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rfc:rfc5913

Internet Engineering Task Force (IETF) S. Turner Request for Comments: 5913 IECA Category: Standards Track S. Chokhani ISSN: 2070-1721 Cygnacom Solutions

                                                             June 2010
      Clearance Attribute and Authority Clearance Constraints
                       Certificate Extension

Abstract

 This document defines the syntax and semantics for the Clearance
 attribute and the Authority Clearance Constraints extension in X.509
 certificates.  The Clearance attribute is used to indicate the
 clearance held by the subject.  The Clearance attribute may appear in
 the subject directory attributes extension of a public key
 certificate or in the attributes field of an attribute certificate.
 The Authority Clearance Constraints certificate extension values in a
 Trust Anchor (TA), in Certification Authority (CA) public key
 certificates, and in an Attribute Authority (AA) public key
 certificate in a certification path for a given subject constrain the
 effective Clearance of the subject.

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/rfc5913.

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
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect

Turner & Chokhani Standards Track [Page 1] RFC 5913 Clearance and Authority Clearance Constraints June 2010

 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1. Introduction ....................................................3
    1.1. Terminology ................................................4
    1.2. ASN.1 Syntax Notation ......................................4
 2. Clearance Attribute .............................................4
 3. Authority Clearance Constraints Certificate Extension ...........5
 4. Processing Clearance and Authority Clearance Constraints
    in a PKC ........................................................6
    4.1. Collecting Constraints .....................................7
         4.1.1. Certification Path Processing .......................7
                4.1.1.1. Inputs .....................................8
                4.1.1.2. Initialization .............................8
                4.1.1.3. Basic Certificate Processing ...............8
                4.1.1.4. Preparation for Certificate i+1 ............9
                4.1.1.5. Wrap-up Procedure ..........................9
                         4.1.1.5.1. Wrap Up Clearance ...............9
                4.1.1.6. Outputs ...................................10
 5. Clearance and Authority Clearance Constraints
    Processing in AC ...............................................10
    5.1. Collecting Constraints ....................................11
         5.1.1. Certification Path Processing ......................11
                5.1.1.1. Inputs ....................................11
                5.1.1.2. Initialization ............................11
                5.1.1.3. Basic PKC Processing ......................12
                5.1.1.4. Preparation for Certificate i+1 ...........12
                5.1.1.5. Wrap-up Procedure .........................12
                         5.1.1.5.1. Wrap Up Clearance ..............12
                5.1.1.6. Outputs ...................................12
 6. Computing the Intersection of permitted-clearances and
    Authority Clearance Constraints Extension ......................12
 7. Computing the Intersection of securityCategories ...............13
 8. Recommended securityCategories .................................15
 9. Security Considerations ........................................15
 10. References ....................................................16
    10.1. Normative References .....................................16
    10.2. Informative References ...................................16
 Appendix A. ASN.1 Module ..........................................17
 Acknowledgments ...................................................19

Turner & Chokhani Standards Track [Page 2] RFC 5913 Clearance and Authority Clearance Constraints June 2010

