def __init__(self, algorithm, length, sharedinfo, backend):

        max_len = algorithm.digest_size * (2 ** 32 - 1)
        if length > max_len:
            raise ValueError(
                "Can not derive keys larger than {0} bits.".format(max_len))
        if not (sharedinfo is None or isinstance(sharedinfo, bytes)):
            raise TypeError("sharedinfo must be bytes.")
        self._algorithm = algorithm
        self._length = length
        self._sharedinfo = sharedinfo

        if not isinstance(backend, HashBackend):
            raise UnsupportedAlgorithm(
                "Backend object does not implement HashBackend.",
                _Reasons.BACKEND_MISSING_INTERFACE
            )
        self._backend = backend
        self._used = False
dataset/ETHPy150Open cloudera/hue/desktop/core/ext-py/cryptography-1.3.1/src/cryptography/hazmat/primitives/kdf/x963kdf.py/X963KDF.__init__/original