GCHQ Challenge Solution Explained

First time accepted submitter DrDevil writes "The British spy agency GCHQ recently published a puzzle at canyoucrackit.co.uk (as featured on Slashdot), now just a few days later an academic at the University of Greenwich in England has posted a full video explanation of the puzzle. The puzzle has three stages and is not at all simple — likely to challenge even the best computer science graduates."

Comments

Re:Opaque

[pushing-robot]

Well, "DEADBEEF" is a bit of a giveaway.

Re:Opaque

[marcansoft]

Recognizing unknown architecture binaries is an important skill to have when reverse engineering, especially for embedded systems. Very often you'll get a firmware file and you have to figure out what it is. Each architecture has its peculiarities, so it doesn't take long to get a feeling for what their opcodes look like. For example, 32-bit ARM code sticks out like a sore thumb (no pun intended :) due to the condition code field, which means that every 32-bit word almost always starts with 0xEx (and whether that's the first or last byte in the word tells you the endianness). Variable length architectures like x86 look very different from RISC ISAs with a fixed instruction length like PowerPC.

My solution with full writeup

[ncw]

I was going to hold this back until the competition was finished, but it seems the cat is out of the bag!

Here is my solution and a writeup of exactly how I got there.

http://www.craig-wood.com/nick/articles/how-i-solved-the-gchq-challenge/