1. Introduction

 Organizations that have implemented a security policy can issue
 certificates that include an indication of the clearance values held
 by the subject.  The Clearance attribute indicates the security
 policy, the clearance levels held by the subject, and additional
 authorization information held by the subject.  This specification
 makes use of the ASN.1 syntax for clearance from [RFC5912].
 The Clearance attribute may be placed in the subject directory
 attributes extension of a Public Key Certificate (PKC) or may be
 placed in a separate attribute certificate (AC).
 The placement of the Clearance attribute in PKCs is suitable 1) when
 the clearance information is relatively static and can be verified as
 part of the PKC issuance process (e.g., using local databases) or 2)
 when the credentials such as PKCs need to be revoked when the
 clearance information changes.  The Clearance attribute may also be
 included to simplify the infrastructure, to reduce the infrastructure
 design cost, or to reduce the infrastructure operations cost.  An
 example of placement of the Clearance attribute in PKCs in
 operational Public Key Infrastructure (PKI) is the Defense Messaging
 Service.  An example of placement of attributes in PKCs is Qualified
 Certificates [RFC3739].
 The placement of Clearance attributes in ACs is desirable when the
 clearance information is relatively dynamic and changes in the
 clearance information do not require revocation of credentials such
 as PKCs, or the clearance information cannot be verified as part of
 the PKC issuance process.
 Since [RFC5755] does not permit a chain of ACs, the Authority
 Clearance Constraints extension may only appear in the PKCs of a
 Certification Authority (CA) or Attribute Authority (AA).  The
 Authority Clearance Constraints extension may also appear in a trust
 anchor (TA) or may be associated with a TA.
 Some organizations have multiple TAs, CAs, and/or AAs, and these
 organizations may wish to indicate to relying parties which clearance
 values from a particular TA, CA, or AA should be accepted.  For
 example, consider the security policies described in [RFC3114], where
 a security policy has been defined for Amoco with three security
 classification values (HIGHLY CONFIDENTIAL, CONFIDENTIAL, and
 GENERAL).  To constrain a CA for just one security classification,
 the Authority Clearance Constraints certificate extension would be
 included in the CA's PKC.

Turner & Chokhani Standards Track [Page 3] RFC 5913 Clearance and Authority Clearance Constraints June 2010

 Cross-certified domains can also make use of the Authority Clearance
 Constraints certificate extension to indicate which clearance values
 should be acceptable to relying parties.
 This document augments the certification path validation rules for
 PKCs (in [RFC5280]) and ACs (in [RFC5755]).

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 [RFC2119].

1.2. ASN.1 Syntax Notation

 All X.509 PKC [RFC5280] extensions are defined using ASN.1 [X.680].
 All X.509 AC [RFC5755] extensions are defined using ASN.1 [X.680].
 Note that [X.680] is the 2002 version of ASN.1, which is the most
 recent version with freeware compiler support.

2. Clearance Attribute

 The Clearance attribute in a certificate indicates the clearances
 held by the subject.  It uses the clearance attribute syntax, whose
 semantics are defined in [RFC5755], in the Attributes field.  A
 certificate MUST include either zero or one instance of the Clearance
 attribute.  If the Clearance attribute is present, it MUST contain a
 single value.
 The following object identifier identifies the Clearance attribute
 (either in the subject directory attributes extension of a PKC or in
 the Attributes field of an AC):
   id-at-clearance OBJECT IDENTIFIER ::= { joint-iso-ccitt(2)
     ds(5) attributeTypes(4) clearance(55) }
 The ASN.1 syntax for the Clearance attribute is defined in [RFC5912]
 and that RFC provides the normative definition.  The ASN.1 syntax for
 Clearance attribute is as follows:
   Clearance  ::=  SEQUENCE {
     policyId            OBJECT IDENTIFIER,
     classList           ClassList DEFAULT {unclassified},
     securityCategories  SET OF SecurityCategory
                           {{ SupportedSecurityCategories }} OPTIONAL
   }

Turner & Chokhani Standards Track [Page 4] RFC 5913 Clearance and Authority Clearance Constraints June 2010

   ClassList  ::=  BIT STRING {
     unmarked       (0),
     unclassified   (1),
     restricted     (2),
     confidential   (3),
     secret         (4),
     topSecret      (5)
   }
   SECURITY-CATEGORY ::= TYPE-IDENTIFIER
   SecurityCategory { SECURITY-CATEGORY:Supported }::= SEQUENCE {
     type  [0] IMPLICIT SECURITY-CATEGORY.&id({Supported}),
     value [1] EXPLICIT SECURITY-CATEGORY.&Type
                                      ({Supported}{@type})
   }
 NOTE: SecurityCategory is shown exactly as it is in [RFC5912].  That
 module is an EXPLICIT tagged module, whereas the module contained in
 this document is an IMPLICIT tagged module.
 The Clearance attribute takes its meaning from Section 4.4.6 of
 [RFC5755], which is repeated here for convenience:
  1. policyId identifies the security policy to which the clearance

relates. The policyId indicates the semantics of the classList

     and securityCategories fields.
  1. classList identifies the security classifications. Six basic

values are defined in bit positions 0 through 5, and more may be

     defined by an organizational security policy.
  1. securityCategories provides additional authorization information.
 If a trust anchor's public key is used directly, then the Clearance
 associated with the trust anchor, if any, should be used as the
 effective clearance (also defined as effective-clearance for a
 certification path).

