EasyCTF_2018: Reverse Engineering

Category: Intro Points: 30 Description:

What does this Python program do? And more specifically, what input would give this output? 6538c2937cc3bb20c3983ac2ab4901c38fc297161fc2a6c3b3c281c28107c2a7c2a03fc3956ec3b55350

Write-up

This one was a simple one as well, just with a twist of UTF-8. Solving this is just knowing that XOR operations are entirely reversable and therefore, we just have to input our output back in to get our input.

cipher = b"6538c2937cc3bb20c3983ac2ab4901c38fc297161fc2a6c3b3c281c28107c2a7c2a03fc3956ec3b55350"
def unmystery(s):
    r = ""
    for i, c in enumerate(s):
        r += chr(ord(c) ^ ((i * ord(key[i % len(key)])) % 256))
    return r

print(unmystery(binascii.unhexlify(cipher).decode("utf-8")))

The full script is available here.

root@ctf:~/downloads# ./mystery.py 
easyctf{char_by_char_41d6D3}

Therefore, the flag is easyctf{char_by_char_41d6D3}.

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