arc4-drop8192-rot256.py

import sys
def ksa(key):
	s = []
	for i in range(0, 256):
		s.append(i)
	j = 0
	for i in range(0, 256):
		j = ((j + s[i] + ord(key[i % len(key)])) % 256)
		t = s[i]
		s[i] = s[j]
		s[j] = t
	return s
def rot(inp, num):
	t = (inp + num)
	while (t > 255):
		t = (t % 256)
	while (t < 0):
		t = (t + 256)
	return t
def cipher(inp, key, dir):
	i = 0
	j = 0
	x = 0
	s = ksa(key)
	o = ""
	while (x < 8192):
		i = ((i + 1) % 256)
		j = ((j + s[i]) % 256)
		t = s[i]
		s[i] = s[j]
		s[j] = t
		x += 1
	x = 0
	while (x < len(inp)):
		i = ((i + 1) % 256)
		j = ((j + s[i]) % 256)
		t = s[i]
		s[i] = s[j]
		s[j] = t
		k = s[(s[i] + s[j]) % 256]
		o = (o + chr(rot(ord(inp[x]), dir * k)))
		x += 1
	return o
sys.stdout.write(cipher(sys.stdin.read(), sys.argv[1], int(sys.argv[2])))

$ echo "secret messageAAAA" | python arc4-drop8192-rot256.py "secret kez" "1" | hexdump -C
00000000  0c 06 1d 6c 9f 24 cb 6e  10 5f 5d 87 10 2e 2b 9a  |...l.$.n._]...+.|
00000010  90 1c b2                                          |...|
00000013
$ echo "secret messageAAAA" | python arc4-drop8192-rot256.py "secret kez" "1" | python arc4-drop8192-rot256.py "secret kez" "-1"
secret messageAAAA