ECDSA signature verification fails for valid tokens

[mark-adams]

ruby-jwt fails with a Signature verification failed (JWT::VerificationError) error when decoding a valid JWT signed with ES512. I believe this is due to an issue with improper ECDSA signature serialization format where the signature parameters are not concatenated in the proper order. The improper serialization results in an implementation being able to successfully verify its own JWTs but not those generated by other conforming libraries.

We encountered the same issue and fixed it on PyJWT a little while ago but some users are now reporting issues with ruby-jwt validating ECDSA-signed tokens and I wanted to pass along the information. PyJWT now has tests validating it against RFC 7520 test vectors to ensure all algorithms serialize properly. It may be a good idea to do a similar thing here.

Ruby test (using ruby-jwt):

require 'jwt'

token = 'eyJhbGciOiJFUzUxMiIsInR5cCI6IkpXVCJ9.eyJoZWxsbyI6IndvcmxkIn0.AQx1MqdTni6KuzfOoedg2-7NUiwe-b88SWbdmviz40GTwrM0Mybp1i1tVtmTSQ91oEXGXBdtwsN6yalzP9J-sp2YATX_Tv4h-BednbdSvYxZsYnUoZ--ZUdL10t7g8Yt3y9hdY_diOjIptcha6ajX8yzkDGYG42iSe3f5LywSuD6FO5c'
pubkey_pem = "-----BEGIN PUBLIC KEY-----\nMIGbMBAGByqGSM49AgEGBSuBBAAjA4GGAAQAcpkss6wI7PPlxj3t7A1RqMH3nvL4\nL5Tzxze/XeeYZnHqxiX+gle70DlGRMqqOq+PJ6RYX7vK0PJFdiAIXlyPQq0B3KaU\ne86IvFeQSFrJdCc0K8NfiH2G1loIk3fiR+YLqlXk6FAeKtpXJKxR1pCQCAM+vBCs\nmZudf1zCUZ8/4eodlHU=\n-----END PUBLIC KEY-----"

pubkey = OpenSSL::PKey::EC.new pubkey_pem
JWT.decode token, pubkey

Python test (using pyjwt):

import jwt

token = 'eyJhbGciOiJFUzUxMiIsInR5cCI6IkpXVCJ9.eyJoZWxsbyI6IndvcmxkIn0.AQx1MqdTni6KuzfOoedg2-7NUiwe-b88SWbdmviz40GTwrM0Mybp1i1tVtmTSQ91oEXGXBdtwsN6yalzP9J-sp2YATX_Tv4h-BednbdSvYxZsYnUoZ--ZUdL10t7g8Yt3y9hdY_diOjIptcha6ajX8yzkDGYG42iSe3f5LywSuD6FO5c'
pubkey_pem = '-----BEGIN PUBLIC KEY-----\nMIGbMBAGByqGSM49AgEGBSuBBAAjA4GGAAQAcpkss6wI7PPlxj3t7A1RqMH3nvL4\nL5Tzxze/XeeYZnHqxiX+gle70DlGRMqqOq+PJ6RYX7vK0PJFdiAIXlyPQq0B3KaU\ne86IvFeQSFrJdCc0K8NfiH2G1loIk3fiR+YLqlXk6FAeKtpXJKxR1pCQCAM+vBCs\nmZudf1zCUZ8/4eodlHU=\n-----END PUBLIC KEY-----'

jwt.decode(token, pubkey_pem)

[excpt]

@jurriaan Thanks for your contribution! 👍