# RC2

Warning

Use AES instead. This module is provided only for legacy purposes.

RC2 (Rivest’s Cipher version 2) is a symmetric block cipher designed by Ron Rivest in 1987. The cipher started as a proprietary design, that was reverse engineered and anonymously posted on Usenet in 1996. For this reason, the algorithm was first called Alleged RC2 (ARC2), since the company that owned RC2 (RSA Data Inc.) did not confirm whether the details leaked into public domain were really correct.

The company eventually published its full specification in RFC2268.

RC2 has a fixed data block size of 8 bytes. Length of its keys can vary from 8 to 128 bits. One particular property of RC2 is that the actual cryptographic strength of the key (effective key length) can be reduced via a parameter.

Even though RC2 is not cryptographically broken, it has not been analyzed as thoroughly as AES, which is also faster than RC2.

As an example, encryption can be done as follows:

    >>> from Crypto.Cipher import ARC2
    >>>
    >>> key = b'Sixteen byte key'
    >>> cipher = ARC2.new(key, ARC2.MODE_CFB)
    >>> msg = cipher.iv + cipher.encrypt(b'Attack at dawn')

Module’s constants for the modes of operation supported with ARC2:

var MODE_ECB:

  Electronic Code Book (ECB)

var MODE_CBC:

  Cipher-Block Chaining (CBC)

var MODE_CFB:

  Cipher FeedBack (CFB)

var MODE_OFB:

  Output FeedBack (OFB)

var MODE_CTR:

  CounTer Mode (CTR)

var MODE_OPENPGP:

  OpenPGP Mode

var MODE_EAX:

  EAX Mode

Crypto.Cipher.ARC2.new(key, mode, *args, **kwargs)

  Create a new RC2 cipher.

  Parameters:

    - key (bytes, bytearray, memoryview) – The secret key to use in the symmetric cipher. Its length can vary from 5 to 128 bytes; the actual search space (and the cipher strength) can be reduced with the `effective_keylen` parameter.

    - mode (One of the supported `MODE_*` constants) – The chaining mode to use for encryption or decryption.

  Keyword Arguments:

    - iv (bytes, bytearray, memoryview) – (Only applicable for `MODE_CBC`, `MODE_CFB`, `MODE_OFB`, and `MODE_OPENPGP` modes).

      The initialization vector to use for encryption or decryption.

      For `MODE_CBC`, `MODE_CFB`, and `MODE_OFB` it must be 8 bytes long.

      For `MODE_OPENPGP` mode only, it 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).

      If not provided, a random byte string is generated (you must then read its value with the `iv` attribute).

    - nonce (bytes, bytearray, memoryview) – (Only applicable for `MODE_EAX` and `MODE_CTR`).

      A value that must never be reused for any other encryption done with this key.

      For `MODE_EAX` there are no restrictions on its length (recommended: 16 bytes).

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

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

    - effective_keylen (integer) – Optional. Maximum strength in bits of the actual key used by the ARC2 algorithm. If the supplied `key` parameter is longer (in bits) of the value specified here, it will be weakened to match it. If not specified, no limitation is applied.

    - 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.

    - mac_len : (integer) – (Only `MODE_EAX`) Length of the authentication tag, in bytes. It must be no longer than 8 (default).

    - initial_value : (integer) – (Only `MODE_CTR`). The initial value for the counter within the counter block. By default it is 0.

  Return:

    an ARC2 object, of the applicable mode.