JWK: support the registered secp256k1 curve name for EC keys #786

Problem

RFC 8812, section 3.1 registers the JWK crv value for the secp256k1 curve as "secp256k1". JWT::JWK::EC only recognises "P-256K", a name from earlier drafts, in lib/jwt/jwk/ec.rb:

• to_openssl_curve maps 'P-256K' to 'secp256k1' and raises JWT::JWKError, 'Invalid curve provided' for anything else.
• keypair_components exports secp256k1 keys with crv: 'P-256K'.

As a result, ES256K tokens can't be verified against a JWKS that uses the registered name, and keys exported by ruby-jwt aren't readable by implementations that only accept "secp256k1".

Reproduction (jwt 3.3.0)

key = OpenSSL::PKey::EC.generate("secp256k1")
jwk = JWT::JWK.new(key)
jwk.export[:crv] # => "P-256K"

token = JWT.encode({ sub: "1" }, key, "ES256K", kid: jwk.kid)
standard = jwk.export.merge(crv: "secp256k1")

JWT.decode(token, nil, true, algorithms: ["ES256K"], jwks: { keys: [standard] })
# => JWT::JWKError: Invalid curve provided

JWT.decode(token, nil, true, algorithms: ["ES256K"], jwks: { keys: [jwk.export] })
# => works with "P-256K"

This also shows up with real JWKS. The Connect2id demo JWKS publishes an EC key with crv: "secp256k1". Importing it works because the key is built lazily, but verify_key, private? and export raise, so JWT::JWK::Set#export fails for the whole set. Looking up other keys by kid isn't affected.

Suggested fix

1. Accept both "secp256k1" and "P-256K" in to_openssl_curve. This is backwards compatible and could go into a patch release.
2. Export secp256k1 keys with crv: "secp256k1". This changes exported JWKs and their thumbprints for existing secp256k1 keys, so it might fit better in a minor or major release, with a CHANGELOG note.

Posted by Claude on behalf of @anakinj.