JWK Thumbprint URI
A URN prefix that lets a JWK Thumbprint be used wherever an identifier must be a URI.
Jones, M. and K. Yasuda, "JWK Thumbprint URI", RFC 9278, DOI 10.17487/RFC9278, August 2022, <https://www.rfc-editor.org/info/rfc9278>.
@misc{rfc9278,
series = {Request for Comments},
number = 9278,
howpublished = {RFC 9278},
publisher = {RFC Editor},
doi = {10.17487/RFC9278},
url = {https://www.rfc-editor.org/info/rfc9278},
author = {M. Jones and K. Yasuda},
title = {{JWK Thumbprint URI}},
pagetotal = 6,
year = 2022,
month = aug,
} RFC text
Contents
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.
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
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to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
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: mbj@microsoft.com
URI: https://self-issued.info/
Kristina Yasuda
Microsoft
Email: kryasuda@microsoft.com
URI: https://twitter.com/kristinayasuda
AI ELI5
On this page
§1Why a Thumbprint Needs a URI Form
A JWK Thumbprint from RFC 7638 is a URL-safe representation of a hash over a JSON Web Key (JWK, RFC 7517). This RFC defines a URI prefix that says the rest of the URI is such a thumbprint. That lets thumbprints travel in contexts that require URIs, including specific JSON Web Token (RFC 7519) claims.
The motivating case is the SIOPv2 specification, which uses a JWK Thumbprint URI as a subject identifier derived from a public key. Writing it as a thumbprint URI differentiates it from other kinds of URI identifiers, such as Decentralized Identifiers (DIDs).
Your fingerprint identifies you, but some forms only accept a web-style address. This gives the fingerprint a proper address form with a label saying what it is.
§3The URI Syntax
The prefix is urn:ietf:params:oauth:jwk-thumbprint. To make the hash algorithm explicit, the prefix is followed by a hash algorithm identifier and then the thumbprint value, each separated by a colon.
So the full shape is the prefix, a colon, the hash name, a colon, and the thumbprint. Putting the algorithm in the URI means a reader never has to guess how the thumbprint was computed.
It is a label with three parts separated by colons: what kind of thing it is, how the fingerprint was made, and the fingerprint itself.
§4Hash Algorithm Identifiers
The hash algorithm identifier MUST be a value from the Hash Name String column of the IANA Named Information Hash Algorithm Registry. A URI whose identifier is not in that registry is not valid, and applications need to detect and handle that error.
For interoperability, the SHA-256 hash algorithm is mandatory to implement (Section 5), so any two implementations have at least one hash in common.
The fingerprint method has to come from an official list. Everyone must know the most common method on the list, so they can always understand each other.
§6A Worked Example
Section 3.1 of RFC 7638 contains this JWK Thumbprint value: NzbLsXh8uDCcd-6MNwXF4W_7noWXFZAfHkxZsRGC9Xs. Combining it with the SHA-256 algorithm identifier gives the complete URI shown in the diagram below.
Take a fingerprint code, add the label and the method name, and you have the finished address.
urn:ietf:params:oauth:jwk-thumbprint:sha-256:
NzbLsXh8uDCcd-6MNwXF4W_7noWXFZAfHkxZsRGC9Xs§7Security Considerations
The security considerations of RFC 7638 also apply. In addition, some cryptographic algorithms have several public keys that correspond to the same private key. RFC 7748 warns that using a public key as an identifier, with knowledge of a shared secret as proof of ownership and without including the public keys in the key derivation, might lead to subtle vulnerabilities.
The same caution applies to JWK Thumbprint URIs used as identifiers. A recommended way to confirm that the URI matches the actual public key in use is to sign a message that contains the correct public key with the private key. That message may also contain the thumbprint URI, although it can be computed directly from the public key.
Sometimes different-looking keys behave the same. So if someone claims a fingerprint is theirs, ask them to sign something with the matching key to prove it.
§8IANA Registration
The RFC registers one value in the IANA OAuth URI registry established by RFC 6755: the URN urn:ietf:params:oauth:jwk-thumbprint, with common name JWK Thumbprint URI. The change controller is the IESG and the specification document is RFC 9278.
The new label was written in an official list so nobody else will use it for something different.