Verify that the activation key is valid and within the
        permitted activation time window, returning the username if
        valid or ``None`` if not.

def validate_key(self, activation_key):
        """
        Verify that the activation key is valid and within the
        permitted activation time window, returning the username if
        valid or ``None`` if not.

        """
        try:
            username = signing.loads(
                activation_key,
                salt=self.key_salt,
                max_age=conf.get('ACCOUNT_ACTIVATION_DAYS') * 86400
            )
            return username
        # SignatureExpired is a subclass of BadSignature, so this will
        # catch either one.
        except signing.BadSignature:
            return None