This is my writeup for the thirty-sixth challenge in the PlaidCTF 2011 competition. The information for the challenge was:
“AED came up with a secret sharing program that looks like innocent food ordering program.
However, there is an information that if you are able to order the following set of food, you can get the secret key.
IMPORTANT: SOUND is VERY VERY IMPORTANT for this mission!!!! MAKE THE VOLUME LARGE before you actually do stuff…
Reverse the program to find out the key!
10 Regular Hamburgers
17 French Fries
8 Hot Dogs
20 Regular Coke”
Taking a quick look at the challenge with IDA Pro and OllyDbg respectively I could see that it’s packed, and that it uses miscellaneous anti-debugging and anti-dumping techniques. To get acquainted with the application I tried to make the order, which gave me the following error message after adding 10 regular hamburgers, 5 cheeseburgers and 11 french fries: “You cannot have more than 25 items in your cart.”
When clicking OK and then the Order-button, I got: “Your order confirmation code is Th3m1d4_iS_s!cK”. At this point I couldn’t imagine that I’ve already found the real key, so I continued with trying to reverse-engineer the program for a while before attempting a different order, which resulted in a completely scrambled string as the order confirmation code. Turns out that the developers for this mission messed something up bigtime, and that “Th3m1d4_iS_s!cK” was the actual key. Might look into actually reversing the program someday, but for now I settle with the key. :D
I’m glad that our team would have won the competition even without these 250 points though, wouldn’t have felt fair if this would have been the difference between winning and losing. :)