rc4.py

from binascii import hexlify, unhexlify
from hexdump import hexdump

def rotate(l, n):
    return l[-n:] + l[:-n]

def convert_key(s):
    return [ord(c) for c in s]

def KSA(key, rot):
    keylength = len(key)

    S = rotate(range(256), rot)

    j = 0
    for i in range(256):
        j = (j + S[i] + key[i % keylength]) % 256
        S[i], S[j] = S[j], S[i]  # swap

    return S


def PRGA(S):
    i = 0
    j = 0
    while True:
        i = (i + 1) % 256
        j = (j + S[i]) % 256
        S[i], S[j] = S[j], S[i]  # swap

        K = S[(S[i] + S[j]) % 256]
        yield K


def RC4(key, rot):
    S = KSA(key, rot)
    return PRGA(S)


if __name__ == '__main__':
    # test vectors are from http://en.wikipedia.org/wiki/RC4

    # ciphertext should be BBF316E8D940AF0AD3
    #key = 'Key'
    #plaintext = 'Plaintext'
    key = '4b6579'.decode('hex')
    plaintext = '506c61696e74657874'.decode('hex')
    print key, plaintext

    # encrypt
    key = convert_key(key)
    keystream = RC4(key, 0)

    output = ''
    for c in plaintext:
        char = ord(c) ^ keystream.next()
        output += "%02X" % char
    print 'verifying encryption. output should be BBF316E8D940AF0AD3'
    hexdump(unhexlify(output))
    print

    # decrypt
    keystream = RC4(key, 0)
    ciphertext = 'BBF316E8D940AF0AD3'.decode('hex')

    output = ''
    for c in ciphertext:
        char = ord(c) ^ keystream.next()
        output += "%02X" % char
    print 'verifying decryption. output should be Plaintext'
    hexdump(unhexlify(output))
    print
    
    key = 'C28222B7B746D25A8F180FFCFADCFF11'.decode('hex')
    msg = '6cd022c2da6dc500105a9274cab92177177527d683fad205fb002071d7931f67170a90080f92'.decode('hex')

    key = convert_key(key)

    for i in range(256):
        keystream = RC4(key, i)

        output = ''
        for c in msg:
            char = ord(c) ^ keystream.next()
            output += "%02X" % char

        print 'offset: %d' % i
        hexdump(unhexlify(output))
        print