Web Authorization Protocol F. Skokan Internet-Draft Okta Intended status: Standards Track 27 February 2026 Expires: 31 August 2026 Additional Hash Algorithms for OAuth 2.0 PKCE and Proof-of-Possession draft-skokan-oauth-additional-hashes-03 Abstract This document defines SHA-512 as an additional hash algorithm for OAuth 2.0 Proof Key for Code Exchange (PKCE), mutual-TLS certificate- bound access tokens, and Demonstrating Proof of Possession (DPoP), for use in deployments operating under security policies that prohibit the use of SHA-256, which is otherwise mandated or the only option in these mechanisms. About This Document This note is to be removed before publishing as an RFC. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-skokan-oauth-additional- hashes/. Source for this draft and an issue tracker can be found at https://github.com/panva/draft-oauth-additional-hashes. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 31 August 2026. Skokan Expires 31 August 2026 [Page 1] Internet-Draft Additional Hashes for OAuth PoP and PKCE February 2026 Copyright Notice Copyright (c) 2026 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 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 . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3 3. Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . 3 4. PKCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4.1. S512 Code Challenge Method . . . . . . . . . . . . . . . 4 4.2. Authorization Server Metadata . . . . . . . . . . . . . . 4 5. Mutual-TLS . . . . . . . . . . . . . . . . . . . . . . . . . 4 5.1. x5t#S512 Confirmation Method . . . . . . . . . . . . . . 5 5.2. Resource Server Metadata . . . . . . . . . . . . . . . . 5 6. DPoP . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 6.1. Authorization Code Binding Methods . . . . . . . . . . . 6 6.1.1. dpop_jkt_method Authorization Request Parameter . . . 6 6.1.2. Authorization Server Metadata . . . . . . . . . . . . 7 6.2. SHA-512 Hash Algorithms . . . . . . . . . . . . . . . . . 7 6.2.1. jkt#S512 Confirmation Method . . . . . . . . . . . . 7 6.2.2. ath#S512 Access Token Hash . . . . . . . . . . . . . 8 6.2.3. Resource Server Metadata . . . . . . . . . . . . . . 8 7. Security Considerations . . . . . . . . . . . . . . . . . . . 9 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 8.1. PKCE Code Challenge Method Registration . . . . . . . . . 9 8.2. DPoP Authorization Code Binding Methods Registry . . . . 10 8.3. OAuth Parameters Registrations . . . . . . . . . . . . . 10 8.4. OAuth Authorization Server Metadata Registration . . . . 10 8.5. JWT Claims Registration . . . . . . . . . . . . . . . . . 11 8.6. OAuth Protected Resource Metadata Registrations . . . . . 11 8.7. JWT Confirmation Methods Registrations . . . . . . . . . 12 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 9.1. Normative References . . . . . . . . . . . . . . . . . . 12 9.2. Informative References . . . . . . . . . . . . . . . . . 13 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 14 Document History . . . . . . . . . . . . . . . . . . . . . . . . 14 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15 Skokan Expires 31 August 2026 [Page 2] Internet-Draft Additional Hashes for OAuth PoP and PKCE February 2026 1. Introduction Several OAuth 2.0 mechanisms exclusively mandate the use of SHA-256: Proof Key for Code Exchange (PKCE) [RFC7636], mutual-TLS certificate- bound access tokens [RFC8705], and Demonstrating Proof of Possession (DPoP) [RFC9449]. Security policies, such as the US Commercial National Security Algorithm (CNSA 2.0) Suite [cnsafaq], prohibit the use of SHA-256 and require SHA-384 or SHA-512. This prevents the deployment of these OAuth 2.0 mechanisms in such environments. This document addresses this gap by defining SHA-512 alternatives for each of these mechanisms, for use in deployments operating under such constrained policies. For PKCE, a new S512 code challenge method is defined. For mutual-TLS certificate-bound access tokens, a new x5t#S512 confirmation method is defined. For DPoP, this document defines SHA-512 alternatives for the JWK Thumbprint confirmation method (jkt#S512) and the access token hash claim (ath#S512), as well as an extensible framework for authorization code binding and access token hash algorithm negotiation. [[TODO: The hash algorithm chosen by this document is currently SHA- 512. The working group should determine whether to define SHA-384 or SHA-512.]] 2. Conventions and Definitions 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. All references to "CNSA 2.0" in this document refer to CNSA 2.0 [cnsafaq], unless stated otherwise. 