Network Working Group S. Ashmore
Internet-Draft National Security Agency
Intended status: Informational C. Wallace
Expires: September 4, 2010 Cygnacom Solutions
March 3, 2010
Using Trust Anchor Constraints During Certification Path Processing
draft-wallace-using-ta-constraints-02
Abstract
This document describes how to use information associated with a
trust anchor public key when validating certification paths. This
information can be used to constrain the usage of a trust anchor.
Typically, constraints are used to limit the certificate policies and
names that can appear in certification paths validated using a trust
anchor.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Identifying trust anchor constraints . . . . . . . . . . . . . 5
3. Using trust anchor constraints during certification path
processing . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Inputs . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Initialization . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Basic Certificate Processing . . . . . . . . . . . . . . . 7
3.4. Preparation for Certificate i+1 . . . . . . . . . . . . . 7
3.5. Wrap-up procedure . . . . . . . . . . . . . . . . . . . . 8
4. Relationship to RFC 5280 . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
Trust anchors are widely used to verify digital signatures and
validate certification paths [RFC5280][X.509]. They are required
when validating certification paths. The Trust Anchor Format (TAF)
specification [I-D.draft-ietf-pkix-ta-format] defines means for
limiting the scope in which a trust anchor may be used. [RFC5280]
describes how to validate a certification path, including the usage
of a trust anchor name and public key. This document describes how
to use other trust anchor information during certification path
processing.
1.1. Terminology
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 RFC 2119 [RFC2119].
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2. Identifying trust anchor constraints
TAF supports three formats for representating trust anchor
information: TrustAnchorInfo, Certificate and TBSCertificate. In all
three cases, trust anchor constraints may be represented as
extensions. In the TrustAnchorInfo structure, certificate policies,
policy constraints, name constraints, inhibit any policy and basic
constraints do not appear as extensions and instead appear as part of
the CertPathControls field.
Extensions may be marked critical or not critical. When trust anchor
constraints are enforced, clients MUST reject certification paths
containing a trust anchor with unrecognized critical extensions.
When trust anchor constraints are not enforced, clients MAY accept
certification paths containing a trust anchor with unrecognized
critical extensions. Information appearing in the CertPathControls
field of a TrustAnchorInfo object MUST be processed, ensuring
enforcement of the constraints indicated by this field in all cases.
For some types of trust anchor constraints there is a type mismatch
between the input parameters for the certification path validation
algorithm and the extension that contains the constraint. The
certification path validation algorithm essentially defines the
initial-any-policy-inhibit, initial-policy-mapping-inhibit and
initial-explicit-policy as boolean values. The inhibitAnyPolicy and
policyConstraints extensions that correspond to these inputs are
expressed as integer values. In the steps described below, presence
of an inhibitAnyPolicy extension results in the initial-any-policy-
inhibit value being set to true. If a policyConstraints extension is
present and contains a requireExplicitPolicy field, the initial-
explicit-policy value is set to true. If a policyConstraints
extension is present and contains a inhibitPolicyMapping field, the
initial-policy-mapping-inhibit value is set to true.
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3. Using trust anchor constraints during certification path processing
3.1. Inputs
This algorithm assumes the nine inputs defined in RFC 5280 are
provided to the path processing logic plus one additional variable:
o enforceTrustAnchorConstraints: indicates if trust anchor
constraints should be enforced
Conforming implementations are not required to support the setting of
the enforceTrustAnchorConstraints input. If a conforming
implementation does not support the setting of this flag, it MUST
validate all certification paths using a value of TRUE for
enforceTrustAnchorConstraints.
3.2. Initialization
If enforceTrustAnchorConstraints is false, no additional
initialization steps are required.
If enforceTrustAnchorConstraints is true, perform the following
intialization steps described below. These steps (or equivalent)
MUST be performed prior to initialization steps described in RFC
5280.
o If no subject distinguished name is associated with the trust
anchor, path validation fails. The name may appear in the subject
field of a Certificate or TBSCertificate structure or in the
taName field of CertPathControls in a TrustAnchorInfo structure.
o If name constraints are associated with the trust anchor, set the
initial-permitted-subtrees variable equal to the intersection of
the permitted subtrees from the trust anchor and the user provided
initial-permitted-subtrees. If one of these two inputs is not
provided, the initial-permitted-subtrees variable is set to the
value that is available. If neither is provided, the initial-
permitted-subtrees variable is set to an infinite set.
o If name constraints are associated with the trust anchor, set the
initial-excluded-subtrees variable equal to the union of the
excluded subtrees from the trust anchor and the user provided
initial-excluded-subtrees. If one of these two inputs is not
provided, the initial-excluded-subtrees variable is set to the
value that is available. If neither is provided, the initial-
excluded-subtrees variable is set to an empty set.
