Slashdot Mirror
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."
I didn't give the challenge a serious go, but stage 1 just seems convoluted - why is it the mark of a good code cracker to recognise x86 bytecode?
To prevent this day from getting worse, I'll just read ERROR as GOOD TH
My opinion, as someone who has both solved and organized several challenges of this sort, is that the challenge is neither hard (at least by the standards of the ones I've dealt with) nor well designed. In fact, it kind of degenerates: it starts out OK but the ending is terrible.
Stage 1 is interesting: it combines recognizing executable code (the first thing I thought when I stared at that hex dump is "this looks like x86 code", but being able to recognize binary architectures is a valuable skill) combined with some steganography (fishing out the rest of the required data from the PNG. Fair enough, and OK for a first round.
Stage 2 starts out well: virtual machines are used for obfuscation and make fun challenges. However, the execution is backwards. Being given VM bytecode and a specificiation to implement a VM isn't a hacking or reverse engineering challenge; it's just work ("go implement this for me"). A much better challenge would be to be given either the spec or (preferably) code that implements it, and then have to reverse engineer the bytecode itself to solve the puzzle. That involves writing a custom disassembler, which is a much more interesting task.
Stage 3 is a clusterfuck. It's just an executable that checks for a few constants in a file and then builds a URL out of the rest of it. There's a hash (old-school DES crypt() salted password) that the input has to match, but even though it's crackable using a dictionary, you don't even have to do that because the URL includes the hash (which is in the executable), not the plaintext! The rest of the URL isn't checked, and it's basically a guessing game where you have to fish out constants from previous levels. It's just a glorified way of saying "okay, now take a wild guess as to what numbers to stick in the URL". It's not realistic in the slightest.
Anyone interested in a "better stage 2" might want to check out a level that I put together for the Hack-It competition at the 18th Euskal Encounter (2010). Your goal is to figure out the 64-bit input key that works (if you don't know what "works" means, compile and run the code and it should be obvious). The full set of challenges can be found here: 2010 2011 (unfortunately, the website / problem statements are in Spanish, but I'm sure you can work it out with a bit of copy/pasting into Google Translate - if there's enough interest I'll translate them to English).
So if you can't crack it, but you can bypass the challenge, do you still win?
http://www.canyoucrackit.co.uk/soyoudidit.asp
I'm aware that the solution has been leaking out onto the net...
Starting later than most, in spare time, I've trudged through stages One and Two... I've been playing with the stage-3 executable and have disassembled it... though there remains further tedious trudging for me to demonstrate by sensible sequential steps how to go about solving stage-3.
I'm finding it difficult to convince myself that it's worth the effort... I'm sure I can fathom any remaining steps - based upon the fact that there has been little about stages one and two that was actually 'challenging'. It seems silly to plod onwards without 'cheating'.
I was interested principally to try and find out what sort of skills GCHQ actually want... I never assumed I'd be (one of the) first to solve it. The experience has left me wondering what sort of job this sort of tom-foolery would suit one for. Sure debugging and OS-level skills can be valuable - but the challenge is most time consuming as one is required to guess the objective - identifying the intentions of the challenge setter rather than to address real-world issues.
I haven't looked at the video yet, because I still want to see how far I can get with just the spoilers in the comments.
Grr...now I'm mad I didn't recognize the byte swapped DEADBEEF.
For better or for worse, modern intelligence agencies are much more dependent on people who can RE software and develop exploits, than they are on pure cryptographers.
This is a consequence of the rolling disaster that is software security, combined with the fact that crypto folks have (mostly) gotten their act together.
GCHQ has just announced that they are to lead the UK's cybersecurity push. I guess they need some reverse engineering skills in a hurry.
This is an intelligence agency, and network intrusion programs pumping executable code in the attempt at smashing a stack and jumping execution are pretty common.
Perhaps they want people who can quickly spot x86 assembly payloads from raw packet traces as part of a counter aggression op?
If we assume that their network stack isn't riddled with exploitable stack variables or pointers, and that they successfully prevent the code from running, but log the unrequested network access and dump the binary packets to file for analysis, then having people that can "at a glance" determine what kind of data is in those dumps would be valuable.
Being able to determine what it actually is supposed to do even more so.
With the recent hysteria over scada system cyber attacks (I hate that phrase btw..),setting up a fake scada system as a honeypot and seeing what the cat drags in could also make use of this skillset.
So, the obvious questions:
Does the UK fear it has poorly secured scada systems, or does it fear network worm intrusion on some network segement, and if so, what segments or systems are those?
This sounds like they are more interested in recruiting people to analyze stuff like Stuxnet
Yes, that's probably exactly what they're after.
Pirate Party UK
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/
Every man for himself, all in favour say "I"
If they're really looking for people who can do that, they should be looking at people who crack DRM. Oh, are most of the people like that keeping their heads down and unlikely to be considering government jobs? Gee golly, I can't imagine why...
http://recordmydesktop.sourceforge.net/about.php
The Z80 is not a segmented memory model either; you might be thinking of some of the embedded versions such as the HD64180. It was the x86 architecture that was really afflicted with these segment registers.