3. Purpose and Scope The sole purpose of this document is to enable deployments operating under security policies that prohibit SHA-256 to use PKCE, mutual-TLS certificate-bound access tokens, and DPoP. In such constrained deployments, the SHA-512 alternatives defined herein are used in place of their SHA-256 counterparts, since those deployments cannot use SHA-256 at all. Skokan Expires 31 August 2026 [Page 3] Internet-Draft Additional Hashes for OAuth PoP and PKCE February 2026 This document does not deprecate the SHA-256 based methods defined in existing specifications. The SHA-256 based methods remain the widely deployed, interoperable and recommended defaults for all mechanisms addressed by this document. Deployments that are not subject to such security policies SHOULD NOT offer or use the SHA-512 based methods defined herein. The negotiation mechanisms defined herein may however facilitate a broader transition away from SHA-256 in the future, should that become necessary. 4. PKCE Proof Key for Code Exchange (PKCE) [RFC7636] defines plain and S256 as code challenge methods, with S256 being the only method that applies a cryptographic hash to the code verifier. The specification establishes the "PKCE Code Challenge Methods" registry, which this document uses to register the S512 code challenge method. 4.1. S512 Code Challenge Method This document defines a new code challenge method for use with PKCE [RFC7636]. The client creates a code challenge derived from the code verifier by using the following transformation on the code verifier: S512: code_challenge = BASE64URL(SHA-512(ASCII(code_verifier))) The server-side verification of the code verifier follows Section 4.6 of [RFC7636], using SHA-512 as the hash algorithm. 4.2. Authorization Server Metadata An Authorization Server that supports the S512 code challenge method MUST advertise its support in its Authorization Server metadata (e.g., [RFC8414] or [OpenID.Discovery]) by including S512 in the code_challenge_methods_supported metadata parameter value as defined in [RFC8414]. 5. Mutual-TLS OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound Access Tokens [RFC8705] exclusively uses SHA-256 for certificate- bound access tokens via the x5t#S256 confirmation method. No alternative hash algorithms or extension points for hash algorithm negotiation are defined. This document defines the x5t#S512 confirmation method and a Resource Server metadata parameter for negotiating the confirmation method. Skokan Expires 31 August 2026 [Page 4] Internet-Draft Additional Hashes for OAuth PoP and PKCE February 2026 5.1. x5t#S512 Confirmation Method RFC 8705 [RFC8705] defines the x5t#S256 confirmation method member for binding access tokens to a client certificate using a SHA-256 hash of the DER-encoded X.509 certificate. This document defines an analogous confirmation method member x5t#S512 that uses SHA-512 as the hash algorithm: x5t#S512: The value is a base64url-encoded SHA-512 hash of the DER encoding of the X.509 certificate. When using x5t#S512, the Authorization Server computes the SHA-512 hash of the client certificate presented during mutual-TLS and includes the result as the x5t#S512 member of the cnf claim in the access token (for JWT access tokens) or associates it with the token for later retrieval via token introspection [RFC7662]. The Resource Server MUST compute the SHA-512 hash of the client certificate presented during mutual-TLS and compare it with the x5t#S512 value in the cnf claim. If the values do not match, the Resource Server MUST reject the request. The choice of x5t#S512 over x5t#S256 is a deployment decision. It can be configured out of band or by the Authorization Server using the Resource Server's metadata (Section 5.2). [[TODO: Section 3.1 of [RFC7800] does not preclude the presence of both x5t#S256 and x5t#S512 in the same cnf claim. Including both would not represent confirmations for two different keys but rather two different hash confirmations of the same certificate. This may actually be useful during a transition period in possible future non- constrained deployment scenarios. The working group should determine whether to prohibit or allow this.]] 5.2. Resource Server Metadata This document defines the mtls_confirmation_methods_supported Resource Server metadata parameter [RFC9728]. Its value is a JSON array containing the mutual-TLS confirmation method names that the Resource Server supports. Defined values are x5t#S256 and x5t#S512. If omitted, the default is ["x5t#S256"]. Skokan Expires 31 August 2026 [Page 5] Internet-Draft Additional Hashes for OAuth PoP and PKCE February 2026 6. DPoP OAuth 2.0 Demonstrating Proof of Possession (DPoP) [RFC9449] exclusively uses SHA-256 for all of its hash operations: the jkt confirmation method, the ath access token hash claim, and the dpop_jkt authorization code binding parameter. No