Security and Cryptography in Python - Stream Ciphers（2）
Implementation of the Authenticity problem with 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))])
# This is Sender
key = KeyStream(10)
message = "Send Bob: 10$".encode()
print(message)
cipher = encrypt(key, message)
print(cipher)
# This is Bob
cipher = modification(cipher)
# This is bank
key = KeyStream(10)
message = encrypt(key, cipher)
print(message)
Running Result:
