Tuesday, December 06, 2011

A back channel confirms that I'm right... sort of

Through a little circuitous back channel I received an unofficial follow up to a blog post about the unused machine code in the GCHQ Code Challenge part 2. The follow up assured me that the unused code was left over after some clean up was done, and that the rest of the data in the file was random filler (as I'd already heard).

But as I'd already determined that it was not actually random (at least at one level) I sent a carrier pigeon out with a question about my follow up blog post.

Listening for secret messages transmitted during The Archers I received a reply indicating that I had indeed 'broken' the encryption on that stream of ASCII data (by guessing the algorithm and reverse engineering the key stream), but that the underlying data was actually random ASCII generated and encrypted using the following Python code:
b = ''.join([chr(randint(0x20, 0x7f)) for i in range(0, 16 * 7)])
c = codec(b, 0x1f, s=5)
The first part generates 112 bytes of random ASCII text and the codec function is apparently the encryption function with the key I had identified (see the 0x1f and 5).

There are a couple of oddities about this code. The randint could generate an 0x7F which is non-printable (and doesn't appear in the decrypted text) and it generates precisely 112 bytes of data (whereas the part 2 actually contains two blocks of 112 and one of 102).

I questioned that by leaving a note in a tree in St. James's Park and received a response by exchanging briefcases at King's Cross Station to the effect that the 102 byte block had simply been truncated to make the code look interesting.

Of course, that could all be completely non-suspicious, but having been told that first it was random filler and then that actually it was blocks of encrypted random printable ASCII filler it wouldn't surprise me if the truth was even more complex (and perhaps rather mundane).

But why go to all this trouble on 'random filler' text? And why update the web site to say "The Challenge Continues"?

I am left slightly baffled, which I'd imagine is just how people working in secret places would like it to be.

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