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Security JOSE Working Group Cryptographic Algorithm Identifiers JSON Object Signing and Encryption (JOSE) CBOR Object Signing and Encryption (COSE) Internet-Draft 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.

The IANA algorithm registries for JOSE and COSE contain two kinds of algorithm identifiers: 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. 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 uses negotiation parameters like these (from an example in the specification):

OpenID Connect Discovery likewise negotiates supported algorithms using alg and enc values. W3C Web Authentication and FIDO Client to Authenticator Protocol (CTAP) 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:

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.

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 when, and only when, they appear in all capitals, as shown here.

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

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 . (The corresponding JOSE registrations in are full-specified.) The following fully-specified COSE algorithms are defined: 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

defines the current use of the Edwards-Curve Digital Signature Algorithm (EdDSA) by JOSE and defines its current use by COSE. Both register polymorphic EdDSA algorithm identifiers. The following fully-specified JOSE and COSE algorithms are defined: 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

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

Algorithm Name: Ed25519 Algorithm Description: EdDSA using Ed25519 curve Algorithm Usage Locations: alg JOSE Implementation Requirements: Optional Change Controller: IESG Reference: of [[ this specification ]] Algorithm Analysis Document(s):   Algorithm Name: Ed448 Algorithm Description: EdDSA using Ed448 curve Algorithm Usage Locations: alg JOSE Implementation Requirements: Optional Change Controller: IESG Reference: of [[ this specification ]] Algorithm Analysis Document(s):

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):

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

Name: ESP256 Value: TBD (requested assignment -9) Description: ECDSA using P-256 curve and SHA-256 Reference: of this document Recommended: Yes   Name: ESP384 Value: TBD (requested assignment -48) Description: ECDSA using P-384 curve and SHA-384 Reference: of this document Recommended: Yes   Name: ESP512 Value: TBD (requested assignment -49) Description: ECDSA using P-521 curve and SHA-512 Reference: of this document Recommended: Yes   Name: Ed25519 Value: TBD (requested assignment -50) Description: EdDSA using Ed25519 curve Reference: of this document Recommended: Yes   Name: Ed448 Value: TBD (requested assignment -51) Description: EdDSA using Ed448 curve Reference: of this document Recommended: Yes

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

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 in Section 7.1 of have been updated to include an additional review criterion: Only fully-specified algorithm identifiers may be registered. Polymorphic algorithm identifiers must not be registered.

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 in Section 10.4 of have been updated to include an additional review criterion: Only fully-specified algorithm identifiers may be registered. Polymorphic algorithm identifiers must not be registered.

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 , except that the alg value would be Ed25519 rather than EdDSA, if included.

Both JOSE and COSE have operations that take multiple algorithms as parameters. Encrypted objects in JOSE 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.

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

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.

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.

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.

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 , for EdDSA in , and for ECDSA and EdDSA in apply.

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[[ to be removed by the RFC Editor before publication as an RFC ]] -01 Included additional instructions for IANA. Added text on KEMs and Encapsulated keys. Added the section Fully-Specified Computations Using Multiple Algorithms. -00 Created initial working group version based on draft-jones-jose-fully-specified-algorithms-02.

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