challenge3.py

#!/usr/bin/env python3
import binascii


def single_byte_xor(ciphertext):
    results = {}
    for i in range(0, 256):
        c = chr(i)
        plaintext = bytearray([a ^ b for a, b in zip(ciphertext, [i]*len(ciphertext))])
        score = compute_score(plaintext.lower())
        results[score] = plaintext
        # print(c, ": error: ", score, ' plaintext: ', plaintext)
    if len(results) > 0:
        best = max(results)
        return best, results[best]


def compute_score(plaintext):
    score = 0
    # From http://www.data-compression.com/english.html
    frequencies = {
        'a': 0.0651738, 'b': 0.0124248, 'c': 0.0217339,
        'd': 0.0349835, 'e': 0.1041442, 'f': 0.0197881,
        'g': 0.0158610, 'h': 0.0492888, 'i': 0.0558094,
        'j': 0.0009033, 'k': 0.0050529, 'l': 0.0331490,
        'm': 0.0202124, 'n': 0.0564513, 'o': 0.0596302,
        'p': 0.0137645, 'q': 0.0008606, 'r': 0.0497563,
        's': 0.0515760, 't': 0.0729357, 'u': 0.0225134,
        'v': 0.0082903, 'w': 0.0171272, 'x': 0.0013692,
        'y': 0.0145984, 'z': 0.0007836, ' ': 0.1918182}
    for letter in plaintext:
        c = chr(letter).lower()
        if c in frequencies:
            score += frequencies[c]
        else:
            score -= 0.5
    return score


if __name__ == '__main__':
    output = single_byte_xor(binascii.unhexlify('1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736'))
    print(output)