CSS-cipher/main.py

#!/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():
    print("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 generated 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[::-1], 2)
        y = int(y_b2[::-1], 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

def test_encrypt():
    print("test encryption: text 0xffffffffff, key 0x0 ∈ {0, 1}^40")
    cipher = css_encrypt(0xffffffffff, [0]*40)
    print(f'cipher: {hex(cipher)}')
    clear = css_encrypt(cipher, [0]*40)
    print(f'decrypted: {hex(clear)}')
    print(f'original text and decrypted message are the same: {clear==0xffffffffff}')

test_lfsr17()
print()
test_encrypt()