def __init__(self, backend, cipher, mode, operation):
        self._backend = backend
        self._cipher = cipher
        self._mode = mode
        self._operation = operation
        # There is a bug in CommonCrypto where block ciphers do not raise
        # kCCAlignmentError when finalizing if you supply non-block aligned
        # data. To work around this we need to keep track of the block
        # alignment ourselves, but only for alg+mode combos that require
        # block alignment. OFB, CFB, and CTR make a block cipher algorithm
        # into a stream cipher so we don't need to track them (and thus their
        # block size is effectively 1 byte just like OpenSSL/CommonCrypto
        # treat RC4 and other stream cipher block sizes).
        # This bug has been filed as rdar://15589470
        self._bytes_processed = 0
        if (isinstance(cipher, ciphers.BlockCipherAlgorithm) and not
                isinstance(mode, (OFB, CFB, CFB8, CTR))):
            self._byte_block_size = cipher.block_size // 8
        else:
            self._byte_block_size = 1

        registry = self._backend._cipher_registry
        try:
            cipher_enum, mode_enum = registry[type(cipher), type(mode)]
        except __HOLE__:
            raise UnsupportedAlgorithm(
                "cipher {0} in {1} mode is not supported "
                "by this backend.".format(
                    cipher.name, mode.name if mode else mode),
                _Reasons.UNSUPPORTED_CIPHER
            )

        ctx = self._backend._ffi.new("CCCryptorRef *")
        ctx = self._backend._ffi.gc(ctx, self._backend._release_cipher_ctx)

        if isinstance(mode, modes.ModeWithInitializationVector):
            iv_nonce = mode.initialization_vector
        elif isinstance(mode, modes.ModeWithNonce):
            iv_nonce = mode.nonce
        else:
            iv_nonce = self._backend._ffi.NULL

        if isinstance(mode, CTR):
            mode_option = self._backend._lib.kCCModeOptionCTR_BE
        else:
            mode_option = 0

        res = self._backend._lib.CCCryptorCreateWithMode(
            operation,
            mode_enum, cipher_enum,
            self._backend._lib.ccNoPadding, iv_nonce,
            cipher.key, len(cipher.key),
            self._backend._ffi.NULL, 0, 0, mode_option, ctx)
        self._backend._check_cipher_response(res)

        self._ctx = ctx
dataset/ETHPy150Open cloudera/hue/desktop/core/ext-py/cryptography-1.3.1/src/cryptography/hazmat/backends/commoncrypto/ciphers.py/_CipherContext.__init__