Internet Engineering Task Force (IETF)                        R. Housley
Request for Comments: 9118                                Vigil Security
Updates: 8226                                                August 2021
Category: Standards Track
ISSN: 2070-1721


  Enhanced JSON Web Token (JWT) Claim Constraints for Secure Telephone
                 Identity Revisited (STIR) Certificates

Abstract

   RFC 8226 specifies the use of certificates for Secure Telephone
   Identity Credentials; these certificates are often called "Secure
   Telephone Identity Revisited (STIR) Certificates".  RFC 8226 provides
   a certificate extension to constrain the JSON Web Token (JWT) claims
   that can be included in the Personal Assertion Token (PASSporT), as
   defined in RFC 8225.  If the PASSporT signer includes a JWT claim
   outside the constraint boundaries, then the PASSporT recipient will
   reject the entire PASSporT.  This document updates RFC 8226; it
   provides all of the capabilities available in the original
   certificate extension as well as an additional way to constrain the
   allowable JWT claims.  The enhanced extension can also provide a list
   of claims that are not allowed to be included in the PASSporT.

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 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9118.

Copyright Notice

   Copyright (c) 2021 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
   (https://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
   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
   2.  Terminology
   3.  Enhanced JWT Claim Constraints Syntax
   4.  Usage Examples
   5.  Certificate Extension Example
   6.  Guidance to Certification Authorities
   7.  IANA Considerations
   8.  Security Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Appendix A.  ASN.1 Module
   Acknowledgements
   Author's Address

1.  Introduction

   The use of certificates [RFC5280] in establishing authority over
   telephone numbers is described in [RFC8226].  These certificates are
   often called "STIR Certificates".  STIR certificates are an important
   element of the overall system that prevents the impersonation of
   telephone numbers on the Internet.

   Section 8 of [RFC8226] provides a certificate extension to constrain
   the JSON Web Token (JWT) claims that can be included in the Personal
   Assertion Token (PASSporT) [RFC8225].  If the PASSporT signer
   includes a JWT claim outside the constraint boundaries, then the
   PASSporT recipient will reject the entire PASSporT.

   This document defines an enhanced JWTClaimConstraints certificate
   extension, which provides all of the capabilities available in the
   original certificate extension as well as an additional way to
   constrain the allowable JWT claims.  That is, the enhanced extension
   can provide a list of claims that are not allowed to be included in
   the PASSporT.

   The Enhanced JWT Claim Constraints certificate extension is needed to
   limit the authority when a parent STIR certificate delegates to a
   subordinate STIR certificate.  For example, [RFC9060] describes the
   situation where service providers issue a STIR certificate to
   enterprises or other customers to sign PASSporTs, and the Enhanced
   JWT Claim Constraints certificate extension can be used to prevent
   specific claims from being included in PASSporTs and accepted as
   valid by the PASSporT recipient.

   The JWT Claim Constraints certificate extension defined in [RFC8226]
   provides a list of claims that must be included in a valid PASSporT
   as well as a list of permitted values for selected claims.  The
   Enhanced JWT Claim Constraints certificate extension defined in this
   document includes those capabilities and adds a list of claims that
   must not be included in a valid PASSporT.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Enhanced JWT Claim Constraints Syntax

   The Enhanced JWT Claim Constraints certificate extension is non-
   critical, applicable only to end-entity certificates, and defined
   with ASN.1 [X.680].  The syntax of the JWT claims in a PASSporT is
   specified in [RFC8225].

   The Enhanced JWT Claim Constraints certificate extension is optional,
   but, when present, it constrains the JWT claims that authentication
   services may include in the PASSporT objects they sign.  Constraints
   are applied by certificate issuers and enforced by recipients when
   validating PASSporT claims as follows:

   1.  mustInclude indicates JWT claims that MUST appear in the PASSporT
       in addition to the iat, orig, and dest claims.  The baseline
       PASSporT claims ("iat", "orig", and "dest") are considered to be
       required by [RFC8225], and these claims SHOULD NOT be part of the
       mustInclude list.  If mustInclude is absent, the iat, orig, and
       dest claims MUST appear in the PASSporT.

