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Discount Challenge - November 2018

Congratulations to all those that obtained the hidden coupon code! Thank you to everyone that participated - we had over 2,000 attempts. The full solution to this challenge is posted below. We hope to do additional challenges like this in the future.

  1. This challenge could be solved with basic reverse engineering knowledge and character encoding knowledge. First, open the libvn_crypt.so binary in IDA or Radare2.
  2. We used IDA for this solution, which identified the binary as being a 64-bit ELF file.
  3. Taking a look at the list of functions contained in this binary, we see several pertaining to base32 character encoding.
  4. Revewing the contents of some of the functions shows an unnamed function, sub_1930, to be of interest.
  5. In that same function, we also see a loop structure moving single characters into rsp ("the stack") sequentially.
  6. If you guess that this string is important and use a base32 decoder on that string, you will get the coupon code!