def _verifyKey(self, pubKey, credentials):
        """
        Checks whether the credentials themselves are valid, now that we know
        if the key matches the user.

        @param pubKey: the key in the credentials (just to prevent it from
            having to be calculated again)
        @type pubKey: L{twisted.conch.ssh.keys.Key}

        @param credentials: the credentials offered by the user
        @type credentials: L{ISSHPrivateKey} provider

        @raise UnauthorizedLogin: If the key signature is invalid or there
            was any error verifying the signature.

        @return: The user's username, if authentication was successful
        @rtype: L{bytes}
        """
        try:
            if pubKey.verify(credentials.signature, credentials.sigData):
                return credentials.username
        except:  # Any error should be treated as a failed login
            log.err()
            raise UnauthorizedLogin('Error while verifying key')

        raise UnauthorizedLogin("Key signature invalid.")

        Checks whether the credentials themselves are valid, now that we know
        if the key matches the user.

        @param pubKey: the key in the credentials (just to prevent it from
            having to be calculated again)
        @type pubKey: L{twisted.conch.ssh.keys.Key}

        @param credentials: the credentials offered by the user
        @type credentials: L{ISSHPrivateKey} provider

        @raise UnauthorizedLogin: If the key signature is invalid or there
            was any error verifying the signature.

        @return: The user's username, if authentication was successful
        @rtype: L{bytes}
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/conch/checkers.py