#! /usr/bin/env python
# -*- coding: utf-8 -*-
import sys

def init_sbox(key):
    s = range(256)
    j = 0
    for i in range(256):
        j = (j + s[i] + ord(key[i % len(key)])) % 256
        s[i], s[j] = s[j], s[i]
    return s

def crypt(text, key):
    buf = list(text)
    i = j = 0
    s = init_sbox(key)
    for n in xrange(len(text)):
        i = (i + 1) % 256
        j = (j + s[i]) % 256
        s[i], s[j] = s[j], s[i]
        rand = s[(s[i] + s[j]) % 256]
        buf[n] = chr(rand ^ ord(buf[n]))

    return "".join(buf)

try:
    with open(sys.argv[1], "r+b") as data:
        new_content = crypt(data.read(), sys.argv[2])
        data.seek(0)
        data.write(new_content)
except IndexError:
    print "usage: rc4.py file key"