HackFest Final 2016 / Crypto 100

In the final hackfest tunisia ctf 2016 we have a crypto task (100pts)

crypto100

so first we open the folder and we get a python code that do encryption and decryption of any file gived with a specific password we noticed that there is a another files code in the py folder that used to do this job

after some analyse of the code we get that its a xor encryption with a key (password)

and also we found a log file

computing using ([105, 108, 111, 118, 101, 99, 114, 121, 112, 116, 111, 103, 114, 97, 112, 104, 121])

convert this we get “ilovecryptography” we used to decrypt enc_mail and its work but not with our picture “enc_photo.png”

so what we should do here is a guess of the password used but we know the original header of any png file

then i just write a simple script python to get the key by xoring this 8 byte with the 8 byte of the encrypted png file

#!/usr/bin/env python

f = open('enc_photo.png')

enc_data = f.read()[:8]
data = '89504E470D0A1A0A'.decode('hex')
key = "".join(chr(ord(x) ^ ord(y)) for x, y in zip(data, enc_data))
print "key is : " + key

and we get key is : Garfield :D

so the last step is to xor the encryption png file with this key and we will get our original png with the flag

there is a helpful tools xortool

$xortool-xor -f enc_photo.png -s "Garfield" > flag.png

and pwn :D we get the flag :D

FLAG{GARFIELD_LOVES_LASAGNES}