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18 October 2018

PicoCTF 2018 - QuackMe WriteUp

by XxcoralloxX


This is an easy reverse challenge. The program just wants a “password”.

Looking with ida we see a “doMagic” function It reads the password form the user, and store in ebp+PASSWORD AltText

Here this password is used in a for loop.


In particular, editing some names to make it more clear, and zooming in the interesting part:

8048858h is a memory address called “sekrutBuffer” containing a string

1 2 3) Put a char form sekrutBuffer[i] into ecx.

4 5 6 7) Put a char from PASSWORD[i] into eax.

8) a xor is performed between sekretBuffer[i] and PASSWORD[i].

9) The resoult is moved into var_1D

10 11 12 13 14) The resoult is compared with greetingMessage[i].


It is quite simple. So, if Password[i]^sekrutBuffer[i] has to bee equal to greetingMessage[i] we can get Password reversing the process. password[i]=sekrutBuffer[i]^greetingMessage[i].


And that’s it picoCTF{qu4ckm3_5f8d9**7}

tags: reverse  PicoCTF2018