One Time Pad

2023-03-09

One Time Pad

一次性密码本：明文与一串随机的比特序列进行XOR运算

#!/usr/bin/env python3

# "BabyOneTimePad" challenge for KalmarCTF 2023
# based on "EasyOneTimePad"
# original challenge by shalaamum
# idea to make this easier variant by killerdog

import os

PASS_LENGTH_BYTES = 128

def encrypt_otp(cleartext, key = os.urandom(PASS_LENGTH_BYTES)):
    ciphertext = bytes([key[i % len(key)] ^ x for i,x in enumerate(cleartext.hex().encode())])
    return ciphertext, key


if __name__ == '__main__':
    print('According to Wikipedia:')
    print('"In cryptography, the one-time pad (OTP) is an encryption technique that cannot be cracked, but requires the use of a single-use pre-shared key that is not smaller than the message being sent."')
    print('So have fun trying to figure out my password!')
    password = os.urandom(PASS_LENGTH_BYTES)

    enc, _ = encrypt_otp(password)
    print(f'Here is my password encrypted with a one-time pad: {enc.hex()}')
    print('Actually, I will give you my password encrypted another time.')
    print('This time you are allowed to permute the password first')
    permutation = input('Permutation: ')
    try:
        permutation = [int(x) for x in permutation.strip().split(',')]
        assert set(permutation) == set(range(PASS_LENGTH_BYTES))
        enc, _ = encrypt_otp(bytes([password[permutation[i]] for i in range(PASS_LENGTH_BYTES)]))
        print(f'Here is the permuted password encrypted with another one-time pad: {enc.hex()}')
    except:
        print('Something went wrong!')
        exit(1)
    password_guess = input('What is my password: ')
    try:
        password_guess = bytes.fromhex(password_guess)
    except:
        print('Something went wrong!')
        exit(1)
    if password_guess == password:
        with open('flag.txt', 'r') as f:
            flag = f.read()
            print(f'The flag is {flag}')
    else:
        print('Nope.')

一次性密码本：明文与一串随机的比特序列进行XOR运算

Python默认形参特性，默认形参值会在执行函数定义时按从左至右顺序被求值，即当函数被定义时将对表达式求值的“预计算”值将在每次调用时被使用。

enumerate() 函数用于将一个可遍历的数据对象(如列表、元组或字符串)组合为一个索引序列，同时列出数据和数据下标

1
2
3

>>> seasons = ['Spring', 'Summer', 'Fall', 'Winter']
>>> list(enumerate(seasons))
[(0, 'Spring'), (1, 'Summer'), (2, 'Fall'), (3, 'Winter')]

cleartext.hex().encode()

使得长度发生变化

cleartext 
	b'A' * 128 
cleartext.hex() 
	'41' * 128 
cleartext.hex().encode() 
	b'41' * 128 
len(cleartext) == 128 == len(key) len(cleartext.hex().encode()) == 256

长度为256的password，前一半和后一半加密用的key是一样的。

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