





Internet Engineering Task Force (IETF)                        R. Housley
Request for Comments: 5914                           Vigil Security, LLC
Category: Standards Track                                     S. Ashmore
ISSN: 2070-1721                                 National Security Agency
                                                              C. Wallace
                                                      Cygnacom Solutions
                                                               June 2010


                          Trust Anchor Format

Abstract

   This document describes a structure for representing trust anchor
   information.  A trust anchor is an authoritative entity represented
   by a public key and associated data.  The public key is used to
   verify digital signatures, and the associated data is used to
   constrain the types of information or actions for which the trust
   anchor is authoritative.  The structures defined in this document are
   intended to satisfy the format-related requirements defined in Trust
   Anchor Management Requirements.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5914.
















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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Without obtaining an adequate license from the person(s) controlling
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   than English.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Trust Anchor Information Syntax  . . . . . . . . . . . . . . .  3
     2.1.  Version  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.2.  Public Key . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.3.  Key Identifier . . . . . . . . . . . . . . . . . . . . . .  4
     2.4.  Trust Anchor Title . . . . . . . . . . . . . . . . . . . .  4
     2.5.  Certification Path Controls  . . . . . . . . . . . . . . .  4
     2.6.  Extensions . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.  Trust Anchor List  . . . . . . . . . . . . . . . . . . . . . .  8
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     5.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Appendix A.  ASN.1 Modules . . . . . . . . . . . . . . . . . . . . 11
     A.1.  ASN.1 Module Using 2002 Syntax . . . . . . . . . . . . . . 11
     A.2.  ASN.1 Module Using 1988 Syntax . . . . . . . . . . . . . . 12
       A.2.1.  ASN.1 Module . . . . . . . . . . . . . . . . . . . . . 12




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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.  Though widely used, there is no
   standard format for representing trust anchor information.  This
   document describes the TrustAnchorInfo structure.  This structure is
   intended to satisfy the format-related requirements expressed in
   Trust Anchor Management Requirements [TA-MGMT-REQS] and is expressed
   using ASN.1 [X.680].  It can provide a more compact alternative to
   X.509 certificates for exchanging trust anchor information and
   provides a means of associating additional or alternative constraints
   with certificates without breaking the signature on the certificate.

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].

2.  Trust Anchor Information Syntax

   This section describes the TrustAnchorInfo structure.

   TrustAnchorInfo ::= SEQUENCE {
      version   TrustAnchorInfoVersion DEFAULT v1,
      pubKey    SubjectPublicKeyInfo,
      keyId     KeyIdentifier,
      taTitle   TrustAnchorTitle OPTIONAL,
      certPath  CertPathControls OPTIONAL,
      exts      [1] EXPLICIT Extensions   OPTIONAL,
      taTitleLangTag   [2] UTF8String OPTIONAL }

   TrustAnchorInfoVersion ::= INTEGER { v1(1) }

2.1.  Version

   version identifies the version of TrustAnchorInfo.  Future updates to
   this document may include changes to the TrustAnchorInfo structure,
   in which case the version number should be incremented.  However, the
   default value, v1, cannot be changed.

2.2.  Public Key

   pubKey identifies the public key and algorithm associated with the
   trust anchor using the SubjectPublicKeyInfo structure [RFC5280].  The
   SubjectPublicKeyInfo structure contains the algorithm identifier
   followed by the public key itself.  The algorithm field is an



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   AlgorithmIdentifier, which contains an object identifier and OPTIONAL
   parameters.  The object identifier names the public key algorithm and
   indicates the syntax of the parameters, if present, as well as the
   format of the public key.  The public key is encoded as a BIT STRING.

2.3.  Key Identifier

   keyId contains the public key identifier of the trust anchor public
   key.  See Section 4.2.1.2 of [RFC5280] for a description of common
   key identifier calculation methods.

2.4.  Trust Anchor Title

    TrustAnchorTitle ::= UTF8String (SIZE (1..64))

   taTitle is OPTIONAL.  When it is present, it provides a human-
   readable name for the trust anchor.  The text is encoded in UTF-8
   [RFC3629], which accommodates most of the world's writing systems.
   The taTitleLangTag field identifies the language used to express the
   taTitle.  When taTitleLangTag is absent, English ("en" language tag)
   is used.  The value of the taTitleLangTag should be a language tag as
   described in [RFC5646].