3. Authority Clearance Constraints Certificate Extension

 The Authority Clearance Constraints certificate extension indicates
 to the relying party what clearances should be acceptable for the
 subject of the AC or the subject of the last certificate in a PKC
 certification path.  It is only meaningful in a trust anchor, a CA
 PKC, or an AA PKC.  A trust anchor, CA PKC, or AA PKC MUST include

Turner & Chokhani Standards Track [Page 5] RFC 5913 Clearance and Authority Clearance Constraints June 2010

 either zero or one instance of the Authority Clearance Constraints
 certificate extension.  The Authority Clearance Constraints
 certificate extension MAY be critical or non-critical.
 Absence of this certificate extension in a TA, a CA PKC, or an AA PKC
 indicates that clearance of the subject of the AC or the subject of
 the last certificate in a PKC certification path containing the TA,
 the CA, or the AA is not constrained by the respective TA, CA, or AA.
 The following object identifier identifies the Authority Clearance
 Constraints certificate extension:
   id-pe-authorityClearanceConstraints OBJECT IDENTIFIER ::= {
     iso(1) identified-organization(3) dod(6) internet(1) security(5)
     mechanisms(5) pkix(7) pe(1) 21 }
 The ASN.1 syntax for the Authority Clearance Constraints certificate
 extension is as follows:
   AuthorityClearanceConstraints ::= SEQUENCE SIZE (1..MAX) OF
                                       Clearance
 The syntax for the Authority Clearance Constraints certificate
 extension contains Clearances that the CA or the AA asserts.  The
 sequence MUST NOT include more than one entry with the same policyId.
 This constraint is enforced during Clearance and Authority Clearance
 Constraints Processing as described below.  If more than one entry
 with the same policyId is present in the Authority Clearance
 Constraints certificate extension, the certification path is
 rejected.

4. Processing of Clearance and Authority Clearance Constraints in a PKC

  This section describes the certification path processing when
  Clearance is asserted in the PKC under consideration.
  User input, the Authority Clearance Constraints certificate
  extension, and Clearance attribute processing determines the
  effective clearance (henceforth called effective-clearance) for the
  end PKC.  User input and the Authority Clearance Constraints
  certificate extension in the TA and in each PKC (up to but not
  including the end PKC) in a PKC certification path impact the
  effective-clearance.  If there is more than one path to the end PKC,
  each path is processed independently.  The process involves two
  steps:

Turner & Chokhani Standards Track [Page 6] RFC 5913 Clearance and Authority Clearance Constraints June 2010

    1) collecting the Authority Clearance Constraints; and
    2) using the Authority Clearance Constraints in the certification
       path and the Clearance in the end PKC to determine the
       effective-clearance for the subject of the end PKC.
 Assuming a certification path consisting of n PKCs, the effective-
 clearance for the subject of the end PKC is the intersection of 1)
 the Clearance attribute in the subject PKC, 2) the Authority
 Clearance Constraints, if present, in the trust anchor, 3) user
 input, and 4) all Authority Clearance Constraints present in n-1
 intermediate PKCs.  Any effective-clearance calculation algorithm
 that performs this calculation and provides the same outcome as the
 one from the algorithm described herein is considered compliant with
 the requirements of this RFC.
 When processing a certification path, Authority Clearance Constraints
 are maintained in one state variable: permitted-clearances.  When
 processing begins, permitted-clearances is initialized to the user
 input value or the special value all-clearances if Authority
 Clearance Constraints user input is not provided.  The permitted-
 clearances state variable is updated by first processing Authority
 Clearance Constraints associated with the trust anchor, and then each
 time an intermediate PKC that contains an Authority Clearance
 Constraints certificate extension in the path is processed.
 When processing the end PKC, the value in the Clearance attribute in
 the end PKC is intersected with the permitted-clearances state
 variable.
 The output of Clearance attribute and Authority Clearance Constraint
 certificate extension processing is the effective-clearance (which
 could also be an empty list), and a status indicator of either
 success or failure.  If the status indicator is failure, then the
 process also returns a reason code.

