def key_specification_from_public_key(public_key):

    """

    Derives key specification (algorithm and associated parameters)

    from the passed-in public key. Key specification can be used for

    generating the private keys via KeyGenerator instances.

    :param public_key: Public

    :type public_key: cryptography.hazmat.primitives.asymmetric.rsa.RSAPublicKey

    :returns: Key algorithm and parameter(s) for generating same type of keys as the passed-in public key.

    :rtype: tuple(str, int)

    :raises ValueError: If algorithm/parameters could not be derived from the passed-in public key.

    """

    if isinstance(public_key, cryptography.hazmat.primitives.asymmetric.rsa.RSAPublicKey):

        return "rsa", public_key.key_size

    raise ValueError("Unsupported public key instance passed-in: \"%s\" (%s)" % (str(public_key), type(public_key)))