2.5.  Certification Path Controls

    CertPathControls ::= SEQUENCE {
      taName           Name,
      certificate      [0] Certificate OPTIONAL,
      policySet        [1] CertificatePolicies OPTIONAL,
      policyFlags      [2] CertPolicyFlags OPTIONAL,
      nameConstr       [3] NameConstraints OPTIONAL,
      pathLenConstraint[4] INTEGER (0..MAX) OPTIONAL}

   certPath is OPTIONAL.  When it is present, it provides the controls
   needed to initialize an X.509 certification path validation algorithm
   implementation (see Section 6 of [RFC5280]).  When absent, the trust
   anchor cannot be used to validate the signature on an X.509
   certificate.

   taName provides the X.500 distinguished name associated with the
   trust anchor, and this distinguished name is used to construct and
   validate an X.509 certification path.  The name MUST NOT be an empty
   sequence.

   certificate provides an OPTIONAL X.509 certificate, which can be used
   in some environments to represent the trust anchor in certification
   path development and validation.  If the certificate is present, the
   subject name in the certificate MUST exactly match the X.500



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   distinguished name provided in the taName field, the public key MUST
   exactly match the public key in the pubKey field, and the
   subjectKeyIdentifier extension, if present, MUST exactly match the
   key identifier in the keyId field.  The complete description of the
   syntax and semantics of the Certificate are provided in [RFC5280].
   Constraints defined in the policySet, policyFlags, nameConstr,
   pathLenConstraint, and exts fields within TrustAnchorInfo replace
   values contained in a certificate or provide values for extensions
   not present in the certificate.  Values defined in these
   TrustAnchorInfo fields are always enforced.  Extensions included in a
   certificate are enforced only if there is no corresponding value in
   the TrustAnchorInfo.  Correspondence between extensions within
   certificate and TrustAnchorInfo fields is defined as follows:

   o  an id-ce-certificatePolicies certificate extension corresponds to
      the CertPathControls.policySet field.

   o  an id-ce-policyConstraints certificate extension corresponds to
      the CertPolicyFlags.inhibitPolicyMapping and
      CertPolicyFlags.requireExplicitPolicy fields.

   o  an id-ce-inhibitAnyPolicy certificate extension corresponds to the
      CertPolicyFlags.inhibitAnyPolicy field.

   o  an id-ce-nameConstraints certificate extension corresponds to the
      CertPathControls.nameConstr field.

   o  the pathLenConstraint field of an id-ce-basicConstraints
      certificate extension corresponds to the
      CertPathControls.pathLenConstraint field (the presence of a
      CertPathControls structure corresponds to a TRUE value in the cA
      field of a BasicConstraints extension).

   o  any other certificate extension corresponds to the same type of
      extension in the TrustAnchorInfo.exts field.

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

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

    CertPolicyId ::= OBJECT IDENTIFIER

   policySet is OPTIONAL.  When present, it contains a sequence of
   certificate policy identifiers to be provided as inputs to the
   certification path validation algorithm.  When absent, the special



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   value any-policy is provided as the input to the certification path
   validation algorithm.  The complete description of the syntax and
   semantics of the CertificatePolicies are provided in [RFC5280],
   including the syntax for PolicyInformation.  In this context, the
   OPTIONAL policyQualifiers structure MUST NOT be included.

    CertPolicyFlags ::= BIT STRING {
      inhibitPolicyMapping   (0),
      requireExplicitPolicy  (1),
      inhibitAnyPolicy       (2) }

   policyFlags is OPTIONAL.  When present, three Boolean values for
   input to the certification path validation algorithm are provided in
   a BIT STRING.  When absent, the input to the certification path
   validation algorithm is { FALSE, FALSE, FALSE }, which represents the
   most liberal setting for these flags.  The three bits are used as
   follows:

      inhibitPolicyMapping indicates if policy mapping is allowed in the
      certification path.  When set to TRUE, policy mapping is not
      permitted.  This value represents the initial-policy-mapping-
      inhibit input value to the certification path validation algorithm
      described in Section 6.1.1 of [RFC5280].

      requireExplicitPolicy indicates if the certification path MUST be
      valid for at least one of the certificate policies in the
      policySet.  When set to TRUE, all certificates in the
      certification path MUST contain an acceptable policy identifier in
      the certificate policies extension.  This value represents the
      initial-explicit-policy input value to the certification path
      validation algorithm described in Section 6.1.1 of [RFC5280].  An
      acceptable policy identifier is a member of the policySet or the
      identifier of a policy that is declared to be equivalent through
      policy mapping.  This bit MUST be set to FALSE if policySet is
      absent.

      inhibitAnyPolicy indicates whether the special anyPolicy policy
      identifier, with the value { 2 5 29 32 0 }, is considered an
      explicit match for other certificate policies.  This value
      represents the initial-any-policy-inhibit input value to the
      certification path validation algorithm described in Section 6.1.1
      of [RFC5280].

