JWK Thumbprint URI
draft-ietf-oauth-jwk-thumbprint-uri-02

OAuth Working Group                                             M. Jones
Internet-Draft                                                 K. Yasuda
Intended status: Standards Track                               Microsoft
Expires: November 17, 2022                                  May 16, 2022

                           JWK Thumbprint URI
                 draft-ietf-oauth-jwk-thumbprint-uri-02

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

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   This Internet-Draft will expire on November 17, 2022.

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Table of Contents

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

1.  Introduction

   A JSON Web Key (JWK) Thumbprint [RFC7638] is a URL-safe
   representation of a hash value over a JSON Web Key (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 Thumbprints 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
   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.

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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:

   o  "urn:ietf:params:oauth:jwk-thumbprint"

   To make it explicit in a URI which hash algorithm is used, 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
   considered invalid 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:

  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:

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      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

   o  URN: urn:ietf:params:oauth:jwk-thumbprint
   o  Common Name: JWK Thumbprint URI
   o  Change controller: IESG
   o  Specification Document: [[ this specification ]]

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>.

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   [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", Aug 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/named-
              information.xhtml#hash-alg>.

   [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. and M. B. Jones, "Self-Issued OpenID Provider
              v2", December 2021, <https://openid.net/specs/openid-
              connect-self-issued-v2-1_0.html>.

Appendix A.  Acknowledgements

   Use cases for this specification were developed in the OpenID Connect
   Working Group of the OpenID Foundation.  Specifically, it is being
   used 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, Adam Lemmon, Neil Madden, James Manger,
   Aaron Parecki, Gonzalo Salgueiro, Rifaat Shekh-Yusef, Robert Sparks,
   and David Waite.

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Appendix B.  Document History

   [[ to be removed by the RFC Editor before publication as an RFC ]]

   -02

   o  Addressed IETF last call comments by clarifying the requirement to
      use registered hash algorithm identifiers.

   -01

   o  Added security considerations about multiple public keys
      coresponding to the same private key.
   o  Added hash algorithm identifier after the JWK thumbprint URI
      prefix to make it explicit in a URI which hash algorithm is used.
   o  Added reference to a registry for hash algorithm identifiers.
   o  Added SHA-256 as a mandatory to implement hash algorithm to
      promote interoperability.

   -00

   o  Created initial working group draft from draft-jones-oauth-jwk-
      thumbprint-uri-01.

Authors' Addresses

   Michael B. Jones
   Microsoft

   Email: mbj@microsoft.com
   URI:   https://self-issued.info/

   Kristina Yasuda
   Microsoft

   Email: kryasuda@microsoft.com
   URI:   https://twitter.com/kristinayasuda

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