Security and Cryptography in Python

Security and Cryptography in Python - Stream Ciphers（1）

The practical implementations of One Time Pads

Real-life One Time Pad?

• 1Gb message requires 1Gb key streams
• No resus of key stream
• Key distribution is difficult

Stream Cipher

A stream cipher is like a One Time Pad

• No requirements on key stream
• Hence, not TRUE randomness
• Also, can re-use key stream

Examples:

• A5/1(G2 encryption) - 54 bits
• A5/2(export version) - 17 bits
• RC4(WEP, SSL) - 40-2048 bits

All algorithms kept secret

Challenges

• Authenticity - MAC
• Re-use key
• Low entropy

Implementation of our Stream Cipher

Refer to :https://en.wikipedia.org/wiki/Linear_congruential_generator

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

key = KeyStream()
for i in range(10):
    print(key.get_key_byte())

Running Result:

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))])

key = KeyStream(10)
message = "Hello, World".encode()
cipher = encrypt(key, message)
print(cipher)

key =KeyStream(10)
message = encrypt(key, cipher)
print(message)

Running Result:

Benefit of 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() % 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)

key = KeyStream(10)
message = "Hello, World! I am here to declare that I will take over the universe and become the supreme emperor.".encode()
print(message)
cipher = encrypt(key, message)
print(cipher)

cipher = transmit(cipher, 5)

key = KeyStream(10)
message = encrypt(key, cipher)
print(message)

Running Result: