RC4 generates a pseudorandom stream of bits (a keystream) which, for encryption, is combined with the plaintext using bit-wise exclusive-or; decryption is performed the same way (since exclusive-or is a symmetric operation).
(see http://en.wikipedia.org/wiki/RC4)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 | #!/usr/bin/env python
#
# rc4.py - RC4, ARC4, ARCFOUR algorithm with random salt
#
# Copyright (c) 2009 joonis new media
# Author: Thimo Kraemer <thimo.kraemer@joonis.de>
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
# MA 02110-1301, USA.
#
import random, base64
def crypt(data, key):
"""RC4 algorithm"""
x = 0
box = range(256)
for i in range(256):
x = (x + box[i] + ord(key[i % len(key)])) % 256
box[i], box[x] = box[x], box[i]
x = y = 0
out = []
for char in data:
x = (x + 1) % 256
y = (y + box[x]) % 256
box[x], box[y] = box[y], box[x]
out.append(chr(ord(char) ^ box[(box[x] + box[y]) % 256]))
return ''.join(out)
def encrypt(data, key, encode=base64.standard_b64encode, salt_length=8):
"""RC4 encryption with random salt and final encoding"""
salt = ''
for n in range(salt_length):
salt += chr(random.randrange(256))
data = salt + crypt(data, key + salt)
if encode:
data = encode(data)
return data
def decrypt(data, key, decode=base64.standard_b64decode, salt_length=8):
"""RC4 decryption of encoded data"""
if decode:
data = decode(data)
salt = data[:salt_length]
return crypt(data[salt_length:], key + salt)
if __name__ == '__main__':
for i in range(10):
data = encrypt('secret message', 'my-key')
print data
print decrypt(data, 'my-key')
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