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# -*- coding: utf-8 -*-
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# Cipher/ARC4.py : ARC4
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# ===================================================================
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# The contents of this file are dedicated to the public domain. To
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# the extent that dedication to the public domain is not available,
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# everyone is granted a worldwide, perpetual, royalty-free,
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# non-exclusive license to exercise all rights associated with the
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# contents of this file for any purpose whatsoever.
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# No rights are reserved.
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# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
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# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
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# BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
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# ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
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# CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
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# ===================================================================
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"""ARC4 symmetric cipher
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ARC4_ (Alleged RC4) is an implementation of RC4 (Rivest's Cipher version 4),
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a symmetric stream cipher designed by Ron Rivest in 1987.
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The cipher started as a proprietary design, that was reverse engineered and
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anonymously posted on Usenet in 1994. The company that owns RC4 (RSA Data
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Inc.) never confirmed the correctness of the leaked algorithm.
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Unlike RC2, the company has never published the full specification of RC4,
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of whom it still holds the trademark.
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ARC4 keys can vary in length from 40 to 2048 bits.
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One problem of ARC4 is that it does not take a nonce or an IV. If it is required
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to encrypt multiple messages with the same long-term key, a distinct
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independent nonce must be created for each message, and a short-term key must
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be derived from the combination of the long-term key and the nonce.
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Due to the weak key scheduling algorithm of RC2, the combination must be carried
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out with a complex function (e.g. a cryptographic hash) and not by simply
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concatenating key and nonce.
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New designs should not use ARC4. A good alternative is AES
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(`Crypto.Cipher.AES`) in any of the modes that turn it into a stream cipher (OFB, CFB, or CTR).
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As an example, encryption can be done as follows:
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>>> from Crypto.Cipher import ARC4
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>>> from Crypto.Hash import SHA
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>>> from Crypto import Random
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>>> key = b'Very long and confidential key'
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>>> nonce = Random.new().read(16)
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>>> tempkey = SHA.new(key+nonce).digest()
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>>> cipher = ARC4.new(tempkey)
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>>> msg = nonce + cipher.encrypt(b'Open the pod bay doors, HAL')
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.. _ARC4: http://en.wikipedia.org/wiki/RC4
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:undocumented: __revision__, __package__
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from Crypto.Cipher import _ARC4
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"""ARC4 cipher object"""
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def __init__(self, key, *args, **kwargs):
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"""Initialize an ARC4 cipher object
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See also `new()` at the module level."""
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self._cipher = _ARC4.new(key, *args, **kwargs)
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self.block_size = self._cipher.block_size
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self.key_size = self._cipher.key_size
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def encrypt(self, plaintext):
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"""Encrypt a piece of data.
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plaintext : byte string
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The piece of data to encrypt. It can be of any size.
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:Return: the encrypted data (byte string, as long as the
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return self._cipher.encrypt(plaintext)
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def decrypt(self, ciphertext):
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"""Decrypt a piece of data.
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ciphertext : byte string
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The piece of data to decrypt. It can be of any size.
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:Return: the decrypted data (byte string, as long as the
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return self._cipher.decrypt(ciphertext)
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def new(key, *args, **kwargs):
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"""Create a new ARC4 cipher
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The secret key to use in the symmetric cipher.
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It can have any length, with a minimum of 40 bytes.
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Its cryptograpic strength is always capped to 2048 bits (256 bytes).
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:Return: an `ARC4Cipher` object
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return ARC4Cipher(key, *args, **kwargs)
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#: Size of a data block (in bytes)
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#: Size of a key (in bytes)
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key_size = xrange(1,256+1)
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lib/Crypto/Cipher/ARC4.py