4.1. Collecting Constraints

 Authority Clearance Constraints are collected from the user input,
 the trust anchor, and the intermediate PKCs in a certification path.

4.1.1. Certification Path Processing

 When processing Authority Clearance Constraints certificate
 extensions for the purposes of validating a Clearance attribute in
 the end PKC, the processing described in this section or an
 equivalent algorithm MUST be performed in addition to the
 certification path validation.

Turner & Chokhani Standards Track [Page 7] RFC 5913 Clearance and Authority Clearance Constraints June 2010

 The processing is presented as an addition to the certification path
 validation algorithm described in Section 6 of [RFC5280].  Note that
 this RFC is fully consistent with [RFC5280]; however, it augments
 [RFC5280] with the following steps:
    o  Ability to provide and process Authority Clearance Constraints
       as an additional input to the certification path processing
       engine with Trust anchor information.
    o  Requirement to process Authority Clearance Constraints present
       with trust anchor information.

4.1.1.1. Inputs

 User input may include an Authority Clearance Constraints structure
 or omit it.
 Trust anchor information may include the Authority Clearance
 Constraints structure to specify Authority Clearance Constraints for
 the trust anchor.  In other words, the trust anchor may be
 constrained or unconstrained.

4.1.1.2. Initialization

 If the user input includes Authority Clearance Constraints, set
 permitted-clearances to the input value; otherwise, set permitted-
 clearances to the special value all-clearances.
 Examine the permitted-clearances for the same Policy ID appearing
 more then once.  If a policyId appears more than once in the
 permitted-clearances state variable, set effective-clearance to an
 empty list, set error code to "multiple instances of same clearance",
 and exit with failure.
 If the trust anchor does not contain an Authority Clearance
 Constraints extension, continue at Section 4.1.1.3.  Otherwise,
 execute the procedure described in Section 6 as an in-line macro by
 treating the trust anchor as a PKC.

4.1.1.3. Basic Certificate Processing

 If the PKC is the last PKC (i.e., certificate n), skip the steps
 listed in this section.  Otherwise, execute the procedure described
 in Section 6 as an in-line macro.

Turner & Chokhani Standards Track [Page 8] RFC 5913 Clearance and Authority Clearance Constraints June 2010

4.1.1.4. Preparation for Certificate i+1

 No additional action associated with the Clearance attribute or the
 Authority Clearance Constraints certificate extensions is taken
 during this phase of certification path validation as described in
 Section 6 of [RFC5280].

4.1.1.5. Wrap-up Procedure

 To complete the processing, perform the following steps for the last
 PKC (i.e., certificate n).
 Examine the PKC and verify that it does not contain more than one
 instance of the Clearance attribute.  If the PKC contains more than
 one instance of the Clearance attribute, set effective-clearance to
 an empty list, set the error code to "multiple instances of an
 attribute", and exit with failure.
 If the Clearance attribute is not present in the end PKC, set
 effective-clearance to an empty list and exit with success.
 Set effective-clearance to the Clearance attribute in the end PKC.

4.1.1.5.1. Wrap Up Clearance

 Examine effective-clearance and verify that it does not contain more
 than one value.  If effective-clearance contains more than one value,
 set effective-clearance to an empty list, set error code to "multiple
 values", and exit with failure.
 If permitted-clearances is an empty list, set effective-clearance to
 an empty list and exit with success.
 If permitted-clearances has the special value all-clearances, exit
 with success.
 Let us say policyId in effective-clearance is X.
 If the policyId X in effective-clearance is absent from the
 permitted-clearances, set effective-clearance to an empty list and
 exit with success.
 Assign those classList bits in effective-clearance a value of one (1)
 that have a value of one (1) both in effective-clearance and in the
 clearance structure in permitted-clearances associated with policyId
 X.  Assign all other classList bits in effective-clearance a value of
 zero (0).

