Fix nonce validation

[timhallmann]

Require a valid nonce if the request contained one.

[lilioid]

The specification calls for us to do this validation:

If a nonce value was sent in the Authentication Request, a nonce Claim MUST be present and its value checked to verify that it is the same value as the one that was sent in the Authentication Request. The Client SHOULD check the nonce value for replay attacks. The precise method for detecting replay attacks is Client specific.

The way I interpret this is that if there is a nonce present in the token, we MUST validate it. This means that not passing a nonce to validate_extern() should produce an error.

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I'd be open to adding an additional error message for this specific case or rewording the existing one to be more clear though.

[timhallmann]

We must validate the nonce if one was sent in the Authentication Request (not necessarily the Token Response / ID Token). That's what my patch enforces: If the caller passes a nonce to validate_extern, we'll check the nonce in the ID token. We do not permit an ID token to skip this check by not including a nonce.

Furthermore, if the ID token contains a nonce, but the caller does not pass one to validate_extern, we'll validate anyway and fail, thereby matching the old behavior. AFAIK, this is undefined behavior - but a conforming OP is not supposed to add a nonce on a whim, so it should be fine to be strict.

[lilioid]

I see. Thanks for taking the time to explain it :)