84 lines
2.5 KiB
Python
84 lines
2.5 KiB
Python
#!/usr/bin/env python3
|
|
from random import randint
|
|
from math import log2
|
|
|
|
class lfsr(object):
|
|
def __init__(self, state, taps):
|
|
self.state = state
|
|
self.taps = taps
|
|
|
|
# getters and setters (private attributes)
|
|
def get_state(self):
|
|
return self.__state
|
|
def set_state(self, value):
|
|
self.__state = value
|
|
state = property(fget=get_state, fset=set_state, doc="lfsr state (current bits value in lfsr)")
|
|
|
|
def get_taps(self):
|
|
return self.__taps
|
|
def set_taps(self, valeur):
|
|
self.__taps = valeur
|
|
taps = property(fget=get_taps, fset=set_taps, doc="lfsr taps (bit positions affecting next state)")
|
|
|
|
def shift(self):
|
|
'''
|
|
calculate next state and return the output bit
|
|
'''
|
|
feedback = sum(self.state[tap] for tap in self.taps) % 2
|
|
output = self.state[0]
|
|
self.state = self.state[1:] + [feedback]
|
|
return output
|
|
|
|
|
|
def test_lfsr17():
|
|
key = [randint(0, 1) for _ in range(16)] # first 16 bits
|
|
key.append(1) # prevent initial state from being {0}^17
|
|
taps = [0, 14]
|
|
lfsr17 = lfsr(key, taps)
|
|
states = [lfsr17.state]
|
|
for _ in range(2**17-2):
|
|
lfsr17.shift()
|
|
states.append(lfsr17.state)
|
|
sorted_states = sorted(states, key=lambda x: tuple(x))
|
|
for i in range(2**17-2):
|
|
if sorted_states[i] == sorted_states[i+1]: # compare each state with the next state in the sorted list, if 2 states are identical they should be next to each other
|
|
print(f'state {sorted_states[i]} appears at least 2 times')
|
|
return False
|
|
n_state = len(sorted_states)
|
|
n_state_log = log2(n_state+1)
|
|
print(f'all {n_state} = 2^({n_state_log})-1 states are different')
|
|
return True
|
|
|
|
|
|
def css_encrypt(text, key):
|
|
taps17 = [0, 14]
|
|
lfsr17 = lfsr((key[:16]+[1]), taps17)
|
|
taps25 = [0, 3, 4, 12]
|
|
lfsr25 = lfsr((key[16:]+[1]), taps25)
|
|
cipher_text = 0
|
|
carry = 0
|
|
bytes = text.to_bytes((text.bit_length() + 7) // 8, 'big')
|
|
|
|
for byte in bytes:
|
|
x_b2 = ""
|
|
y_b2 = ""
|
|
# Generate bytes from lfsr17 and lfsr25
|
|
for _ in range(8):
|
|
x_b2 += str(lfsr17.shift())
|
|
y_b2 += str(lfsr25.shift())
|
|
x = int(x_b2, 2)
|
|
y = int(y_b2, 2)
|
|
|
|
z = (x + y + carry) % 256
|
|
carry = 1 if x + y > 255 else 0
|
|
|
|
cipher_byte = z ^ byte
|
|
cipher_text = (cipher_text << 8) | cipher_byte
|
|
return cipher_text
|
|
|
|
test_lfsr17()
|
|
cipher = css_encrypt(0xffffffffff, [0]*40)
|
|
print(hex(cipher))
|
|
clear = css_encrypt(cipher, [0]*40)
|
|
print(hex(clear))
|