Turner & Chokhani Standards Track [Page 9] RFC 5913 Clearance and Authority Clearance Constraints June 2010

 If none of the classList bits have a value of one (1) in effective-
 clearance, set effective-clearance to an empty list and exit with
 success.
 Set the securityCategories in effective-clearance to the intersection
 of securityCategories in effective-clearance and securityCategories
 for policyId X in permitted-clearances using the algorithm described
 in Section 7.  Note that an empty SET is represented by simply
 omitting the SET.
 Exit with success.

4.1.1.6. Outputs

 If certification path validation processing succeeds, effective-
 clearance contains the subject's effective clearance for this
 certification path.  Processing also returns success or failure
 indication and reason for failure, if applicable.

5. Clearance and Authority Clearance Constraints Processing in AC

 This section describes the certification path processing when
 Clearance is asserted in an AC.  Relevant to processing are: one TA;
 0 or more CA PKCs; 0 or 1 AA PKC; and 1 AC.
 User input, Authority Clearance Constraints certificate extension,
 and Clearance attribute processing determine the effective clearance
 (henceforth called effective-clearance) for the subject of the AC.
 User input and the Authority Clearance Constraints certificate
 extensions in the TA and in each PKC (up to and including the AA PKC)
 in a certification path impact the effective-clearance.  If there is
 more than one path to the AA PKC, each path is processed
 independently.  The process involves two steps:
   1) collecting the Authority Clearance Constraints; and
   2) using the Authority Clearance Constraints in the PKC
     certification path and the Clearance in the AC to determine the
     effective-clearance for the subject of the AC.
 The effective-clearance for the subject of the AC is the intersection
 of 1) the Clearance attribute in the subject AC, 2) the Authority
 Clearance Constraints, if present, in trust anchor, 3) user input,
 and 4) all Authority Clearance Constraints present in the PKC
 certification path from the TA to the AA.  Any effective-clearance
 calculation algorithm that performs this calculation and provides the
 same outcome as the one from the algorithm described herein is
 considered compliant with the requirements of this RFC.

Turner & Chokhani Standards Track [Page 10] RFC 5913 Clearance and Authority Clearance Constraints June 2010

 The Authority Clearance Constraints are maintained in one state
 variable: permitted-clearances.  When processing begins, permitted-
 clearances is initialized to user input or the special value all-
 clearances if Authority Clearance Constraints user input is not
 provided.  The permitted-clearances state variable is updated by
 first processing the Authority Clearance Constraints associated with
 the trust anchor, and then each time a PKC (other than AC holder PKC)
 that contains an Authority Clearance Constraints certificate
 extension in the path is processed.
 When processing the AC, the value in the Clearance attribute in the
 AC is intersected with the permitted-clearances state variable.
 The output of Clearance attribute and Authority Clearance Constraint
 certificate extension processing is the effective-clearance, which
 could also be an empty list; and success or failure with a reason
 code for failure.

5.1. Collecting Constraints

 Authority Clearance Constraints are collected from the user input,
 the trust anchor, and all the PKCs in the AA PKC certification path.

5.1.1. Certification Path Processing

 When processing Authority Clearance Constraints certificate
 extensions for the purpose of validating a Clearance attribute in the
 AC, the processing described in this section or an equivalent
 algorithm MUST be performed in addition to the certification path
 validation.  The processing is presented as an addition to the PKC
 certification path validation algorithm described in Section 6 of
 [RFC5280] for the AA PKC certification path and the algorithm
 described in Section 5 of [RFC5755] for the AC validation.  Also see
 the note related to [RFC5280] augmentation in Section 4.1.1.

5.1.1.1. Inputs

 Same as Section 4.1.1.1.
 In addition, let us assume that the PKC certification path for the AA
 consists of n certificates.

5.1.1.2. Initialization

 Same as Section 4.1.1.2.

