Package Cryptodome :: Package Cipher :: Module CAST

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).

Use AES, not CAST. This module is only provided for legacy purposes.

As an example, encryption can be done as follows:

>>> from Cryptodome.Cipher import CAST
>>>
>>> key = b'Sixteen byte key'
>>> cipher = CAST.new(key, CAST.MODE_OPENPGP)
>>> plaintext = b'sona si latine loqueris '
>>> msg = cipher.encrypt(plaintext)
>>>
...
>>> eiv = msg[:CAST.block_size+2]
>>> ciphertext = msg[CAST.block_size+2:]
>>> cipher = CAST.new(key, CAST.MODE_OPENPGP, eiv)
>>> print cipher.decrypt(ciphertext)
Functions
 
new(key, mode, *args, **kwargs)
Create a new CAST-128 cipher
Variables
  MODE_ECB = 1
Electronic Code Book (ECB). See Cryptodome.Cipher._mode_ecb.EcbMode.
  MODE_CBC = 2
Cipher-Block Chaining (CBC). See Cryptodome.Cipher._mode_cbc.CbcMode.
  MODE_CFB = 3
Cipher FeedBack (CFB). See Cryptodome.Cipher._mode_cfb.CfbMode.
  MODE_OFB = 5
Output FeedBack (OFB). See Cryptodome.Cipher._mode_ofb.OfbMode.
  MODE_CTR = 6
CounTer Mode (CTR). See Cryptodome.Cipher._mode_ctr.CtrMode.
  MODE_OPENPGP = 7
OpenPGP Mode. See Cryptodome.Cipher._mode_openpgp.OpenPgpMode.
  MODE_EAX = 9
EAX Mode. See Cryptodome.Cipher._mode_eax.EaxMode.
  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, mode, *args, **kwargs)

 

Create a new CAST-128 cipher

Parameters:
  • key (byte string) - The secret key to use in the symmetric cipher. Its length may vary from 5 to 16 bytes. The recommended length is 16 bytes.
  • mode (a MODE_* constant) - The chaining mode to use for encryption or decryption.
  • iv (byte string) - (Only MODE_CBC, MODE_CFB, MODE_OFB, MODE_OPENPGP).

    The initialization vector to use for encryption or decryption.

    For MODE_OPENPGP, IV must be 8 bytes long for encryption and 10 bytes for decryption (in the latter case, it is actually the encrypted IV which was prefixed to the ciphertext).

    For all other modes, it must be 8 bytes long.

    If not provided, a random byte string will be generated (you can read it back via the iv attribute).

  • nonce (byte string) - (Only MODE_EAX and MODE_CTR) A mandatory value that must never be reused for any other encryption.

    For MODE_CTR, its length must be in the range [0..7].

    For MODE_EAX, there are no restrictions, but it is recommended to use at least 16 bytes.

    If not provided for MODE_EAX, a random 16 byte string will be generated (you can read it back via the nonce attribute).

  • mac_len (integer) - (Only MODE_EAX). Length of the authentication tag, in bytes. It must be no larger than 8 (which is the default).
  • 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 not specified, it will be assumed to be 8.
  • initial_value (integer) - (Only MODE_CTR). The initial value for the counter within the counter block. By default it is 0.
Returns:

a CAST cipher object, of the applicable mode: