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.
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]).
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].
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.
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):
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
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.
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).
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:
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.
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.
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.
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.
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
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].
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.
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.
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.
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.
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.
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:
- 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.
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.
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.
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.
[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.
-- 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.