    NameConstraints ::= SEQUENCE {
      permittedSubtrees  [0] GeneralSubtrees OPTIONAL,
      excludedSubtrees   [1] GeneralSubtrees OPTIONAL }

    GeneralSubtrees ::= SEQUENCE SIZE (1..MAX) OF GeneralSubtree



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    GeneralSubtree ::= SEQUENCE {
      base     GeneralName,
      minimum  [0] BaseDistance DEFAULT 0,
      maximum  [1] BaseDistance OPTIONAL }

    BaseDistance ::= INTEGER (0..MAX)

   nameConstr is OPTIONAL.  It has the same syntax and semantics as the
   Name Constraints certificate extension [RFC5280], which includes a
   list of permitted names and a list of excluded names.  The definition
   of GeneralName can be found in [RFC5280].  When it is present,
   constraints are provided on names (including alternative names) that
   might appear in subsequent X.509 certificates in a certification
   path.  This field is used to set the initial-permitted-subtrees and
   initial-excluded-subtrees input values to the certification path
   validation algorithm described in Section 6.1.1 of [RFC5280].  When
   this field is absent, the initial-permitted-subtrees variable is
   unbounded and the initial-excluded-subtrees variable is empty.

   The pathLenConstraint field gives the maximum number of non-self-
   issued intermediate certificates that may follow this certificate in
   a valid certification path.  (Note: The last certificate in the
   certification path is not an intermediate certificate and is not
   included in this limit.  Usually, the last certificate is an end
   entity certificate, but it can be a CA certificate.)  A
   pathLenConstraint of zero indicates that no non-self-issued
   intermediate certification authority (CA) certificates may follow in
   a valid certification path.  Where it appears, the pathLenConstraint
   field MUST be greater than or equal to zero.  Where pathLenConstraint
   does not appear, no limit is imposed.

   When the trust anchor is used to validate a certification path,
   CertPathControls provides limitations on certification paths that
   will successfully validate.  An application that is validating a
   certification path SHOULD NOT ignore these limitations, but the
   application can impose additional limitations to ensure that the
   validated certification path is appropriate for the intended
   application context.  As input to the certification path validation
   algorithm, an application MAY:

   o  Provide a subset of the certification policies provided in the
      policySet;

   o  Provide a TRUE value, if appropriate, for any of the flags in the
      policyFlags;

   o  Provide a subset of the permitted names provided in the
      nameConstr;



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   o  Provide additional excluded names to the ones that are provided in
      the nameConstr;

   o  Provide a smaller value for pathLenConstraint.

2.6.  Extensions

   exts is OPTIONAL.  When it is present, it can be used to associate
   additional information with the trust anchor using the standard
   Extensions structure.  Extensions that are anticipated to be widely
   used have been included in the CertPathControls structure to avoid
   overhead associated with use of the Extensions structure.  To avoid
   duplication with the CertPathControls field, the following types of
   extensions MUST NOT appear in the exts field and are ignored if they
   do appear: id-ce-certificatePolicies, id-ce-policyConstraints, id-ce-
   inhibitAnyPolicy, or id-ce-nameConstraints.

3.  Trust Anchor List

   TrustAnchorInfo allows for the representation of a single trust
   anchor.  In many cases, it is convenient to represent a collection of
   trust anchors.  The TrustAnchorList structure is defined for this
   purpose.  TrustAnchorList is defined as a sequence of one or more
   TrustAnchorChoice objects.  TrustAnchorChoice provides three options
   for representing a trust anchor.  The certificate option allows for
   the use of a certificate with no additional associated constraints.
   The tbsCert option allows for associating constraints by removing a
   signature on a certificate and changing the extensions field.  The
   taInfo option allows for use of the TrustAnchorInfo structure defined
   in this document.

   TrustAnchorList ::= SEQUENCE SIZE (1..MAX) OF TrustAnchorChoice

   TrustAnchorChoice ::= CHOICE {
      certificate  Certificate,
      tbsCert      [1] EXPLICIT TBSCertificate,
      taInfo       [2] EXPLICIT TrustAnchorInfo }

    trust-anchor-list PKCS7-CONTENT-TYPE ::=
       { TrustAnchorList IDENTIFIED BY id-ct-trustAnchorList }

   The TrustAnchorList structure can be protected using the SignedData
   structure defined in the Cryptographic Message Syntax (CMS)
   [RFC5652].  The id-ct-trustAnchorList object identifier has been
   defined to represent TrustAnchorList payloads with CMS structures.






