Internet-Draft Fully-Specified Algorithms February 2024
Jones & Steele Expires 31 August 2024 [Page]
Workgroup:
JOSE Working Group
Internet-Draft:
draft-ietf-jose-fully-specified-algorithms-01
Updates:
7518, 8037, 9053 (if approved)
Published:
Intended Status:
Standards Track
Expires:
Authors:
M.B. Jones
Self-Issued Consulting
O. Steele
Transmute

Fully-Specified Algorithms for JOSE and COSE

Abstract

This specification refers to cryptographic algorithm identifiers that fully specify the cryptographic operations to be performed, including any curve, key derivation function (KDF), hash functions, etc., as being "fully specified". Whereas, it refers to cryptographic algorithm identifiers that require additional information beyond the algorithm identifier to determine the cryptographic operations to be performed as being "polymorphic". This specification creates fully-specified algorithm identifiers for all registered JOSE and COSE polymorphic algorithm identifiers, enabling applications to use only fully-specified algorithm identifiers.

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 2024.

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Table of Contents

1. Introduction

The IANA algorithm registries for JOSE [IANA.JOSE.Algorithms] and COSE [IANA.COSE.Algorithms] contain two kinds of algorithm identifiers:

Fully Specified
Those that fully determine the cryptographic operations to be performed, including any curve, key derivation function (KDF), hash functions, etc. Examples are RS256 and ES256K in both JOSE and COSE and ES256 in JOSE.
Polymorphic
Those requiring information beyond the algorithm identifier to determine the cryptographic operations to be performed. Such additional information could include the actual key value and a curve that it uses. Examples are EdDSA in both JOSE and COSE and ES256 in COSE.

This matters because many protocols negotiate supported operations using only algorithm identifiers. For instance, OAuth Authorization Server Metadata [RFC8414] uses negotiation parameters like these (from an example in the specification):

  "token_endpoint_auth_signing_alg_values_supported":
    ["RS256", "ES256"]

OpenID Connect Discovery [OpenID.Discovery] likewise negotiates supported algorithms using alg and enc values. W3C Web Authentication [WebAuthn] and FIDO Client to Authenticator Protocol (CTAP) [FIDO2] negotiate using COSE alg numbers.

This does not work for polymorphic algorithms. For instance, with EdDSA, you do not know which of the curves Ed25519 and/or Ed448 are supported! This causes real problems in practice.

WebAuthn contains this de-facto algorithm definition to work around this problem:

  -8 (EdDSA), where crv is 6 (Ed25519)

This redefines the COSE EdDSA algorithm identifier for the purposes of WebAuthn to restrict it to using the Ed25519 curve - making it non-polymorphic so that algorithm negotiation can succeed, but also effectively eliminating the possibility of using Ed448. Other similar workarounds for polymorphic algorithm identifiers are used in practice.

This specification creates fully-specified algorithm identifiers for all registered polymorphic JOSE and COSE algorithms and their parameters, enabling applications to use only fully-specified algorithm identifiers. It furthermore deprecates the practice of registering polymorphic algorithm identifiers.

1.1. 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.

2. Fully-specified Digital Signature Algorithm Identifiers

This section creates fully-specified digital signature algorithm identifiers for all registered polymorphic JOSE and COSE algorithms and their parameters.

2.1. Elliptic Curve Digital Signature Algorithm (ECDSA)

[RFC9053] defines the current use of the Elliptic Curve Digital Signature Algorithm (ECDSA) by COSE. The COSE algorithm registrations for ECDSA are polymorphic, since they do not specify the curve used. For instance, ES256 is defined as "ECDSA w/ SHA-256" in Section 2.1 of [RFC9053]. (The corresponding JOSE registrations in [RFC7518] are full-specified.)

The following fully-specified COSE algorithms are defined:

Table 1: ECDSA Algorithm Values
Name COSE Value Description COSE Recommended
ESP256 TBD (requested assignment -9) ECDSA using P-256 curve and SHA-256 Yes
ESP384 TBD (requested assignment -48) ECDSA using P-384 curve and SHA-384 Yes
ESP512 TBD (requested assignment -49) ECDSA using P-521 curve and SHA-512 Yes

2.2. Edwards-Curve Digital Signature Algorithm (EdDSA)

[RFC8037] defines the current use of the Edwards-Curve Digital Signature Algorithm (EdDSA) by JOSE and [RFC9053] defines its current use by COSE. Both register polymorphic EdDSA algorithm identifiers.

The following fully-specified JOSE and COSE algorithms are defined:

Table 2: EdDSA Algorithm Values
Name COSE Value Description JOSE Implementation Requirements COSE Recommended
Ed25519 TBD (requested assignment -50) EdDSA using Ed25519 curve Optional No
Ed448 TBD (requested assignment -51) EdDSA using Ed448 curve Optional No

3. IANA Considerations

3.1. JOSE Algorithms Registrations

This section registers the following values in the IANA "JSON Web Signature and Encryption Algorithms" registry [IANA.JOSE.Algorithms] established by [RFC7515].

