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XORed

1 minute to read

The program says that someone has used a XOR cipher with a single byte key. This is the output: * 7%8-s70& 61&>.

Since we know that flags start with ictf{, we can get the key using "*" ^ "i" because of XOR cipher properties:

$$ c = m \oplus k \iff k = c \oplus m $$

So we solve the challenge like this:

$ python3 -q
>>> from pwn import xor
>>> c = b'* 7%8-s70& 61&>'  
>>> k = xor(c[0], b'i')
>>> m = xor(c, k)
>>> m
b'ictf{n0tsecure}'