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).
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#!/usr/bin/env python # # RC4, ARC4, ARCFOUR algorithm # # Copyright (c) 2009 joonis new media # Author: Thimo Kraemer <firstname.lastname@example.org> # # 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. # def rc4crypt(data, key): 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 = 0 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)