Package Crypto :: Package Cipher :: Module ARC4
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Module ARC4

ARC4 symmetric cipher

ARC4 (Alleged RC4) is an implementation of RC4 (Rivest's Cipher version 4), a symmetric stream cipher designed by Ron Rivest in 1987.

The cipher started as a proprietary design, that was reverse engineered and anonymously posted on Usenet in 1994. The company that owns RC4 (RSA Data Inc.) never confirmed the correctness of the leaked algorithm.

Unlike RC2, the company has never published the full specification of RC4, of whom it still holds the trademark.

ARC4 keys can vary in length from 40 to 2048 bits.

One problem of ARC4 is that it does not take a nonce or an IV. If it is required to encrypt multiple messages with the same long-term key, a distinct independent nonce must be created for each message, and a short-term key must be derived from the combination of the long-term key and the nonce. Due to the weak key scheduling algorithm of RC2, the combination must be carried out with a complex function (e.g. a cryptographic hash) and not by simply concatenating key and nonce.

New designs should not use ARC4. A good alternative is AES (Crypto.Cipher.AES) in any of the modes that turn it into a stream cipher (OFB, CFB, or CTR).

As an example, encryption can be done as follows:

>>> from Crypto.Cipher import ARC4
>>> from Crypto.Hash import SHA
>>> from Crypto import Random
>>> key = b'Very long and confidential key'
>>> nonce =
>>> tempkey =
>>> cipher =
>>> msg = nonce + cipher.encrypt(b'Open the pod bay doors, HAL')
ARC4 cipher object
new(key, *args, **kwargs)
Create a new ARC4 cipher
  block_size = 1
Size of a data block (in bytes)
  key_size = xrange(1, 257)
Size of a key (in bytes)
Function Details

new(key, *args, **kwargs)

Create a new ARC4 cipher
  • key (byte string) - The secret key to use in the symmetric cipher. It can have any length, with a minimum of 40 bytes. Its cryptograpic strength is always capped to 2048 bits (256 bytes).
an ARC4Cipher object