basic functionalty

modules/Crypto/Cipher/ARC4.py

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+# -*- coding: utf-8 -*-
+#
+#  Cipher/ARC4.py : ARC4
+#
+# ===================================================================
+# The contents of this file are dedicated to the public domain.  To
+# the extent that dedication to the public domain is not available,
+# everyone is granted a worldwide, perpetual, royalty-free,
+# non-exclusive license to exercise all rights associated with the
+# contents of this file for any purpose whatsoever.
+# No rights are reserved.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+# BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+# ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+# CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+# SOFTWARE.
+# ===================================================================
+"""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')
+
+.. _ARC4: http://en.wikipedia.org/wiki/RC4
+
+:undocumented: __revision__, __package__
+"""
+
+__revision__ = "$Id$"
+
+from Crypto.Util.py3compat import *
+from Crypto.Cipher import _ARC4
+
+class ARC4Cipher:
+    """ARC4 cipher object"""
+
+
+    def __init__(self, key, *args, **kwargs):
+        """Initialize an ARC4 cipher object
+        
+        See also `new()` at the module level."""
+
+        if len(args)>0:
+            ndrop = args[0]
+            args = args[1:]
+        else:
+            ndrop = kwargs.get('drop', 0)
+            if ndrop: del kwargs['drop'] 
+        self._cipher = _ARC4.new(key, *args, **kwargs)
+        if ndrop:
+            # This is OK even if the cipher is used for decryption, since encrypt
+            # and decrypt are actually the same thing with ARC4.
+            self._cipher.encrypt(b('\x00')*ndrop)
+
+        self.block_size = self._cipher.block_size
+        self.key_size = self._cipher.key_size
+
+    def encrypt(self, plaintext):
+        """Encrypt a piece of data.
+
+        :Parameters:
+          plaintext : byte string
+            The piece of data to encrypt. It can be of any size.
+        :Return: the encrypted data (byte string, as long as the
+          plaintext).
+        """
+        return self._cipher.encrypt(plaintext)
+
+    def decrypt(self, ciphertext):
+        """Decrypt a piece of data.
+
+        :Parameters:
+          ciphertext : byte string
+            The piece of data to decrypt. It can be of any size.
+        :Return: the decrypted data (byte string, as long as the
+          ciphertext).
+        """
+        return self._cipher.decrypt(ciphertext)
+
+def new(key, *args, **kwargs):
+    """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).
+    :Keywords:
+      drop : integer
+        The amount of bytes to discard from the initial part of the keystream.
+        In fact, such part has been found to be distinguishable from random
+        data (while it shouldn't) and also correlated to key.
+        
+        The recommended value is 3072_ bytes. The default value is 0.
+
+    :Return: an `ARC4Cipher` object
+
+    .. _3072: http://eprint.iacr.org/2002/067.pdf
+    """
+    return ARC4Cipher(key, *args, **kwargs)
+
+#: Size of a data block (in bytes)
+block_size = 1
+#: Size of a key (in bytes)
+key_size = xrange(1,256+1)
+