Internet Engineering Task Force (IETF)                          M. Jones
Request for Comments: 9278                                     K. Yasuda
Category: Standards Track                                      Microsoft
ISSN: 2070-1721                                              August 2022


                          JWK Thumbprint URI

Abstract

  This specification registers a kind of URI that represents a JSON Web
  Key (JWK) Thumbprint value.  JWK Thumbprints are defined in RFC 7638.
  This enables JWK Thumbprints to be used, for instance, as key
  identifiers in contexts requiring URIs.

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/rfc9278.

Copyright Notice

  Copyright (c) 2022 IETF Trust and the persons identified as the
  document authors.  All rights reserved.

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  in the Revised BSD License.

Table of Contents

  1.  Introduction
  2.  Requirements Notation and Conventions
  3.  JWK Thumbprint URI
  4.  Hash Algorithms Identifier
  5.  Mandatory to Implement Hash Algorithm
  6.  Example JWK Thumbprint URI
  7.  Security Considerations
    7.1.  Multiple Public Keys per Private Key
  8.  IANA Considerations
    8.1.  OAuth URI Registration
      8.1.1.  Registry Contents
  9.  References
    9.1.  Normative References
    9.2.  Informative References
  Acknowledgements
  Authors' Addresses

1.  Introduction

  A JSON Web Key (JWK) Thumbprint [RFC7638] is a URL-safe
  representation of a hash value over a JWK [RFC7517].  This
  specification defines a URI prefix indicating that the portion of the
  URI following the prefix is a JWK Thumbprint.  This enables JWK
  Thumbprints to be communicated in contexts requiring URIs, including
  in specific JSON Web Token (JWT) [RFC7519] claims.

  JWK Thumbprint URIs are being used in the [SIOPv2] specification as
  one kind of subject identifier in a context requiring that the
  identifier be a URI.  In this case, the subject identifier is derived
  from a public key represented as a JWK.  Expressing the identifier as
  a JWK Thumbprint URI enables this kind of identifier to be
  differentiated from other kinds of identifiers that are also URIs,
  such as Decentralized Identifiers (DIDs) [DID-Core].

2.  Requirements Notation and Conventions

  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.  JWK Thumbprint URI

  The following URI prefix is defined to indicate that the portion of
  the URI following the prefix is a JWK Thumbprint:

     urn:ietf:params:oauth:jwk-thumbprint

  To make the hash algorithm being used explicit in a URI, the prefix
  is followed by a hash algorithm identifier and a JWK Thumbprint
  value, each separated by a colon character to form a URI representing
  a JWK Thumbprint.

4.  Hash Algorithms Identifier

  Hash algorithm identifiers used in JWK Thumbprint URIs MUST be values
  from the "Hash Name String" column in the IANA "Named Information
  Hash Algorithm Registry" [IANA.Hash.Algorithms].  JWK Thumbprint URIs
  with hash algorithm identifiers not found in this registry are not
  considered valid and applications will need to detect and handle this
  error, should it occur.

5.  Mandatory to Implement Hash Algorithm

  To promote interoperability among implementations, the SHA-256 hash
  algorithm is mandatory to implement.

6.  Example JWK Thumbprint URI

  Section 3.1 of [RFC7638] contains the following example JWK
  Thumbprint value:

     NzbLsXh8uDCcd-6MNwXF4W_7noWXFZAfHkxZsRGC9Xs

  A complete JWK Thumbprint URI using the above JWK Thumbprint and
  SHA-256 hash algorithm is as follows:

     urn:ietf:params:oauth:jwk-thumbprint:sha-256:NzbLsXh8uDCcd-
     6MNwXF4W_7noWXFZAfHkxZsRGC9Xs

7.  Security Considerations

  The security considerations of [RFC7638] also apply when using this
  specification.

7.1.  Multiple Public Keys per Private Key

  There are cryptographic algorithms for which multiple public keys
  correspond to the same private key.  This is described in the
  security considerations of [RFC7748] as follows:

  |  Designers using these curves should be aware that for each public
  |  key, there are several publicly computable public keys that are
  |  equivalent to it, i.e., they produce the same shared secrets.
  |  Thus using a public key as an identifier and knowledge of a shared
  |  secret as proof of ownership (without including the public keys in
  |  the key derivation) might lead to subtle vulnerabilities.

  This consideration for public keys as identifiers equally applies to
  JWK Thumbprint URIs used as identifiers.  A recommended way to ensure
  that the JWK Thumbprint URI corresponds to the actual public key used
  is to sign a message containing the correct public key with the
  private key.  This signed message could also contain the JWK
  Thumbprint URI (although, by definition, it could also be computed
  directly from the public key).

8.  IANA Considerations

8.1.  OAuth URI Registration

  This specification registers the following value in the IANA "OAuth
  URI" registry [IANA.OAuth.Parameters] established by [RFC6755].

8.1.1.  Registry Contents

  URN:  urn:ietf:params:oauth:jwk-thumbprint

  Common Name:  JWK Thumbprint URI

  Change controller:  IESG

  Specification Document:  RFC 9278

9.  References

9.1.  Normative References

  [IANA.OAuth.Parameters]
             IANA, "OAuth Parameters",
             <http://www.iana.org/assignments/oauth-parameters>.

  [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>.

  [RFC7638]  Jones, M. and N. Sakimura, "JSON Web Key (JWK)
             Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September
             2015, <https://www.rfc-editor.org/info/rfc7638>.

  [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>.

9.2.  Informative References

  [DID-Core] Sporny, M., Guy, A., Sabadello, M., and D. Reed,
             "Decentralized Identifiers (DIDs) v1.0", August 2021,
             <https://www.w3.org/TR/2021/PR-did-core-20210803/>.

  [IANA.Hash.Algorithms]
             IANA, "Named Information Hash Algorithm Registry",
             <https://www.iana.org/assignments/named-information>.

  [RFC6755]  Campbell, B. and H. Tschofenig, "An IETF URN Sub-Namespace
             for OAuth", RFC 6755, DOI 10.17487/RFC6755, October 2012,
             <https://www.rfc-editor.org/info/rfc6755>.

  [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517,
             DOI 10.17487/RFC7517, May 2015,
             <https://www.rfc-editor.org/info/rfc7517>.

  [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
             (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
             <https://www.rfc-editor.org/info/rfc7519>.

  [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
             for Security", RFC 7748, DOI 10.17487/RFC7748, January
             2016, <https://www.rfc-editor.org/info/rfc7748>.

  [SIOPv2]   Yasuda, K., Jones, M., and T. Lodderstedt, "Self-Issued
             OpenID Provider v2", June 2022, <https://openid.net/specs/
             openid-connect-self-issued-v2-1_0.html>.

Acknowledgements

  Use cases for this specification were developed in the OpenID Connect
  Working Group of the OpenID Foundation.  Specifically, it is being
  used as a key identifier in the [SIOPv2] specification.

  The following individuals also contributed to the creation of this
  specification: John Bradley, Scott Bradner, Brian Campbell, Roman
  Danyliw, Vladimir Dzhuvinov, Lars Eggert, Warren Kumari, Adam Lemmon,
  Neil Madden, James Manger, Francesca Palombini, Aaron Parecki,
  Gonzalo Salgueiro, Rifaat Shekh-Yusef, Robert Sparks, David Waite,
  Robert Wilton, and Paul Wouters.

Authors' Addresses

  Michael B. Jones
  Microsoft
  Email: [email protected]
  URI:   https://self-issued.info/


  Kristina Yasuda
  Microsoft
  Email: [email protected]
  URI:   https://twitter.com/kristinayasuda

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