This binary allows you to hide a flag of your choice in a file that will be filled with seemingly random data. Here is an example of such a garbage file in which a flag has been hidden using the given binary.The obvious challenge is to recover the flag from this seemingly random file. This requires a bit of reverse engineering before the crypto involved can be attacked. This attack is quite subtle and might be hard to find.
This challenge was meant for the ebCTF during OHM2013. In the end it was deemed too difficult for use in an online time-constrained CTF.