   2.  permittedValues indicates that, if the claim name is present, the
       claim MUST exactly match one of the listed values.

   3.  mustExclude indicates JWT claims that MUST NOT appear in the
       PASSporT.  The baseline PASSporT claims ("iat", "orig", and
       "dest") are always permitted, and these claims MUST NOT be part
       of the mustExclude list.  If one of these baseline PASSporT
       claims appears in the mustExclude list, then the certificate MUST
       be treated as if the extension was not present.

   Following the precedent in [RFC8226], JWT Claim Names MUST be ASCII
   strings, which are also known as strings using the International
   Alphabet No. 5 [ISO646].

   The Enhanced JWT Claim Constraints certificate extension is
   identified by the following object identifier (OID):

       id-pe-eJWTClaimConstraints OBJECT IDENTIFIER ::= { id-pe 33 }

   The Enhanced JWT Claim Constraints certificate extension has the
   following syntax:

       EnhancedJWTClaimConstraints ::= SEQUENCE {
         mustInclude [0] JWTClaimNames OPTIONAL,
           -- The listed claim names MUST appear in the PASSporT
           -- in addition to iat, orig, and dest.  If absent, iat, orig,
           -- and dest MUST appear in the PASSporT.
         permittedValues [1] JWTClaimValuesList OPTIONAL,
           -- If the claim name is present, the claim MUST contain one
           -- of the listed values.
         mustExclude [2] JWTClaimNames OPTIONAL }
           -- The listed claim names MUST NOT appear in the PASSporT.
       ( WITH COMPONENTS { ..., mustInclude PRESENT } |
         WITH COMPONENTS { ..., permittedValues PRESENT } |
         WITH COMPONENTS { ..., mustExclude PRESENT } )

       JWTClaimValuesList ::= SEQUENCE SIZE (1..MAX) OF JWTClaimValues

       JWTClaimValues ::= SEQUENCE {
         claim JWTClaimName,
         values SEQUENCE SIZE (1..MAX) OF UTF8String }

       JWTClaimNames ::= SEQUENCE SIZE (1..MAX) OF JWTClaimName

       JWTClaimName ::= IA5String

4.  Usage Examples

   Consider these usage examples with a PASSporT claim called
   "confidence" with values "low", "medium", and "high".  These examples
   illustrate the constraints that are imposed by mustInclude,
   permittedValues, and mustExclude:

   *  If a certification authority (CA) issues a certificate to an
      authentication service that includes an Enhanced JWT Claim
      Constraints certificate extension that contains the mustInclude
      JWTClaimName "confidence", then an authentication service is
      required to include the "confidence" claim in all PASSporTs it
      generates and signs.  A verification service will treat any
      PASSporT it receives without a "confidence" PASSporT claim as
      invalid.

   *  If a CA issues a certificate to an authentication service that
      includes an Enhanced JWT Claim Constraints certificate extension
      that contains the permittedValues JWTClaimName "confidence" and a
      permitted "high" value, then a verification service will treat any
      PASSporT it receives with a PASSporT "confidence" claim with a
      value other than "high" as invalid.  However, a verification
      service will not treat a PASSporT it receives without a PASSporT
      "confidence" claim at all as invalid, unless "confidence" also
      appears in mustInclude.

   *  If a CA issues a certificate to an authentication service that
      includes an Enhanced JWT Claim Constraints certificate extension
      that contains the mustExclude JWTClaimName "confidence", then a
      verification service will treat any PASSporT it receives with a
      PASSporT "confidence" claim as invalid regardless of the claim
      value.