alternative hash algorithms or extension points for hash algorithm negotiation are defined. Section 11.10 of [RFC9449] anticipated the need for hash algorithm agility and foresaw that a future specification would define a new confirmation method, JWT claim, and authorization request parameter for use as alternatives to their SHA-256 counterparts. This document defines those DPoP mechanisms: the dpop_jkt_method authorization request parameter, the jkt#S512 confirmation method, and the ath#S512 JWT claim. In constrained deployments where SHA-256 is prohibited, these are used in place of their SHA-256 counterparts rather than alongside them. 6.1. Authorization Code Binding Methods 6.1.1. dpop_jkt_method Authorization Request Parameter RFC 9449 [RFC9449] defines the dpop_jkt authorization request parameter as the JWK Thumbprint [RFC7638] of the DPoP public key using SHA-256. This document changes the definition of dpop_jkt to allow alternative hash algorithms indicated by the dpop_jkt_method parameter. This document defines the dpop_jkt_method authorization request parameter, sent alongside dpop_jkt, to indicate the hash algorithm used to compute the JWK Thumbprint. The following method values are defined: S256: JWK Thumbprint [RFC7638] using SHA-256, as originally defined in Section 10 of [RFC9449]. S512: JWK Thumbprint [RFC7638] using SHA-512. For backwards compatibility, when dpop_jkt_method is absent from the authorization request, the Authorization Server MUST assume the value S256. The value of dpop_jkt MUST be computed using the hash algorithm indicated by dpop_jkt_method. Skokan Expires 31 August 2026 [Page 6] Internet-Draft Additional Hashes for OAuth PoP and PKCE February 2026 6.1.2. Authorization Server Metadata This document defines the dpop_jkt_methods_supported Authorization Server metadata parameter. Its value is a JSON array containing the dpop_jkt_method values that the Authorization Server supports. An Authorization Server that supports dpop_jkt_method values beyond S256 MUST advertise its support by including the supported values in the dpop_jkt_methods_supported metadata parameter. 6.2. SHA-512 Hash Algorithms 6.2.1. jkt#S512 Confirmation Method RFC 9449 [RFC9449] defines the jkt confirmation method member for binding access tokens to a DPoP public key using a SHA-256 JWK Thumbprint [RFC7638]. This document defines an analogous confirmation method member jkt#S512 that uses SHA-512 as the hash algorithm: jkt#S512: The value is the base64url encoding of the JWK Thumbprint [RFC7638] computed using SHA-512 of the DPoP public key (in JWK format) to which the access token is bound. When using jkt#S512, the Authorization Server computes the SHA-512 JWK Thumbprint of the DPoP public key and includes the result as the jkt#S512 member of the cnf claim in the access token (for JWT access tokens) or associates it with the token for later retrieval via token introspection [RFC7662]. The Resource Server MUST compute the SHA-512 JWK Thumbprint of the DPoP public key and compare it with the jkt#S512 value in the cnf claim. If the values do not match, the Resource Server MUST reject the request. The choice of jkt#S512 over jkt is a deployment decision. It can be configured out of band or by the Authorization Server using the Resource Server's metadata (Section 6.2.3). [[TODO: Section 3.1 of [RFC7800] does not preclude the presence of both jkt and jkt#S512 in the same cnf claim. Including both would not represent confirmations for two different keys but rather two different hash confirmations of the same key. This may actually be useful during a transition period in possible future non-constrained deployment scenarios. The working group should determine whether to prohibit or allow this.]] Skokan Expires 31 August 2026 [Page 7] Internet-Draft Additional Hashes for OAuth PoP and PKCE February 2026 6.2.2. ath#S512 Access Token Hash RFC 9449 [RFC9449] defines the ath claim in the DPoP proof JWT as the base64url-encoded SHA-256 hash of the ASCII encoding of the access token value. This document defines an analogous claim ath#S512 that uses SHA-512 as the hash algorithm: ath#S512: The value is the base64url encoding of the SHA-512 hash of the ASCII encoding of the associated access token's value. [[TODO: Including both ath and ath#S512 in the same DPoP proof JWT would not represent hashes of two different access tokens but rather two different hash confirmations of the same access token. This may actually be useful during a transition period in possible future non- constrained deployment scenarios. The working group should determine whether to prohibit or allow this.]] The Resource Server MUST compute the SHA-512 hash of the ASCII encoding of the access token value and compare it with the