Python Implementation of Courtois Toy Cipher (CTC)

CTC.py

'''
Python Implementation of Courtois Toy Cipher (CTC)
CTC is a toy cipher presented for sake of cryptanalysis using algebraic attacks in http://eprint.iacr.org/2006/168.pdf.
'''

def xorRoundKey(state,roundkey):
	
	return state ^ roundkey

	
def sLayer(state, B):
	
	SBox = list([7, 6, 0, 4, 2, 5, 1, 3])
	output = 0
	for i in xrange(B):
		output |= SBox[(state>>(i*3))&0x7]<<(i*3)
	return output
		

def pLayer(state, B):
	
	output = (state&0x01)<<2
	for i in xrange(1,3*B):
		output |= (((state>>i)&0x01) ^ ((state>>(i+137)%255)&0x01)) << ( (i*202+2)%255 )
	return output
		

def encrypt(plain, key, B, R):

	assert B>0 and R>0
	assert len(key) == R+1
	assert len(bin(plain))-2 <= 3*B
	for k in key:
		assert len(bin(k))-2 <= 3*B

	state = plain
	for i in xrange(R-1):
		state = xorRoundKey(state,key[i])
		state = sLayer(state, B)
		state = pLayer(state, B)
	cipher = xorRoundKey(state,key[-1])
	return cipher


if __name__ == '__main__':
	
	B = 85	#number of paralleled sboxes
	R = 6	#number of rounds
	key = [0,0,0,0,0,0,0]
	
	from random import randint
	plain = randint(0, (1<<3*B))
	cipher = encrypt(plain, key, B, R)
	
	print 'plain \t', bin(plain)
	print 'cipher \t', bin(cipher)