challenge_5.py

"""
Orel Ben-Reuven
https://cryptopals.com/sets/1/challenges/5

Implement repeating-key XOR
Here is the opening stanza of an important work of the English language:
Burning 'em, if you ain't quick and nimble
I go crazy when I hear a cymbal

Encrypt it, under the key "ICE", using repeating-key XOR.

In repeating-key XOR, you'll sequentially apply each byte of the key; the first byte of plaintext will be XOR'd
against I, the next C, the next E, then I again for the 4th byte, and so on.

It should come out to:

0b3637272a2b2e63622c2e69692a23693a2a3c6324202d623d63343c2a26226324272765272
a282b2f20430a652e2c652a3124333a653e2b2027630c692b20283165286326302e27282f

Encrypt a bunch of stuff using your repeating-key XOR function. Encrypt your mail. Encrypt your password file.
Your .sig file. Get a feel for it. I promise, we aren't wasting your time with this.
"""


def repeating_key_xor(stream: bytes, key: bytes) -> bytes:
    return bytes([letter ^ key[idx % len(key)] for idx, letter in enumerate(stream)])


if __name__ == '__main__':
    stream = b"Burning 'em, if you ain't quick and nimble\nI go crazy when I hear a cymbal"
    key = b'ICE'

    res = repeating_key_xor(stream, key)
    out = '0b3637272a2b2e63622c2e69692a23693a2a3c6324202d623d63343c2a26226324272765272' \
          'a282b2f20430a652e2c652a3124333a653e2b2027630c692b20283165286326302e27282f'
    print(res.hex() == out)