Impossible to know if expired token is valid or not

[brycx]

Because the token expiration check is performed before authentication and decryption, it's impossible to know if an expired token is valid or invalid.

An attacker may modify the timestamp, in the header, to make the token expire but the user would never know that it had been modified.

[tuupola]

Good catch. I seem to have same problem with pybranca. I think I should also mention this in the spec.