Security and Cryptography in Python - Stream Ciphers（4）

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Security and Cryptography in Python - Stream Ciphers（4）

Low entropy - Brute force of our Stream Cipher

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()//2**23) % 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)])

def brute_force(plain, cipher):
    for k in range(2**31):
        bf_key = KeyStream(k)
        for i in range(len(plain)):
            xor_stream = plain[i] ^ cipher[i]
            if xor_stream != bf_key.get_key_byte():
                break
        else:
            return k
    return False

# This is Alice
secret_key = random.randrange(0, 2**20)
print(secret_key)
key = KeyStream(secret_key)
header = "MESSAGE: "
message = header + "My secret message to Bob"
message = message.encode()
print(message)
cipher = encrypt(key, message)
print(cipher)

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

# This is Eve
bf_key = brute_force(header.encode(), cipher)
print("Eve's brute force key:", bf_key)
key = KeyStream(bf_key)
message = encrypt(key, cipher)
print(message)

Running Result:

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