rc4.py

# https://en.wikipedia.org/wiki/RC4
# dsandler 2019

class cipher(object):
  S = None

  def __init__(this, key):
    this.key = key
    this.reset()

  def reset(this):
    key = [ord(x) for x in this.key]
    keylen = len(key)
    this.S = range(256)
    S = this.S
    j = 0
    for i in range(256):
      j = (j + S[i] + key[i % keylen]) % 256
      S[i], S[j] = S[j], S[i]

    this.keystream = this.start_keystream()

  def start_keystream(this):
    S = this.S
    i = 0
    j = 0
    while True:
      i = (i + 1) % 256
      j = (j + S[i]) % 256
      S[i], S[j] = S[j], S[i]
      yield S[(S[i] + S[j]) % 256]

  def code(this, plain):
    return (chr(ord(b) ^ this.keystream.next()) for b in plain)