Single-Byte XOR Cipher (python)

Question

This is for a modern cryptography class that I am currently taking.

The challenge is the cryptopals challenge 3: Single-Byte XOR Cipher, and I am trying to use python 3 to help complete this.

I know that I am supposed to XOR the string and converted to English. The hex string is "1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736" which converts to "806748453371902409051174291875458592743800337585421566549206796642836053682239286" in decimal form.

I have XOR'd this against multiple hex byte combinations (2 hex digits), but I do not know how to convert this into English. Is it just brute force and educated guessing at this point?

I know about ETAOIN SHRDLU, but this hasn't really been that helpful.

Thank you for your time and help.

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ADDED: Additionally, I tried Challenge #4 but this code does not seem to work. But it did work for Challenge #3 so I am confused.

Challenge #3 Challenge #4

Answer 1

You can use binascii.hexlify, binascii.unhexlify to convert byte strings to hexadecimals or vice versa:

>>> import binascii
>>> binascii.hexlify(b'HELLO')  # to Hex
b'48454c4c4f'
>>> binascii.unhexlify('48454c4c4f')  # from Hex
b'HELLO'

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Using str.isprintable, you can filter out non-printable candidates:

>>> 'abcd'.isprintable()
True
>>> '\x00'.isprintable()
False
>>> '\x7f'.isprintable()
False

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import binascii

encoded = binascii.unhexlify('1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736')
for xor_key in range(256):
    decoded = ''.join(chr(b ^ xor_key) for b in encoded)
    if decoded.isprintable():
        print(xor_key, decoded)

Answer 2

Building on @falsetru's answer, but showing just the decoded string with the most space characters:

>>> encoded = '1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736'
>>> import binascii
>>> nums = binascii.unhexlify(encoded)
>>> strings = (''.join(chr(num ^ key) for num in nums) for key in range(256))
>>> max(strings, key=lambda s: s.count(' '))
"Cooking MC's like a pound of bacon"

Instead of counting spaces, you could use ETAOIN SHRDLU ("the approximate order of frequency of the 12 most commonly used letters in the English language") for weights, but it's not necessary here.

Btw, I think it would've been good if you had linked to the challenge.

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Edit: Alternatively, you can try to find the key (or a few most promising keys) and then only decode using that key (or those few keys). For example, assuming that counting the spaces will determine the winner:

>>> encoded = '1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736'
>>> import binascii
>>> nums = binascii.unhexlify(encoded)
>>> key = max(nums, key=nums.count) ^ ord(' ')
>>> ''.join(chr(num ^ key) for num in nums)
"Cooking MC's like a pound of bacon"

This could even easily be done by hand (though the challenge tells you not to do that).