JWK: raise JWT::JWKError for EC coordinates that don't match the curve #788

Problem

When an EC JWK's x/y coordinates aren't a valid point on the curve named in crv, JWT::JWK::EC lets the raw OpenSSL error escape. That error isn't a JWT::Error, so code that rescues JWT::Error or JWT::JWKError around decoding doesn't catch it.

The error comes from create_point in lib/jwt/jwk/ec.rb, which builds an OpenSSL::PKey::EC::Point from the JWK parameters. It's raised when the key is first used, through verify_key, private?, export or JWKS-based decoding, not on import.

Reproduction (main, 3.3.1 dev)

p256 = JWT::JWK.new(OpenSSL::PKey::EC.generate("prime256v1")).export

JWT::JWK.import(p256.merge(crv: "P-384")).verify_key
# => OpenSSL::PKey::EC::Point::Error: EC_POINT_bn2point: invalid encoding

JWT::JWK.import(p256.merge(y: p256[:x])).verify_key
# => OpenSSL::PKey::EC::Point::Error: EC_POINT_bn2point: point is not on curve

token = JWT.encode({ a: 1 }, OpenSSL::PKey::EC.generate("prime256v1"), "ES256", kid: "k")
JWT.decode(token, nil, true, algorithms: ["ES256"], jwks: { keys: [p256.merge(crv: "P-384", kid: "k")] })
# => OpenSSL::PKey::EC::Point::Error, not a JWT::Error

Other invalid JWK input is already wrapped. An invalid base64 coordinate raises JWT::Base64DecodeError, and an unknown crv raises JWT::JWKError.

A JWKS is usually fetched from a remote endpoint, so a malformed or mislabelled key there currently surfaces as an OpenSSL exception in application code.

Suggested fix

Rescue OpenSSL::PKey::EC::Point::Error in create_point and re-raise it as JWT::JWKError (for example "Invalid EC point for curve P-384"), keeping the original as the cause. Add specs for a mislabelled curve and an off-curve point, covering both the import-and-use path and JWT.decode with jwks:.

Found while reviewing #787.

Posted by Claude on behalf of @anakinj.