Turner & Chokhani Standards Track [Page 11] RFC 5913 Clearance and Authority Clearance Constraints June 2010

5.1.1.3. Basic PKC Processing

 Same as Section 4.1.1.3 except that the logic is applied to all n
 PKCs.

5.1.1.4. Preparation for Certificate i+1

 Same as Section 4.1.1.4.

5.1.1.5. Wrap-up Procedure

 To complete the processing, perform the following steps for the AC.
 Examine the AC and verify that it does not contain more than one
 instance of the Clearance attribute.  If the AC contains more than
 one instance of the Clearance attribute, set effective-clearance to
 an empty list, set the error code to "multiple instances of an
 attribute", and exit with failure.
 If the Clearance attribute is not present in the AC, set effective-
 clearance to an empty list and exit with success.
 Set effective-clearance to the Clearance attribute in the AC.

5.1.1.5.1. Wrap Up Clearance

 Same as Section 4.1.1.5.1.

5.1.1.6. Outputs

 Same as Section 4.1.1.6.
 In addition, apply AC processing rules described in Section 5 of
 [RFC5755].

6. Computing the Intersection of permitted-clearances and Authority

  Clearance Constraints Extension
 Examine the PKC and verify that it does not contain more than one
 instance of the Authority Clearance Constraints extension.  If the
 PKC contains more than one instance of Authority Clearance
 Constraints extension, set effective-clearance to an empty list, set
 error code to "multiple extension instances", and exit with failure.
 If the Authority Clearance Constraints certificate extension is not
 present in the PKC, no action is taken, and the permitted-clearances
 value is unchanged.

Turner & Chokhani Standards Track [Page 12] RFC 5913 Clearance and Authority Clearance Constraints June 2010

 If the Authority Clearance Constraints certificate extension is
 present in the PKC, set the variable temp-clearances to the value of
 the Authority Clearance Constraints certificate extension.  Examine
 the temp-clearances for the same Policy ID appearing more then once.
 If a policyId appears more than once in the temp-clearances state
 variable, set effective-clearance to an empty list, set error code to
 "multiple instances of same clearance", and exit with failure.
 If the Authority Clearance Constraints certificate extension is
 present in the PKC and permitted-clearances contains the all-
 clearances special value, then assign permitted-clearances the value
 of temp-clearances.
 If the Authority Clearance Constraints certificate extension is
 present in the PKC and permitted-clearances does not contain the all-
 clearances special value, take the intersection of temp-clearances
 and permitted-clearances by repeating the following steps for each
 clearance in the permitted-clearances state variable:
  1. If the policyId associated with the clearance is absent in the

temp-clearances, delete the clearance structure associated with

     the policyID from the permitted-clearances state variable.
  1. If the policyId is present in temp-clearances:
  1. - For every classList bit, assign the classList bit a value of

one (1) for the policyId in the permitted-clearances state

        variable if the bit is one (1) in both the permitted-
        clearances state variable and the temp-clearances for that
        policyId; otherwise, assign the bit a value of zero (0).
  1. - If no bits are one (1) for the classList, delete the clearance

structure associated with the policyId from the permitted-

        clearances state variable and skip the next step of processing
        securityCategories.
  1. - For the policyId in permitted-clearances, set the

securityCategories to the intersection of securityCategories

        for the policyId in permitted-clearances and in temp-
        clearances using the algorithm described in Section 7.  Note
        that an empty SET is represented by simply omitting the SET.

7. Computing the Intersection of securityCategories

 The algorithm described here has the idempotent, associative, and
 commutative properties.

