def _sanityCheckKey(self, credentials):
        """
        Checks whether the provided credentials are a valid SSH key with a
        signature (does not actually verify the signature).

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

        @raise ValidPublicKey: the credentials do not include a signature. See
            L{error.ValidPublicKey} for more information.

        @raise BadKeyError: The key included with the credentials is not
            recognized as a key.

        @return: the key in the credentials
        @rtype: L{twisted.conch.ssh.keys.Key}
        """
        if not credentials.signature:
            raise error.ValidPublicKey()

        return keys.Key.fromString(credentials.blob)

        Checks whether the provided credentials are a valid SSH key with a
        signature (does not actually verify the signature).

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

        @raise ValidPublicKey: the credentials do not include a signature. See
            L{error.ValidPublicKey} for more information.

        @raise BadKeyError: The key included with the credentials is not
            recognized as a key.

        @return: the key in the credentials
        @rtype: L{twisted.conch.ssh.keys.Key}
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/conch/checkers.py