Package Crypto :: Package Cipher :: Module CAST
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Module CAST

CAST-128 symmetric cipher

CAST-128 (or CAST5) is a symmetric block cipher specified in RFC2144.

It has a fixed data block size of 8 bytes. Its key can vary in length from 40 to 128 bits.

CAST is deemed to be cryptographically secure, but its usage is not widespread. Keys of sufficient length should be used to prevent brute force attacks (128 bits are recommended).

As an example, encryption can be done as follows:

>>> from Crypto.Cipher import CAST
>>> from Crypto import Random
>>> key = b'Sixteen byte key'
>>> iv =
>>> cipher =, CAST.MODE_OPENPGP, iv)
>>> plaintext = b'sona si latine loqueris '
>>> msg = cipher.encrypt(plaintext)
>>> eiv = msg[:CAST.block_size+2]
>>> ciphertext = msg[CAST.block_size+2:]
>>> cipher =, CAST.MODE_OPENPGP, eiv)
>>> print cipher.decrypt(ciphertext)
CAST-128 cipher object
new(key, *args, **kwargs)
Create a new CAST-128 cipher
  MODE_ECB = 1
Electronic Code Book (ECB). See blockalgo.MODE_ECB.
  MODE_CBC = 2
Cipher-Block Chaining (CBC). See blockalgo.MODE_CBC.
  MODE_CFB = 3
Cipher FeedBack (CFB). See blockalgo.MODE_CFB.
  MODE_PGP = 4
This mode should not be used.
  MODE_OFB = 5
Output FeedBack (OFB). See blockalgo.MODE_OFB.
  MODE_CTR = 6
CounTer Mode (CTR). See blockalgo.MODE_CTR.
OpenPGP Mode. See blockalgo.MODE_OPENPGP.
  block_size = 8
Size of a data block (in bytes)
  key_size = xrange(5, 17)
Size of a key (in bytes)
Function Details

new(key, *args, **kwargs)

Create a new CAST-128 cipher
  • key (byte string) - The secret key to use in the symmetric cipher. Its length may vary from 5 to 16 bytes.
  • mode (a MODE_* constant) - The chaining mode to use for encryption or decryption. Default is MODE_ECB.
  • IV (byte string) - The initialization vector to use for encryption or decryption.

    It is ignored for MODE_ECB and MODE_CTR.

    For MODE_OPENPGP, IV must be block_size bytes long for encryption and block_size +2 bytes for decryption (in the latter case, it is actually the encrypted IV which was prefixed to the ciphertext). It is mandatory.

    For all other modes, it must be block_size bytes longs. It is optional and when not present it will be given a default value of all zeroes.

  • counter (callable) - (Only MODE_CTR). A stateful function that returns the next counter block, which is a byte string of block_size bytes. For better performance, use Crypto.Util.Counter.
  • segment_size (integer) - (Only MODE_CFB).The number of bits the plaintext and ciphertext are segmented in. It must be a multiple of 8. If 0 or not specified, it will be assumed to be 8.
an CAST128Cipher object