Security and Cryptography in Python

Security and Cryptography in Python - Stream Ciphers（3）

The problem of re-use of keys in Stream Ciphers

import random

class KeyStream:
    def __init__(self, key=1):
        self.next = key

    def rand(self):
        self.next = (1103515245*self.next + 12345) % 2**31
        return self.next

    def get_key_byte(self):
        return self.rand() % 256

def encrypt(key, message):
    return bytes([message[i]^ key.get_key_byte() for i in range(len(message))])

def transmit(cipher, likely):
    b = []
    for c in cipher:
        if random.randrange(0, likely) == 0:
            c = c ^ 2**random.randrange(0, 8)
        b.append(c)
    return bytes(b)

def modification(cipher):
    mod = [0]*len(cipher)
    mod[10] = ord(' ') ^ ord('1')
    mod[11] = ord(' ') ^ ord('0')
    mod[12] = ord('1') ^ ord('0')
    return bytes([mod[i] ^ cipher[i] for i in range(len(cipher))])

def get_key(message, cipher):
    return bytes([message[i] ^ cipher[i] for i in range(len(cipher))])

def crack(key_stream, cipher):
    length = min(len(key_stream), len(cipher))
    return bytes([key_stream[i] ^ cipher[i] for i in range(length)])

# Eve goes to Alice
eves_message =  "This is Eve's most valued secrets of all her life".encode()

# This is Alice alone
key = KeyStream(10)
message = eves_message
print(message)
cipher = encrypt(key, message)
print(cipher)

# This is Eve(alone) all evil
eves_key_stream = get_key(eves_message, cipher)

# This is Bob
key = KeyStream(10)
message = encrypt(key, cipher)
print(message)

# Alice again
message = "Hi Bob, let's meet a plan our world domination.".encode()
key = KeyStream(10)
cipher = encrypt(key, message)
print(cipher)

# Bob again
key = KeyStream(10)
message = encrypt(key, cipher)
print(message)

# Eve again (more evil than ever)
print(" THIS IS EVE")
print(crack(eves_key_stream, cipher))

Running Result: