Package Crypto :: Package Cipher :: Module ARC2
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Module ARC2

RC2 symmetric cipher

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.

New designs should not use RC2.

As an example, encryption can be done as follows:

>>> from Crypto.Cipher import ARC2
>>> from Crypto import Random
>>> key = b'Sixteen byte key'
>>> iv =
>>> cipher =, ARC2.MODE_CFB, iv)
>>> msg = iv + cipher.encrypt(b'Attack at dawn')
RC2 cipher object
new(key, *args, **kwargs)
Create a new RC2 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(1, 17)
Size of a key (in bytes)
Function Details

new(key, *args, **kwargs)

Create a new RC2 cipher
  • key (byte string) - The secret key to use in the symmetric cipher. Its length can vary from 1 to 128 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.
  • effective_keylen (integer) - Maximum cryptographic strength of the key, in bits. It can vary from 0 to 1024. The default value is 1024.
an RC2Cipher object