CSAW 2012 Writeup - Reversing 100

Run the binary, and see the output:

So what we want to do is to obtain the decrypted key. Load the binary in a debugger:

From the debugger, it is shown that the program exits prematurely before it shows the decrypted key. That's easy, just put a breakpoint at the call to exit() at 0x401106, patch it with nops, and voila!

Key: welcome_to_csaw!

Another way is to load the binary in a disassembler:

Key bytes are being loaded into the buffer.

The decryption routine:

Xor the key bytes with 0xff, and we get welcome_to_csaw!