3.1.1. Fully-Specified JOSE Algorithm Registrations

  • Algorithm Name: Ed25519

  • Algorithm Description: EdDSA using Ed25519 curve

  • Algorithm Usage Locations: alg

  • JOSE Implementation Requirements: Optional

  • Change Controller: IESG

  • Reference: Section 2.2 of [[ this specification ]]

  • Algorithm Analysis Document(s): [RFC8032]

  • Algorithm Name: Ed448

  • Algorithm Description: EdDSA using Ed448 curve

  • Algorithm Usage Locations: alg

  • JOSE Implementation Requirements: Optional

  • Change Controller: IESG

  • Reference: Section 2.2 of [[ this specification ]]

  • Algorithm Analysis Document(s): [RFC8032]

3.1.2. Deprecated Polymorphic JOSE Algorithm Registrations

The following registration is updated to change its status to Deprecated.

  • Algorithm Name: EdDSA

  • Algorithm Description: EdDSA signature algorithms

  • Algorithm Usage Locations: alg

  • JOSE Implementation Requirements: Deprecated

  • Change Controller: IESG

  • Reference: Section 3.1 of RFC8037

  • Algorithm Analysis Document(s): [RFC8032]

3.2. COSE Algorithms Registrations

This section registers the following values in the IANA "COSE Algorithms" registry [IANA.COSE.Algorithms].

3.2.1. Fully-Specified COSE Algorithm Registrations

  • Name: ESP256

  • Value: TBD (requested assignment -9)

  • Description: ECDSA using P-256 curve and SHA-256

  • Reference: Section 2.1 of this document

  • Recommended: Yes

  • Name: ESP384

  • Value: TBD (requested assignment -48)

  • Description: ECDSA using P-384 curve and SHA-384

  • Reference: Section 2.1 of this document

  • Recommended: Yes

  • Name: ESP512

  • Value: TBD (requested assignment -49)

  • Description: ECDSA using P-521 curve and SHA-512

  • Reference: Section 2.1 of this document

  • Recommended: Yes

  • Name: Ed25519

  • Value: TBD (requested assignment -50)

  • Description: EdDSA using Ed25519 curve

  • Reference: Section 2.2 of this document

  • Recommended: Yes

  • Name: Ed448

  • Value: TBD (requested assignment -51)

  • Description: EdDSA using Ed448 curve

  • Reference: Section 2.2 of this document

  • Recommended: Yes

3.2.2. Deprecated Polymorphic COSE Algorithm Registrations

The following registrations are updated to change their status to Deprecated.

  • Name: ES256

  • Value: -7

  • Description: ECDSA w/ SHA-256

  • Reference: RFC 9053

  • Recommended: Deprecated

  • Name: ES384

  • Value: -35

  • Description: ECDSA w/ SHA-384

  • Reference: RFC 9053

  • Recommended: Deprecated

  • Name: ES512

  • Value: -36

  • Description: ECDSA w/ SHA-512

  • Reference: RFC 9053

  • Recommended: Deprecated

  • Name: EdDSA

  • Value: -8

  • Description: EdDSA

  • Reference: RFC 9053

  • Recommended: Deprecated

3.3. Updated Review Instructions for Designated Experts

3.3.1. JSON Web Signature and Encryption Algorithms

IANA is directed to preserve the current reference to RFC 7518, and to add a reference to this section of this document.

The review instructions for the designated experts for the IANA "JSON Web Signature and Encryption Algorithms" registry [IANA.JOSE.Algorithms] in Section 7.1 of [RFC7518] have been updated to include an additional review criterion:

  • Only fully-specified algorithm identifiers may be registered. Polymorphic algorithm identifiers must not be registered.

3.3.2. COSE Algorithms

IANA is directed to preserve the current references to RFC 9053 and RFC 9054, and to add a reference to this section of this document.

The review instructions for the designated experts for the IANA "COSE Algorithms" registry [IANA.COSE.Algorithms] in Section 10.4 of [RFC9053] have been updated to include an additional review criterion:

  • Only fully-specified algorithm identifiers may be registered. Polymorphic algorithm identifiers must not be registered.

4. Key Representations

The key representations for the new fully-specified algorithms defined by this specification are the same as those for the polymorphic algorithms that they replace, other than the alg value, if included. For instance, the representation for a key used with the Ed25519 algorithm is the same as that specified in [RFC8037], except that the alg value would be Ed25519 rather than EdDSA, if included.

5. Fully-Specified Computations Using Multiple Algorithms

Both JOSE and COSE have operations that take multiple algorithms as parameters. Encrypted objects in JOSE [RFC7516] use two algorithm identifiers: the first in the alg (Algorithm) Header Parameter, which specifies how to determine the content encryption key, and the second in the enc (Encryption Algorithm) Header Parameter, which specifies the content encryption algorithm. Likewise, encrypted COSE objects can use multiple algorithms for corresponding purposes.

Each of these multiple algorithms must be independently fully specified. The operations performed by each of them MUST NOT vary when used alongside other algorithms. So for instance, for JOSE, alg values and enc values MUST each be fully specified, and their behaviors MUST NOT depend upon one another.

