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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-----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-----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, Éric
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