Checks that the verifier contains only safe characters
        and is no shorter than lower and no longer than upper.

def check_verifier(self, verifier):
        """Checks that the verifier contains only safe characters
        and is no shorter than lower and no longer than upper.
        """
        lower, upper = self.verifier_length
        return (set(verifier) <= self.safe_characters and
                lower <= len(verifier) <= upper)