6. Notes on Algorithms Not Updated

The working group has discussed some existing algorithms that are not updated by this specification. This section discusses why they have not been updated.

6.1. Algorithms for Signing with RSASSA-PKCS1-v1_5

The working group has discussed whether the RS256, RS384, and RS512 algorithms should be considered fully-specified or not, because they can operate on keys of different sizes. For instance, they can use both 2048- and 4096-bit keys. The same is true of the PS* algorithms.

This is not a problem in practice, because RSA libraries accomodate keys of different sizes without having to use different code. Therefore, for example, there are not known cases in the wild where it would be useful to have different algorithm identifiers for RSASSA-PKCS1-v1_5 with SHA-256 and 2048-bit keys and RSASSA-PKCS1-v1_5 with SHA-256 and 4096-bit keys or 8192-bit keys. Therefore, the RSA signature algorithms are not replaced by this specification.

6.2. ECDH-ES and its Ephemeral Keys

The working group has discussed whether the ECDH-ES key agreement algorithm should be considered fully-specified or not, because it can use ephemeral keys of different key types and algorithms. Indeed, an implementation might work when ECDH-ES is used with a ephemeral keys using the P-256 curve, and not work when used with ephemeral keys using the Ed25519 curve.

One way that protocols can handle this situation is to use a discovery mechanism to declare what ephemeral key types are supported. The alternative would be to introduce new fully-specified algorithm identifiers for choices such as "ECDH-ES with the P-256 Curve", etc. Feedback from deployers would be useful in determining what actions this specification should take in this case.

6.3. KEMs and Encapsulated Keys

All KEM algorithms provide three functions: KeyGen(), Encapsulate(), and Decapsulate(). In order to consider a KEM algorithm fully specified, there MUST be a single KDF used per KEM Algorithm.

7. Security Considerations

Using fully-specified algorithm identifiers reduces the attack surface relative to using polymorphic algorithm identifiers, since it reduces the opportunity for attackers to choose algorithms.

The security considerations for ECDSA in [RFC7518], for EdDSA in [RFC8037], and for ECDSA and EdDSA in [RFC9053] apply.

8. References

8.1. Normative References

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC7515]
Jones, M., Bradley, J., and N. Sakimura, "JSON Web Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, , <https://www.rfc-editor.org/info/rfc7515>.
[RFC7516]
Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)", RFC 7516, DOI 10.17487/RFC7516, , <https://www.rfc-editor.org/info/rfc7516>.
[RFC8037]
Liusvaara, I., "CFRG Elliptic Curve Diffie-Hellman (ECDH) and Signatures in JSON Object Signing and Encryption (JOSE)", RFC 8037, DOI 10.17487/RFC8037, , <https://www.rfc-editor.org/info/rfc8037>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
[RFC9053]
Schaad, J., "CBOR Object Signing and Encryption (COSE): Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053, , <https://www.rfc-editor.org/info/rfc9053>.

8.2. Informative References

[FIDO2]
Bradley, J., Hodges, J., Jones, M., Kumar, A., and J. Johan, "Client to Authenticator Protocol (CTAP)", FIDO Alliance Proposed Standard, , <https://fidoalliance.org/specs/fido-v2.1-ps-20210615/fido-client-to-authenticator-protocol-v2.1-ps-20210615.html>.
[IANA.COSE.Algorithms]
IANA, "COSE Algorithms", <https://www.iana.org/assignments/cose/cose.xhtml#algorithms>.
[IANA.JOSE.Algorithms]
IANA, "JOSE Algorithms", <https://www.iana.org/assignments/jose/jose.xhtml#web-signature-encryption-algorithms>.
[OpenID.Discovery]
Sakimura, N., Bradley, J., Jones, M.B., and E. Jay, "OpenID Connect Discovery 1.0", , <https://openid.net/specs/openid-connect-discovery-1_0.html>.
[RFC7518]
Jones, M., "JSON Web Algorithms (JWA)", RFC 7518, DOI 10.17487/RFC7518, , <https://www.rfc-editor.org/info/rfc7518>.
[RFC8032]
Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital Signature Algorithm (EdDSA)", RFC 8032, DOI 10.17487/RFC8032, , <https://www.rfc-editor.org/info/rfc8032>.
[RFC8414]
Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0 Authorization Server Metadata", RFC 8414, DOI 10.17487/RFC8414, , <https://www.rfc-editor.org/info/rfc8414>.
[WebAuthn]
Hodges, J., Jones, J.C., Jones, M., Kumar, A., and E. Lundberg, "Web Authentication: An API for accessing Public Key Credentials - Level 2", World Wide Web Consortium (W3C) Recommendation, , <https://www.w3.org/TR/2021/REC-webauthn-2-20210408/>.

Appendix A. Document History

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

-01

-00

Acknowledgements

The authors thank John Bradley, Brian Campbell, Ilari Liusvarra, Tobias Looker, and Filip Skokan for their contributions to this specification.

Authors' Addresses

Michael B. Jones
Self-Issued Consulting
URI: https://self-issued.info/
Orie Steele
Transmute