Using Edwards-curve Digital Signature Algorithm (EdDSA) in the Internet Key Exchange (IKEv2)

The Internet Key Exchange protocol can use arbitrary signature algorithms as described in . The latter RFC defines the SIGNATURE_HASH_ALGORITHMS notification where each side of the IKE negotiation lists its supported hash algorithms. This assumes that all signature schemes involve a hashing phase before a signature phase, which makes sense because most signature algorithms either cannot sign messages bigger than their key or truncate messages bigger than their key. defines signature algorithms that do not require pre-hashing of the message. Unlike other methods, these signature algorithms accept arbitrary-sized messages, so no pre-hashing is required. These methods are called Ed25519 and Ed448, which respectively use the Edwards 25519 and the Edwards 448 ("Goldilocks") curves. Although that document also defines pre-hashed versions of these algorithm, those versions are not recommended for protocols where the entire to-be-signed message is available at once. defines the binary format of the signatures that should be used in the "Signature Value" field of the Authentication Data Format in section 3. defined the OIDs for these two signature methods. To signal within IKE that no hashing needs to be done. A new value has to be signalled in the SIGNATURE_HASH_ALGORITHMS notification, one that indicates that no hashing is performed.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

This document defines a new value called "Identity" (value TBA by IANA) in the hash algorithm registry for use in the SIGNATURE_HASH_ALGORITHMS notification. Inserting this value into the notification indicates that the receiver supports at least one signature algorithm that accepts arbitrary-sized messages such as Ed25519 and Ed448. Ed25519 and Ed448 are only defined with the Identity hash, and MUST NOT be sent to a receiver that has not indicated support for the "Identity" hash. The pre-hashed versions of Ed25519 and Ed448 (Ed25519ph and Ed448ph respectively) SHOULD NOT be used in IKE.

The new "Identity" value is needed only for signature algorithms that accept an arbitrary-sized input. It MUST NOT be used if none of the supported algorithms has this property. OTOH there is no good reason to hash where the signature algorithm does not require it (or does it internally), so the "Identity" value SHOULD be the only one used if all of the supported signature algorithms have this property.

IANA is requested to assign a new value from the "IKEv2 Hash Algorithms" registry with name "Identity" and this document as reference.