Turner & Chokhani Standards Track [Page 13] RFC 5913 Clearance and Authority Clearance Constraints June 2010

 This section describes how to compute the intersection of
 securityCategories A and B.  It uses the state variable temp-set.  It
 also uses temporary variables X and Y.
 Set the SET temp-set to empty.
 Set X = A and Y = B.
 If SET X is empty (i.e., securityCategories is absent), return temp-
 set.
 If SET Y is empty (i.e., securityCategories is absent), return temp-
 set.
 For each type OID in X, if all the elements for the type OID in X and
 if and only if all the elements for that type OID in Y are identical,
 add those elements to temp-set and delete those elements from X and
 Y.  Note: identical means that if the element with the type OID and
 given value is present in X, it is also present in Y with the same
 type OID and given value and vice versa.  Delete the elements from X
 and from Y.
 If SET X is empty (i.e., securityCategories is absent), return temp-
 set.
 If SET Y is empty (i.e., securityCategories is absent), return temp-
 set.
 For every element (i.e., SecurityCategory) in the SET X, carry out
 the following steps:
   1. If there is no element in SET Y with the same type OID as the
      type OID in the element from SET X, go to step 5.
   2. If there is an element in SET Y with the same type OID and value
      as in the element in SET X, carry out the following steps:
      a) If the element is not present in the SET temp-set, add an
         element containing the type OID and the value to the SET
         temp-set.
   3. If the processing semantics of type OID in the element in SET X
      is not known, go to step 5.
   4. For each element in SET Y, do the following:
      a) If the type OID of the element in SET Y is not the same as
         the element in SET X being processed, go to step 4.d.

Turner & Chokhani Standards Track [Page 14] RFC 5913 Clearance and Authority Clearance Constraints June 2010

      b) Perform type-OID-specific intersection of the value in the
         element in SET X with the value in the element in SET Y.
      c) If the intersection is not empty, and the element
         representing the type OID and intersection value is not
         already present in temp-set, add the element containing the
         type OID and intersection value as an element to temp-set.
      d) Continue to the next element in SET Y.
   5. If more elements remain in SET X, process the next element
      starting with step 1.
 Return temp-set.

8. Recommended securityCategories

 This RFC also includes a recommended securityCategories object as
 follows:
 recommended-category SECURITY-CATEGORY ::=
   { BIT STRING IDENTIFIED BY OID }
 The above structure is provided as an example.  To use this
 structure, the object identifier (OID) needs to be registered and the
 semantics of the bits in the bit string need to be enumerated.
 Note that type-specific intersection of two values for this type will
 be simply setting the bits that are set in both values.  If the
 resulting intersection has none of the bits set, the intersection is
 considered empty.

9. Security Considerations

 Certificate issuers must recognize that absence of the Authority
 Clearance Constraints in a TA, in a CA certificate, or in an AA
 certificate means that in terms of the clearance, the subject
 Authority is not constrained.
 Absence of the Clearance attribute in a certificate means that the
 subject has not been assigned any clearance.
 If there is no Clearance associated with a TA, it means that the TA
 has not been assigned any clearance.
 If the local security policy considers the clearance held by a
 subject or those supported by a CA or AA to be sensitive, then the
 Clearance attribute or Authority Clearance Constraints should only be

Turner & Chokhani Standards Track [Page 15] RFC 5913 Clearance and Authority Clearance Constraints June 2010

 included if the subject's and Authority's certificates can be privacy
 protected.  Also in this case, distribution of trust anchors and
 associated Authority Clearance Constraints extension or Clearance
 must also be privacy protected.

10. References

10.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC5280]  Cooper, D. et. al., "Internet X.509 Public Key
            Infrastructure Certificate and Certification Revocation
            List (CRL) Profile", RFC 5280, May 2008.
 [RFC5755]  Farrell, S., Housley, R., and S. Turner, "An Internet
            Attribute Certificate Profile for Authorization", RFC
            5755, January 2010.
 [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 (2002) | ISO/IEC 8824-1:2002.
            Information Technology - Abstract Syntax Notation One.

10.2. Informative References

 [RFC3114]  Nicolls, W., "Implementing Company Classification Policy
            with the S/MIME Security Label", RFC 3114, May 2002.
 [RFC3739]  Santesson, S., Nystrom, M., and T. Polk, "Internet X.509
            Public Key Infrastructure: Qualified Certificates
            Profile", RFC 3739, March 2004.