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4.  Security Considerations

   Compromise of a trust anchor private key permits unauthorized parties
   to masquerade as the trust anchor, with potentially severe
   consequences.  Where TA-based constraints are enforced, the
   unauthorized holder of the trust anchor private key will be limited
   by the certification path controls associated with the trust anchor,
   as expressed in the certPath and exts fields.  For example, name
   constraints in the trust anchor will determine the name space that
   will be accepted in certificates that are validated using the
   compromised trust anchor.  Reliance on an inappropriate or incorrect
   trust anchor public key has similar potentially severe consequences.

   The compromise of a CA's private key leads to the same type of
   problems as the compromise of a trust anchor private key.  The
   unauthorized holder of the CA private key will be limited by the
   certification path controls associated with the trust anchor, as
   expressed in the certPath field or as an extension.

   Usage of a certificate independent of the TrustAnchorInfo structure
   that envelopes it must be carefully managed to avoid violating
   constraints expressed in the TrustAnchorInfo.  When enveloping a
   certificate in a TrustAnchorInfo structure, values included in the
   certificate should be evaluated to ensure there is no confusion or
   conflict with values in the TrustAnchorInfo structure.

5.  References

5.1.  Normative References

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

   [RFC3629]       Yergeau, F., "UTF-8, a transformation format of ISO
                   10646", STD 63, RFC 3629, November 2003.

   [RFC5652]       Housley, R., "Cryptographic Message Syntax (CMS)",
                   RFC 5652, September 2009.

   [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.

   [RFC5646]       Phillips, A. and M. Davis, "Tags for Identifying
                   Languages", BCP 47, RFC 5646, September 2009.





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   [RFC5912]       Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
                   Public Key Infrastructure Using X.509 (PKIX)",
                   RFC 5912, June 2010.

   [X.680]         "ITU-T Recommendation X.680: Information Technology -
                   Abstract Syntax Notation One", 2002.

5.2.  Informative References

   [TA-MGMT-REQS]  Reddy, R. and C. Wallace, "Trust Anchor Management
                   Requirements", Work in Progress, March 2010.

   [X.509]         "ITU-T Recommendation X.509 - The Directory -
                   Authentication Framework", 2000.





































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Appendix A.  ASN.1 Modules

A.1.  ASN.1 Module Using 2002 Syntax

   Appendix A.1 provides the normative ASN.1 definitions for the
   structures described in this specification using ASN.1 as defined in
   [X.680].  It includes definitions imported from [RFC5280] and
   [RFC5912].

   TrustAnchorInfoModule
   { joint-iso-ccitt(2) country(16) us(840) organization(1)
      gov(101) dod(2) infosec(1) modules(0) 33 }

   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN

   IMPORTS
   Certificate, Name, SubjectPublicKeyInfo, TBSCertificate
   FROM PKIX1Explicit-2009 -- from [RFC5912]
       {iso(1) identified-organization(3) dod(6) internet(1) security(5)
       mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-explicit-02(51)}
   CertificatePolicies, KeyIdentifier, NameConstraints
   FROM PKIX1Implicit-2009 -- from [RFC5912]
       {iso(1) identified-organization(3) dod(6) internet(1) security(5)
       mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-implicit-02(59)}
   Extensions{}
   FROM PKIX-CommonTypes-2009 -- from [RFC5912]
       { iso(1) identified-organization(3) dod(6) internet(1)
       security(5) mechanisms(5) pkix(7) id-mod(0)
       id-mod-pkixCommon-02(57) }         ;

   TrustAnchorInfo ::= SEQUENCE {
      version   TrustAnchorInfoVersion DEFAULT v1,
      pubKey    SubjectPublicKeyInfo,
      keyId     KeyIdentifier,
      taTitle   TrustAnchorTitle OPTIONAL,
      certPath  CertPathControls OPTIONAL,
      exts      [1] EXPLICIT Extensions {{...}}   OPTIONAL,
      taTitleLangTag   [2] UTF8String OPTIONAL }

   TrustAnchorInfoVersion ::= INTEGER { v1(1) }

   TrustAnchorTitle ::= UTF8String (SIZE (1..64))

   CertPathControls ::= SEQUENCE {
     taName           Name,
     certificate      [0] Certificate OPTIONAL,
     policySet        [1] CertificatePolicies OPTIONAL,



