'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}'

def _sanityCheckKey(self, credentials):

    if (not credentials.signature):
        raise error.ValidPublicKey()
    return keys.Key.fromString(credentials.blob)