ath#S512 value in the DPoP proof JWT. If the values do not match, the Resource Server MUST reject the request. A Resource Server MAY signal the acceptable access token hash methods by including the ath_methods parameter in the WWW-Authenticate: DPoP challenge. The value of ath_methods is a space-delimited list of access token hash claim names that the Resource Server supports, analogous to the algs parameter defined in Section 7.1 of [RFC9449]. When ath_methods is absent, the Client MUST use ath. When ath_methods is present, the Client MUST use one of the listed methods. Additionally, Resource Server metadata for the supported access token hash methods is defined in Section 6.2.3. The following is a non-normative example of an HTTP response signalling the client to use ath#S512: HTTP/1.1 401 Unauthorized WWW-Authenticate: DPoP algs="Ed25519", ath_methods="ath#S512" 6.2.3. Resource Server Metadata This document defines the following Resource Server metadata parameters [RFC9728]: dpop_confirmation_methods_supported: JSON array containing the DPoP Skokan Expires 31 August 2026 [Page 8] Internet-Draft Additional Hashes for OAuth PoP and PKCE February 2026 confirmation method names that the Resource Server supports. Defined values are jkt and jkt#S512. If omitted, the default is ["jkt"]. dpop_access_token_hash_methods_supported: JSON array containing the access token hash claim names that the Resource Server supports. Defined values are ath and ath#S512. If omitted, the default is ["ath"]. 7. Security Considerations The S512 code challenge method provides the same structural security properties as S256. It is a one-way transformation of the code verifier that prevents an attacker who intercepts the authorization code from computing the code verifier needed to exchange it for tokens. The x5t#S512 confirmation method provides the same structural security properties as x5t#S256 defined in [RFC8705]. The jkt#S512 confirmation method, dpop_jkt combined with dpop_jkt_method parameter, and ath#S512 claim provide the same structural security properties as their SHA-256 counterparts defined in DPoP [RFC9449]. SHA-512 provides a 256-bit collision resistance and 512-bit preimage resistance, exceeding the 128-bit and 256-bit levels provided by SHA- 256. The use of SHA-512 is suitable for deployments with elevated security requirements. Deployments that do not have restrictions on use of SHA-256 do not need to migrate away from the established SHA-256 based mechanisms. 8. IANA Considerations 8.1. PKCE Code Challenge Method Registration This document requests registration of the following value in the "PKCE Code Challenge Methods" registry established by Section 6.2 of [RFC7636]: Code Challenge Method Parameter Name: S512 Change Controller: IETF Specification Document(s): Section 4.1 of this document Skokan Expires 31 August 2026 [Page 9] Internet-Draft Additional Hashes for OAuth PoP and PKCE February 2026 8.2. DPoP Authorization Code Binding Methods Registry This document establishes the "DPoP Authorization Code Binding Methods" registry for dpop_jkt_method values. New entries are registered using the Specification Required policy [RFC5226]. The initial contents of the registry are: Method Name: S256 Change Controller: IETF Specification Document(s): Section 10 of [RFC9449] Method Name: S512 Change Controller: IETF Specification Document(s): Section 6.1.1 of this document 8.3. OAuth Parameters Registrations This document requests registration of the following value in the "OAuth Parameters" registry established by [RFC6749]: Parameter Name: dpop_jkt_method Parameter Usage Location: authorization request Change Controller: IETF Specification Document(s): Section 6.1.1 of this document 8.4. OAuth Authorization Server Metadata Registration This document requests registration of the following value in the "OAuth Authorization Server Metadata" registry established by [RFC8414]: Metadata Name: dpop_jkt_methods_supported Metadata Description: JSON array containing a list of the dpop_jkt_method values supported by the Authorization Server Change Controller: IETF Skokan Expires 31 August 2026 [Page 10] Internet-Draft Additional Hashes for OAuth PoP and PKCE February 2026 Specification Document(s): Section 6.1.2 of this document 8.5. JWT Claims Registration This document requests registration of the following value in the "JSON Web Token Claims" registry established by [RFC7519]: Claim Name: ath#S512 Claim Description: The base64url-encoded SHA-512 hash of the ASCII encoding of the associated access token's value Change Controller: IETF Specification Document(s): Section 6.2.2 of this document 8.6. OAuth Protected Resource Metadata Registrations This document requests registration of the following values in the "OAuth Protected Resource Metadata" registry established by [RFC9728]: Metadata Name: dpop_confirmation_methods_supported Metadata Description: JSON array containing a list of the DPoP confirmation method names supported by the Resource Server Change Controller: IETF Specification Document(s): Section 6.2.3 of this document Metadata Name: dpop_access_token_hash_methods_supported Metadata Description: JSON array containing a list of the access token hash claim names supported by the Resource Server Change Controller: IETF Specification Document(s): Section 6.2.3 of this document Metadata Name: mtls_confirmation_methods_supported Metadata Description: JSON array containing a list of the mutual-TLS confirmation method names supported by the Resource Server Change Controller: IETF Specification Document(s): Section 5.2 of this document Skokan Expires 31 August 2026 [Page 11] Internet-Draft Additional Hashes for OAuth PoP and PKCE February 2026 8.7. JWT Confirmation Methods Registrations This document requests registration of the following values in the "JWT Confirmation Methods" registry established by [RFC7800]: Confirmation Method Value: x5t#S512 Confirmation Method Description: X.509 Certificate SHA-512 Thumbprint Change Controller: IETF Specification Document(s): Section 5.1 of this document Confirmation Method Value: jkt#S512 Confirmation Method Description: JWK SHA-512 Thumbprint Change Controller: IETF Specification Document(s): Section 6.2.1 of this document 9. References 9.1. Normative References [OpenID.Discovery] Sakimura, N., Bradley, J., Jones, M., and E. Jay, "OpenID Connect Discovery 1.0 incorporating errata set 2", December 2023, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 5226, DOI 10.17487/RFC5226, May 2008, . [RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", RFC 6749, DOI 10.17487/RFC6749, October 2012, . Skokan Expires 31 August 2026 [Page 12] Internet-Draft Additional Hashes for OAuth PoP and PKCE February 2026 [RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015, . [RFC7636] Sakimura, N., Ed., Bradley, J., and N. Agarwal, "Proof Key for Code Exchange by OAuth Public Clients", RFC 7636, DOI 10.17487/RFC7636, September 2015, . [RFC7638] Jones, M. and N. Sakimura, "JSON Web Key (JWK) Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September 2015, . [RFC7800] Jones, M., Bradley, J., and H. Tschofenig, "Proof-of- Possession Key Semantics for JSON Web Tokens (JWTs)", RFC 7800, DOI 10.17487/RFC7800, April 2016, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0 Authorization Server Metadata", RFC 8414, DOI 10.17487/RFC8414, June 2018, . [RFC8705] Campbell, B., Bradley, J., Sakimura, N., and T. Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound Access Tokens", RFC 8705, DOI 10.17487/RFC8705, February 2020, . [RFC9449] Fett, D., Campbell, B., Bradley, J., Lodderstedt, T., Jones, M., and D. Waite, "OAuth 2.0 Demonstrating Proof of Possession (DPoP)", RFC 9449, DOI 10.17487/RFC9449, September 2023, . [RFC9728] Jones, M.B., Hunt, P., and A. Parecki, "OAuth 2.0 Protected Resource Metadata", RFC 9728, DOI 10.17487/RFC9728, April 2025, . 9.2. Informative References Skokan Expires 31 August 2026 [Page 13] Internet-Draft Additional Hashes for OAuth PoP and PKCE February 2026 [cnsafaq] National Security Agency, "The Commercial National Security Algorithm Suite 2.0 and Quantum Computing FAQ", December 2024, . [RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection", RFC 7662, DOI 10.17487/RFC7662, October 2015, . Acknowledgments TODO acknowledge. Document History draft-skokan-oauth-additional-hashes-03 * Added Document History * Changed ath_method to ath_methods (plural, space-delimited list), analogous to the algs parameter in Section 7.1 of [RFC9449] * Removed premature "in place of ath" language for ath#S512, pending resolution of the dual-hash coexistence TODO draft-skokan-oauth-additional-hashes-02 * Removed client-side MUST NOT requirements for using unadvertised PKCE and DPoP authorization code binding methods draft-skokan-oauth-additional-hashes-01 * Changed hash algorithm from SHA-384 to SHA-512 * Added Purpose and Scope section * Added Mutual-TLS section with x5t#S512 confirmation method and mtls_confirmation_methods_supported RS metadata * Added dpop_confirmation_methods_supported RS metadata for DPoP * Added WWW-Authenticate challenge parameter for access token hash method signalling * Added TODO notes for dual-hash coexistence questions * Expanded Security Considerations Skokan Expires 31 August 2026 [Page 14] Internet-Draft Additional Hashes for OAuth PoP and PKCE February 2026 draft-skokan-oauth-additional-hashes-00 * Initial draft Author's Address Filip Skokan Okta Email: panva.ip@gmail.com Skokan Expires 31 August 2026 [Page 15]