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     policyFlags      [2] CertPolicyFlags OPTIONAL,
     nameConstr       [3] NameConstraints OPTIONAL,
     pathLenConstraint[4] INTEGER (0..MAX) OPTIONAL}

   CertPolicyFlags ::= BIT STRING {
     inhibitPolicyMapping    (0),
     requireExplicitPolicy   (1),
     inhibitAnyPolicy        (2) }

   TrustAnchorList ::= SEQUENCE SIZE (1..MAX) OF TrustAnchorChoice

   TrustAnchorChoice ::= CHOICE {
     certificate  Certificate,
     tbsCert      [1] EXPLICIT TBSCertificate,
     taInfo       [2] EXPLICIT TrustAnchorInfo }

   id-ct-trustAnchorList      OBJECT IDENTIFIER ::= { iso(1)
       member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
       id-smime(16) id-ct(1) 34 }

   PKCS7-CONTENT-TYPE ::= TYPE-IDENTIFIER

   trust-anchor-list PKCS7-CONTENT-TYPE ::=
       { TrustAnchorList IDENTIFIED BY id-ct-trustAnchorList }

   END

A.2.  ASN.1 Module Using 1988 Syntax

   Appendix A.2 provides the normative ASN.1 definitions for the
   structures described in this specification using ASN.1 as defined in
   [X.680].

A.2.1.  ASN.1 Module

   TrustAnchorInfoModule-88
       { joint-iso-ccitt(2) country(16) us(840) organization(1)
         gov(101) dod(2) infosec(1) modules(0) 37 }

   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN

   IMPORTS
   Certificate, Name, Extensions,
   SubjectPublicKeyInfo, TBSCertificate
      FROM PKIX1Explicit88 -- from [RFC5280]
           { iso(1) identified-organization(3) dod(6) internet(1)
             security(5) mechanisms(5) pkix(7) id-mod(0)



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             id-pkix1-explicit(18) }
   CertificatePolicies, KeyIdentifier, NameConstraints
      FROM PKIX1Implicit88 -- [RFC5280]
           { iso(1) identified-organization(3) dod(6) internet(1)
             security(5) mechanisms(5) pkix(7) id-mod(0)
             id-pkix1-implicit(19) }
    ;

   TrustAnchorInfo ::= SEQUENCE {
      version   TrustAnchorInfoVersion DEFAULT v1,
      pubKey    SubjectPublicKeyInfo,
      keyId     KeyIdentifier,
      taTitle   TrustAnchorTitle OPTIONAL,
      certPath  CertPathControls OPTIONAL,
      exts      [1] EXPLICIT Extensions   OPTIONAL,
      taTitleLangTag   [2] UTF8String OPTIONAL }

   TrustAnchorInfoVersion ::= INTEGER { v1(1) }

   TrustAnchorTitle ::= UTF8String (SIZE (1..64))

   CertPathControls ::= SEQUENCE {
     taName           Name,
     certificate      [0] Certificate OPTIONAL,
     policySet        [1] CertificatePolicies OPTIONAL,
     policyFlags      [2] CertPolicyFlags OPTIONAL,
     nameConstr       [3] NameConstraints OPTIONAL,
     pathLenConstraint[4] INTEGER (0..MAX) OPTIONAL}

   CertPolicyFlags ::= BIT STRING {
     inhibitPolicyMapping    (0),
     requireExplicitPolicy   (1),
     inhibitAnyPolicy        (2) }

   TrustAnchorList ::= SEQUENCE SIZE (1..MAX) OF TrustAnchorChoice

   TrustAnchorChoice ::= CHOICE {
     certificate  Certificate,
     tbsCert      [1] EXPLICIT TBSCertificate,
     taInfo       [2] EXPLICIT TrustAnchorInfo }

   id-ct-trustAnchorList      OBJECT IDENTIFIER ::= { iso(1)
       member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
       id-smime(16) id-ct(1) 34 }

   END





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Authors' Addresses

   Russ Housley
   Vigil Security, LLC
   918 Spring Knoll Drive
   Herndon, VA  20170

   EMail: housley@vigilsec.com


   Sam Ashmore
   National Security Agency
   Suite 6751
   9800 Savage Road
   Fort Meade, MD  20755

   EMail: srashmo@radium.ncsc.mil


   Carl Wallace
   Cygnacom Solutions
   Suite 5400
   7925 Jones Branch Drive
   McLean, VA  22102

   EMail: cwallace@cygnacom.com

























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