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o If certificate policies are associated with the trust anchor, set
the user-initial-policy-set variable equal to the intersection of
the certificate policies associated with the trust anchor and the
user provided user-initial-policy-set. If one of these two inputs
is not provided, the user-initial-policy-set variable is set to
the value that is available. If neither is provided, the user-
initial-policy-set variable is set to any-policy.
o If an inhibit any policy value of true is associated with the
trust anchor (either in a CertPathControls or in an
InhibitAnyPolicy extension) and the initial-any-policy-inhibit
value is false, set the initial-any-policy-inhibit to true.
o If a require explicit policy value of true is associated with the
trust anchor (either in a CertPathControls or in a
PolicyConstraints extension) and the initial-explicit-policy value
is false, set the initial-explicit-policy to true.
o If an inhibit policy mapping value of true is associated with the
trust anchor (either in a CertPathControls or in a
PolicyConstraints extension) and the initial-policy-mapping-
inhibit value is false, set the initial-policy-mapping-inhibit to
true.
o If a basic constraints extension is associated with the trust
anchor and contains a pathLenConstraint value, set the
max_path_length state variable equal to the pathLenConstraint
value from the basic constraints extension.
3.3. Basic Certificate Processing
This document does not require any augmentation of the basic
certificate processing steps described in RFC 5280. However, some
types of trust anchor constraints may have defined additional steps,
for example, CMS Content Constraints or Authority Clearance
Constraints.
3.4. Preparation for Certificate i+1
This document does not require any augmentation of the basic
certificate processing steps described in RFC 5280. However, some
types of trust anchor constraints may have defined additional steps,
for example, CMS Content Constraints or Authority Clearance
Constraints.
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3.5. Wrap-up procedure
This document does not require any augmentation of the basic
certificate processing steps described in RFC 5280. However, some
types of trust anchor constraints may have defined additional steps,
for example, CMS Content Constraints or Authority Clearance
Constraints.
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4. Relationship to RFC 5280
The processing described above can be incorporated into an RFC 5280
implementation or be implemented as pre-processing of RFC 5280 inputs
and post-processing of RFC 5280 outputs, i.e., as a wrapper around an
RFC 5280 compliant implementation.
For name constraints and policy-related constraints, pre-processing
can be used, provided the RFC 5280 implementation allows
configuration of the user-initial-policy-set, initial-policy-mapping-
inhibit, initial-explicit-policy, initial-any-policy-inhibit,
initial-permitted-subtrees and initial-excluded-subtrees input
values. RFC 5280 does not define an input for path length
constraints, so basic constraints can not be implemented using pre-
preprocessing. It can be implemented as post-processing, provided
the RFC 5280 implementation returns the certification path to enable
the post-processor to perform the length check.
Some types of trust anchor constraints may impose additional
requirements on an RFC 5280 implementation to support pre-
preprocessing or post-processing to enforce trust anchor constraints.
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5. Security Considerations
Implementations that enforce trust anchor constraints may reject some
certification paths accepted by implementations that do not enforce
trust anchor constraints.
Trust anchor information must be securely stored. Changes to trust
anchor information can cause acceptance of certificates that should
be rejected.
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6. IANA Considerations
There are no IANA considerations. Please delete this section prior
to RFC publication.
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7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
7.2. Informative References
[I-D.draft-ietf-pkix-ta-format]
Housley, R., Wallace, C., and S. Ashmore, "Trust Anchor
Format", draft-ietf-pkix-ta-format (work in progress).
[X.509] "ITU-T Recommendation X.509 - The Directory -
Authentication Framework", 2000.
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Authors' Addresses
Sam Ashmore
National Security Agency
Suite 6751
9800 Savage Road
Fort Meade, MD 20755
Email: srashmo@radium.ncsc.mil
Carl Wallace
Cygnacom Solutions
Suite 5200
7925 Jones Branch Drive
McLean, VA 22102
Email: cwallace@cygnacom.com
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