Keycloakauthenticator bug: signature verification failed when keycloak jwks_uri return multiple certs

[drJabber]

Hi team,

there's a bug in keycloak authenticator. When keycloak jwks_uri returns multiple certs and cert with index 0 is e.g. cert for encryption - client fails signature verification.

The error in in here:

jupyterhub-extensions/KeyCloakAuthenticator/keycloakauthenticator/auth.py

Line 176 in 0997d9b

self.public_key = RSAAlgorithm(RSAAlgorithm.SHA256).from_jwk(jwk_data['keys'][0])

I think it could be fixed with code like this:

                    for key in jwk_data['keys']:
                        if key['use'] == 'sig':
                            self.public_key = RSAAlgorithm(RSAAlgorithm.SHA256).from_jwk(key)
                            self.log.info(f"aquired public key from {jwks_uri}")
                            break