5.  Certificate Extension Example

   A certificate containing an example of the
   EnhancedJWTClaimConstraints certificate extension is provided in
   Figure 1.  The certificate is provided in the format described in
   [RFC7468].  The example of the EnhancedJWTClaimConstraints extension
   from the certificate is shown in Figure 2.  The example imposes three
   constraints:

   1.  The "confidence" claim must be present in the PASSporT.

   2.  The "confidence" claim must have a value of "high" or "medium".

   3.  The "priority" claim must not be present in the PASSporT.

   -----BEGIN CERTIFICATE-----
   MIICpzCCAk2gAwIBAgIUH7Zd3rQ5AsvOlzLnzUHhrVhDSlswCgYIKoZIzj0EAwIw
   KTELMAkGA1UEBhMCVVMxGjAYBgNVBAMMEUJPR1VTIFNIQUtFTiBST09UMB4XDTIx
   MDcxNTIxNTIxNVoXDTIyMDcxNTIxNTIxNVowbDELMAkGA1UEBhMCVVMxCzAJBgNV
   BAgMAlZBMRAwDgYDVQQHDAdIZXJuZG9uMR4wHAYDVQQKDBVCb2d1cyBFeGFtcGxl
   IFRlbGVjb20xDTALBgNVBAsMBFZvSVAxDzANBgNVBAMMBlNIQUtFTjBZMBMGByqG
   SM49AgEGCCqGSM49AwEHA0IABNR6C6nBWRA/fXTglV03aXkXy8hx9oBttVLhsTZ1
   IYVRBao4OZhVf/Xv1a3xLsZ6KfdhuylSeAKuCoSbVGojYDGjggEOMIIBCjAMBgNV
   HRMBAf8EAjAAMA4GA1UdDwEB/wQEAwIHgDAdBgNVHQ4EFgQUDlG3dxHyzKL/FZfS
   PI7rpuueRbswHwYDVR0jBBgwFoAUlToKtrQeFrwwyXpMj1qu3TQEeoEwQgYJYIZI
   AYb4QgENBDUWM1RoaXMgY2VydGlmaWNhdGUgY2Fubm90IGJlIHRydXN0ZWQgZm9y
   IGFueSBwdXJwb3NlLjAWBggrBgEFBQcBGgQKMAigBhYEMTIzNDBOBggrBgEFBQcB
   IQRCMECgDjAMFgpjb25maWRlbmNloSAwHjAcFgpjb25maWRlbmNlMA4MBGhpZ2gM
   Bm1lZGl1baIMMAoWCHByaW9yaXR5MAoGCCqGSM49BAMCA0gAMEUCIQCbNR4QK1um
   +0vq2CE1B1/W3avYeREsPi/7RKHffL+5eQIgarHot+X9Rl7SOyNBq5X5JyEMx0SQ
   hRLkCY3Zoz2OCNQ=
   -----END CERTIFICATE-----

                       Figure 1: Example Certificate

     0  64: SEQUENCE {
     2  14:   [0] {
     4  12:     SEQUENCE {
     6  10:       IA5String 'confidence'
          :       }
          :     }
    18  32:   [1] {
    20  30:     SEQUENCE {
    22  28:       SEQUENCE {
    24  10:         IA5String 'confidence'
    36  14:         SEQUENCE {
    38   4:           UTF8String 'high'
    44   6:           UTF8String 'medium'
          :           }
          :         }
          :       }
          :     }
    52  12:   [2] {
    54  10:     SEQUENCE {
    56   8:       IA5String 'priority'
          :       }
          :     }
          :   }

          Figure 2: Example EnhancedJWTClaimConstraints Extension

6.  Guidance to Certification Authorities

   The EnhancedJWTClaimConstraints extension specified in this document
   and the JWTClaimConstraints extension specified in [RFC8226] MUST NOT
   both appear in the same certificate.

   If the situation calls for mustExclude constraints, then the
   EnhancedJWTClaimConstraints extension is the only extension that can
   express the constraints.

   On the other hand, if the situation does not call for mustExclude
   constraints, then either the EnhancedJWTClaimConstraints extension or
   the JWTClaimConstraints extension can express the constraints.  Until
   such time as support for the EnhancedJWTClaimConstraints extension
   becomes widely implemented, the use of the JWTClaimConstraints
   extension may be more likely to be supported.  This guess is based on
   the presumption that the first specified extension will be
   implemented more widely in the next few years.

