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Posted By

weilawei on 05/10/12


Tagged

stream Encryption cipher alleged RC4 ARC4 arcfour


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ARC4

 / Published in: Python
 

This is a pure Python implementation of ARC4 as a generator to highlight its nature as a stream cipher. Several improvements can be made, for instance, it could take a nonce, use multiple state spaces (parallelizable), automatically discard the first 4K of the state space(s), use a more complex transformation than a simple swap, limit the # of bytes encrypted per nonce, etc.. The size of the state space is a parameter. The size of the key must not exceed the size of the state space, as additional key data will not be mixed into the prepared state.

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  1. #!/usr/bin/env python3
  2.  
  3. def prepare_state(key, state_size=256):
  4.     key_size    = len(key)
  5.     state       = [i for i in range(state_size)]
  6.     j           = 0
  7.  
  8.     for i in range(state_size):
  9.         j = (j + state[i] + key[i % key_size]) % state_size
  10.  
  11.         state[i] ^= state[j]
  12.         state[j] ^= state[i]
  13.         state[i] ^= state[j]
  14.  
  15.     return state
  16.  
  17. def pseudorandom_generator(state):
  18.     size    = len(state)
  19.     i       = 0
  20.     j       = 0
  21.  
  22.     while True:
  23.         i = (i + 1) % size
  24.         j = (j + state[i]) % size
  25.  
  26.         state[i] ^= state[j]
  27.         state[j] ^= state[i]
  28.         state[i] ^= state[j]
  29.  
  30.         yield state[(state[i] + state[j]) % size]
  31.  
  32. def ARC4(key, text):
  33.     state       = prepare_state(key)
  34.     keystream   = pseudorandom_generator(state)
  35.  
  36.     for key_byte, text_byte in zip(keystream, text):
  37.         yield key_byte ^ text_byte
  38.  
  39. #   Test Vectors for ARC4
  40. #   ---------------------
  41. #
  42. #   key:        b'Key'
  43. #   plaintext:  b'Plaintext'
  44. #   ciphertext: b'\xbb\xf3\x16\xe8\xd9@\xaf\n\xd3'
  45. #
  46. #   key:        b'Wiki'
  47. #   plaintext:  b'pedia'
  48. #   ciphertext: b'\x10!\xbf\x04 '
  49. #
  50. #   key:        b'Secret'
  51. #   plaintext:  b'Attack at dawn'
  52. #   ciphertext: b'E\xa0\x1fd_\xc3[85RTK\x9b\xf5'
  53.  
  54. #   Example usage
  55. if (__name__ == "__main__"):
  56.     from pprint import pprint as pp
  57.  
  58.     encryption  = ARC4(b'Key', b'Plaintext')
  59.     ciphertext  = bytes([next(encryption) for i in range(len(b'Plaintext'))])
  60.     pp(ciphertext)
  61.  
  62.     decryption  = ARC4(b'Key', ciphertext)
  63.     plaintext   = bytes([next(decryption) for i in range(len(ciphertext))])
  64.     pp(plaintext)

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