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



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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







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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.





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  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
    }





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    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:

    - policyId identifies the security policy to which the clearance
      relates.  The policyId indicates the semantics of the classList
      and securityCategories fields.

    - 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.

    - 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





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  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:






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     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.



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  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.







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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).




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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.



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  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.





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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.




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  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:

    - 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.

    - If the policyId is present in temp-clearances:

      -- 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).

      -- 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.

      -- 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.





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  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.



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       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



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  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.
















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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

  -- EXPORTS ALL --

  IMPORTS

  -- 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)
     }

  -- 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)
     }
  ;

  -- Clearance attribute OID and syntax

  -- The following is a 2002 ASN.1 version for clearance.
  -- It is included for convenience.

  -- id-at-clearance OBJECT IDENTIFIER ::=
  --  { joint-iso-ccitt(2) ds(5) attributeTypes(4) clearance (55) }

  -- Clearance  ::=  SEQUENCE {
  --   policyId            OBJECT IDENTIFIER,
  --   classList           ClassList DEFAULT {unclassified},
  --   securityCategories  SET OF SecurityCategory



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  --                         {{SupportSecurityCategories }} OPTIONAL
  -- }

  -- ClassList  ::=  BIT STRING {
  --   unmarked      (0),
  --   unclassified  (1),
  --   restricted    (2),
  --   confidential  (3),
  --   secret        (4),
  --   topSecret     (5)
  -- }

  -- SECURITY-CATEGORY ::= TYPE-IDENTIFIER

  -- NOTE that the module SecurityCategory is taken from a module
  -- that uses EXPLICIT tags [RFC5912].  If Clearance was not imported
  -- from [RFC5912] and the comments were removed from the ASN.1
  -- contained herein, then the IMPLICIT in type could also be removed
  -- with no impact on the encoding.

  -- SecurityCategory { SECURITY-CATEGORY:Supported } ::= SEQUENCE {
  --   type  [0] IMPLICIT SECURITY-CATEGORY.&id({Supported}),
  --   value [1] EXPLICIT SECURITY-CATEGORY.&Type
  --                                    ({Supported}{@type})
  -- }

  -- Authority Clearance Constraints certificate extension OID
  -- 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










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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: [email protected]


  Santosh Chokhani
  CygnaCom Solutions, Inc.

  EMail: [email protected]



























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