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 = Random.new().read(16)
>>> tempkey = SHA.new(key+nonce).digest()
>>> cipher = ARC4.new(tempkey)
>>> msg = nonce + cipher.encrypt(b'Open the pod bay doors, HAL')
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new(key,
*args,
**kwargs)
Create a new ARC4 cipher |
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block_size = 1
Size of a data block (in bytes)
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key_size = xrange(1, 257)
Size of a key (in bytes)
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new(key,
*args,
**kwargs)
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Create a new ARC4 cipher
- Parameters:
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).
- Returns:
- an ARC4Cipher object
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