7.  IANA Considerations

   This document makes use of object identifiers for the Enhanced JWT
   Claim Constraints certificate extension defined in Section 3 and the
   ASN.1 module identifier defined in Appendix A.  Therefore, IANA has
   made the following assignments within the "Structure of Management
   Information (SMI) Numbers (MIB Module Registrations)" registry.

   For the Enhanced JWT Claim Constraints certificate extension in the
   "SMI Security for PKIX Certificate Extension" (1.3.6.1.5.5.7.1)
   registry:

                 +=========+============================+
                 | Decimal | Description                |
                 +=========+============================+
                 | 33      | id-pe-eJWTClaimConstraints |
                 +---------+----------------------------+

                                 Table 1

   For the ASN.1 module identifier in the "SMI Security for PKIX Module
   Identifier" (1.3.6.1.5.5.7.0) registry:

              +=========+==================================+
              | Decimal | Description                      |
              +=========+==================================+
              | 101     | id-mod-eJWTClaimConstraints-2021 |
              +---------+----------------------------------+

                                 Table 2

8.  Security Considerations

   For further information on certificate security and practices, see
   [RFC5280], especially the Security Considerations section.

   Since non-critical certificate extensions are ignored by
   implementations that do not recognize the extension object identifier
   (OID), constraints on PASSporT validation will only be applied by
   relying parties that recognize the EnhancedJWTClaimConstraints
   extension.

   The Enhanced JWT Claim Constraints certificate extension can be used
   by certificate issuers to provide limits on the acceptable PASSporTs
   that can be accepted by verification services.  Enforcement of these
   limits depends upon proper implementation by the verification
   services.  The digital signature on the PASSporT data structure will
   be valid even if the limits are violated.

   Use of the Enhanced JWT Claim Constraints certificate extension
   permittedValues constraint is most useful when the claim definition
   allows a specified set of values.  In this way, all of the values
   that are not listed in the JWTClaimValuesList are prohibited in a
   valid PASSporT.

   Certificate issuers must take care when imposing constraints on the
   PASSporT claims and the claim values that can be successfully
   validated; some combinations can prevent any PASSporT from being
   successfully validated by the certificate.  For example, an entry in
   mustInclude and an entry in mustExclude for the same claim will
   prevent successful validation on any PASSporT.

   Certificate issuers SHOULD NOT include an entry in mustExclude for
   the "rcdi" claim for a certificate that will be used with the
   PASSporT Extension for Rich Call Data defined in [STIR-PASSPORT-RCD].
   Excluding this claim would prevent the integrity protection mechanism
   from working properly.

   Certificate issuers must take care when performing certificate
   renewal [RFC4949] to include exactly the same Enhanced JWT Claim
   Constraints certificate extension in the new certificate as the old
   one.  Renewal usually takes place before the old certificate expires,
   so there is a period of time where both the new certificate and the
   old certificate are valid.  If different constraints appear in the
   two certificates with the same public key, some PASSporTs might be
   valid when one certificate is used and invalid when the other one is
   used.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5912]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
              Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
              DOI 10.17487/RFC5912, June 2010,
              <https://www.rfc-editor.org/info/rfc5912>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8225]  Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
              Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
              <https://www.rfc-editor.org/info/rfc8225>.

   [RFC8226]  Peterson, J. and S. Turner, "Secure Telephone Identity
              Credentials: Certificates", RFC 8226,
              DOI 10.17487/RFC8226, February 2018,
              <https://www.rfc-editor.org/info/rfc8226>.

   [X.680]    ITU-T, "Information technology - Abstract Syntax Notation
              One (ASN.1): Specification of basic notation", ITU-T
              Recommendation X.680, February 2021.

9.2.  Informative References

   [ISO646]   ISO, "Information technology - ISO 7-bit coded character
              set for information interchange", ISO/IEC 646:1991,
              December 1991.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/info/rfc4949>.

