 15 }

   AcPoliciesSyntax ::=     SEQUENCE SIZE (1..MAX) OF PolicyInformation

   PolicyInformation ::=    SEQUENCE {
      policyIdentifier         AcPolicyId,
      policyQualifiers         SEQUENCE SIZE (1..MAX) OF
                               PolicyQualifierInfo OPTIONAL }

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   AcPolicyId ::=           OBJECT IDENTIFIER

   PolicyQualifierInfo ::=  SEQUENCE {
      policyQualifierId        PolicyQualifierId,
      qualifier                ANY DEFINED BY policyQualifierId }

   PolicyQualifierId ::=
      OBJECT IDENTIFIER               ( id-qt-acps | id-qt-acunotice )
   -- ACPS pointer qualifier

   ACPSuri ::=         IA5String
   -- ACP statement user notice qualifier

   ACUserNotice ::=    UserNotice
   -- UserNotice is defined in [RFC3280]


Authors' Addresses

   Christopher S. Francis
   1501 72nd Street North, MS 25
   St. Petersburg, Florida  33764

   EMail: Chris_S_Francis@Raytheon.com

   Denis Pinkas
   Rue Jean Jaures
   78340 Les Clayes-sous-Bois

   EMail: Denis.Pinkas@bull.net

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