• Convert hex to base64
• Fixed XOR
• Find the key

The string:

49276d206b696c6c696e6720796f757220627261696e206c696b65206120706f69736f6e6f7573206d757368726f6f6d

Should produce:

SSdtIGtpbGxpbmcgeW91ciBicmFpbiBsaWtlIGEgcG9pc29ub3VzIG11c2hyb29t

As simple as that, hexadecimal decode it and later base64 encode it as follows:

python -c "
    '49276d206b696c6c696e6720796f757220627261696e206c696b65206120706f69736f6e6f7573206d757368726f6f6d'
    .decode('hex')
    .encode('base64')
    "

Write a function that takes two equal-length buffers and produces their XOR combination. If your function works properly, then when you feed it the string:

1c0111001f010100061a024b53535009181c

... after hex decoding, and when XOR'd against:

686974207468652062756c6c277320657965

... should produce:

746865206b696420646f6e277420706c6179

Solution:

import sys

    key = 686974207468652062756c6c277320657965'
    data = 1c0111001f010100061a024b53535009181c'
    for i in range(len(key)):
        sys.stdout.write(str(int(str(int(data[i], 16) ^ int(key[i], 16)), 16)))
    print

The hex encoded string:

1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736

Has been XOR'd against a single character. Find the key, decrypt the message.

with open("english_words.txt") as word_file:
        english_words = set(word.strip().lower() for word in word_file)

        def decrypt(chiper_text, key):
            return ''.join(chr(ord(t) ^ key) for t in chiper_text)

        def is_english_word(word):
            return word.lower() in english_words

        def score(sentence):
            return sum(1 for word in sentence.split(' ') if is_english_word(word))

        hex_input = '1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736'.decode('hex')
        best_score = -1
        best_key = -1
        for key in range(256):
            key_score = score(decrypt(hex_input, key))
            if key_score > best_score:
                best_score = key_score
                best_key = key

        print 'Key[%d] score[%d] plaintext[%s] ' % (best_score, best_key, decrypt(hex_input, best_key))