   [RFC7468]  Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
              PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468,
              April 2015, <https://www.rfc-editor.org/info/rfc7468>.

   [RFC9060]  Peterson, J., "Secure Telephone Identity Revisited (STIR)
              Certificate Delegation", RFC 9060, DOI 10.17487/RFC9060,
              August 2021, <https://www.rfc-editor.org/rfc/rfc9060>.

   [STIR-PASSPORT-RCD]
              Wendt, C. and J. Peterson, "PASSporT Extension for Rich
              Call Data", Work in Progress, Internet-Draft, draft-ietf-
              stir-passport-rcd-12, 12 July 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-stir-
              passport-rcd-12>.

Appendix A.  ASN.1 Module

   This appendix provides the ASN.1 [X.680] definitions for the Enhanced
   JWT Claim Constraints certificate extension.  The module defined in
   this appendix is compatible with the ASN.1 specifications published
   in 2015.

   This ASN.1 module imports ASN.1 from [RFC5912].

   <CODE BEGINS>
   EnhancedJWTClaimConstraints-2021
     { iso(1) identified-organization(3) dod(6) internet(1)
       security(5) mechanisms(5) pkix(7) id-mod(0)
       id-mod-eJWTClaimConstraints-2021(101) }

   DEFINITIONS EXPLICIT TAGS ::= BEGIN

   IMPORTS

   id-pe
   FROM PKIX1Explicit-2009  -- From RFC 5912
     { iso(1) identified-organization(3) dod(6) internet(1)
       security(5) mechanisms(5) pkix(7) id-mod(0)
       id-mod-pkix1-explicit-02(51) }

   EXTENSION
   FROM PKIX-CommonTypes-2009  -- From RFC 5912
     { iso(1) identified-organization(3) dod(6) internet(1)
       security(5) mechanisms(5) pkix(7) id-mod(0)
       id-mod-pkixCommon-02(57) } ;

   -- Enhanced JWT Claim Constraints Certificate Extension

   ext-eJWTClaimConstraints EXTENSION ::= {
     SYNTAX EnhancedJWTClaimConstraints
     IDENTIFIED BY id-pe-eJWTClaimConstraints }

   id-pe-eJWTClaimConstraints OBJECT IDENTIFIER ::= { id-pe 33 }

   EnhancedJWTClaimConstraints ::= SEQUENCE {
     mustInclude [0] JWTClaimNames OPTIONAL,
       -- The listed claim names MUST appear in the PASSporT
       -- in addition to iat, orig, and dest.  If absent, iat, orig,
       -- and dest MUST appear in the PASSporT.
     permittedValues [1] JWTClaimValuesList OPTIONAL,
       -- If the claim name is present, the claim MUST contain one
       -- of the listed values.
     mustExclude [2] JWTClaimNames OPTIONAL }
       -- The listed claim names MUST NOT appear in the PASSporT.
   ( WITH COMPONENTS { ..., mustInclude PRESENT } |
     WITH COMPONENTS { ..., permittedValues PRESENT } |
     WITH COMPONENTS { ..., mustExclude PRESENT } )

   JWTClaimValuesList ::= SEQUENCE SIZE (1..MAX) OF JWTClaimValues

   JWTClaimValues ::= SEQUENCE {
     claim JWTClaimName,
     values SEQUENCE SIZE (1..MAX) OF UTF8String }

   JWTClaimNames ::= SEQUENCE SIZE (1..MAX) OF JWTClaimName

   JWTClaimName ::= IA5String

   END
   <CODE ENDS>

Acknowledgements

   Many thanks to Chris Wendt for his insight into the need for the for
   the Enhanced JWT Claim Constraints certificate extension.

   Thanks to Ben Campbell, Theresa Enghardt, Ben Kaduk, Erik Kline, √Čric
   Vyncke, and Rob Wilton for their thoughtful review and comments.  The
   document is much better as a result of their efforts.

Author's Address

   Russ Housley
   Vigil Security, LLC
   516 Dranesville Road
   Herndon, VA 20170
   United States of America

   Email: housley@vigilsec.com