def _cbRequestAvatarId(self, validKey, credentials):
        """
        Check whether the credentials themselves are valid, now that we know
        if the key matches the user.

        @param validKey: A boolean indicating whether or not the public key
            matches a key in the user's authorized_keys file.

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

        @raise UnauthorizedLogin: (as a failure) if the key does not match the
            user in C{credentials}. Also raised if the user provides an invalid
            signature.

        @raise ValidPublicKey: (as a failure) if the key matches the user but
            the credentials do not include a signature. See
            L{error.ValidPublicKey} for more information.

        @return: The user's username, if authentication was successful.
        """
        if not validKey:
            return failure.Failure(UnauthorizedLogin("invalid key"))
        if not credentials.signature:
            return failure.Failure(error.ValidPublicKey())
        else:
            try:
                pubKey = keys.Key.fromString(credentials.blob)
                if pubKey.verify(credentials.signature, credentials.sigData):
                    return credentials.username
            except: # any error should be treated as a failed login
                log.err()
                return failure.Failure(UnauthorizedLogin('error while verifying key'))
        return failure.Failure(UnauthorizedLogin("unable to verify key"))

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

        @param validKey: A boolean indicating whether or not the public key
            matches a key in the user's authorized_keys file.

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

        @raise UnauthorizedLogin: (as a failure) if the key does not match the
            user in C{credentials}. Also raised if the user provides an invalid
            signature.

        @raise ValidPublicKey: (as a failure) if the key matches the user but
            the credentials do not include a signature. See
            L{error.ValidPublicKey} for more information.

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