Turner & Chokhani Standards Track [Page 16] RFC 5913 Clearance and Authority Clearance Constraints June 2010

Appendix A. ASN.1 Module

 This appendix provides the normative ASN.1 definitions for the
 structures described in this specification using ASN.1 as defined in
 X.680.
 ClearanceConstraints { iso(1) identified-organization(3) dod(6)
 internet(1) security(5) mechanisms(5) pkix(7) mod(0) 46 }
 DEFINITIONS IMPLICIT TAGS ::=
 BEGIN
  1. - EXPORTS ALL –
 IMPORTS
  1. - IMPORTS from [RFC5912]
 id-at-clearance, Clearance
    FROM PKIXAttributeCertificate-2009
    { iso(1) identified-organization(3) dod(6) internet(1)
      security(5) mechanisms(5) pkix(7) id-mod(0)
      id-mod-attribute-cert-02(47)
    }
  1. - IMPORTS from [RFC5912]
 EXTENSION, SECURITY-CATEGORY
   FROM PKIX-CommonTypes-2009
    { iso(1) identified-organization(3) dod(6) internet(1)
      security(5) mechanisms(5) pkix(7) id-mod(0)
      id-mod-pkixCommon-02(57)
    }
 ;
  1. - Clearance attribute OID and syntax
  1. - The following is a 2002 ASN.1 version for clearance.
  2. - It is included for convenience.
  1. - id-at-clearance OBJECT IDENTIFIER ::=
  2. - { joint-iso-ccitt(2) ds(5) attributeTypes(4) clearance (55) }
  1. - Clearance ::= SEQUENCE {
  2. - policyId OBJECT IDENTIFIER,
  3. - classList ClassList DEFAULT {unclassified},
  4. - securityCategories SET OF SecurityCategory

Turner & Chokhani Standards Track [Page 17] RFC 5913 Clearance and Authority Clearance Constraints June 2010

  1. - }
  1. - ClassList ::= BIT STRING {
  2. - unmarked (0),
  3. - unclassified (1),
  4. - restricted (2),
  5. - confidential (3),
  6. - secret (4),
  7. - topSecret (5)
  8. - }
  1. - SECURITY-CATEGORY ::= TYPE-IDENTIFIER
  1. - NOTE that the module SecurityCategory is taken from a module
  2. - that uses EXPLICIT tags [RFC5912]. If Clearance was not imported
  3. - from [RFC5912] and the comments were removed from the ASN.1
  4. - contained herein, then the IMPLICIT in type could also be removed
  5. - with no impact on the encoding.
  1. - SecurityCategory { SECURITY-CATEGORY:Supported } ::= SEQUENCE {
  2. - type [0] IMPLICIT SECURITY-CATEGORY.&id({Supported}),
  3. - value [1] EXPLICIT SECURITY-CATEGORY.&Type
  4. - ({Supported}{@type})
  5. - }
  1. - Authority Clearance Constraints certificate extension OID
  2. - and syntax
 id-pe-clearanceConstraints OBJECT IDENTIFIER ::=
   { iso(1) identified-organization(3) dod(6) internet(1) security(5)
     mechanisms(5) pkix(7) pe(1) 21 }
 authorityClearanceConstraints EXTENSION ::= {
   SYNTAX         AuthorityClearanceConstraints
   IDENTIFIED BY  id-pe-clearanceConstraints
 }
 AuthorityClearanceConstraints ::= SEQUENCE SIZE (1..MAX) OF Clearance
 END

Turner & Chokhani Standards Track [Page 18] RFC 5913 Clearance and Authority Clearance Constraints June 2010

Acknowledgments

 Many thanks go out to Mark Saaltink for his valuable contributions to
 this document.
 We would also like to thank Francis Dupont, Pasi Eronen, Adrian
 Farrel, Dan Romascanu, and Stefan Santesson for their reviews and
 comments.

Authors' Addresses

 Sean Turner
 IECA, Inc.
 3057 Nutley Street, Suite 106
 Fairfax, VA 22031
 USA
 EMail: turners@ieca.com
 Santosh Chokhani
 CygnaCom Solutions, Inc.
 EMail: SChokhani@cygnacom.com

Turner & Chokhani Standards Track [Page 19]

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