Network Working Group C. Francis
Request for Comments: 4476 Raytheon
Category: Standards Track D. Pinkas
Bull
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).
Abstract
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
ACPs.
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RFC 4476 AC Policies Extension May 2006
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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
flag.
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}
AcPolicyId ::= OBJECT IDENTIFIER
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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
incident.
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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RFC 4476 AC Policies Extension May 2006
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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RFC 4476 AC Policies Extension May 2006
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) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
-- EXPORTS ALL --
IMPORTS
-- Imports from RFC 3280 [RFC3280], Appendix A
UserNotice
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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RFC 4476 AC Policies Extension May 2006
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]
END
Authors' Addresses
Christopher S. Francis
Raytheon
1501 72nd Street North, MS 25
St. Petersburg, Florida 33764
EMail: Chris_S_Francis@Raytheon.com
Denis Pinkas
Bull
Rue Jean Jaures
78340 Les Clayes-sous-Bois
FRANCE
EMail: Denis.Pinkas@bull.net
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RFC 4476 AC Policies Extension May 2006
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Francis & Pinkas Standards Track [Page 11]
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