Silly Science Electronics, Programming and Hacking About

Can you crack it? Stage 2 Solution

There has been a lot of news about the "Can you crack it?" challenge that is currently to publicise GCHQ recruiting. Here is my solution for stage 2 - the implementation of the Virtual Machine, or emulator. It's actually pretty straight forward when you get a hold of a few of the nuances. I'll put a list of hints here, and then present the full solution after the fold.

1. As of stage 1 - everything is centred around the x86 architecture, this means that your instructions need to mirror the behaviour of their x86 equivalents
2. Firmware seems to be irrelevant - don't worry about it.
3. There is no real trickery here - it's just a straight forward instruction set simulator implementation
4. The programme will finish on a HALT instruction

This is actually quite a neat little program that works by decrypting itself further programme code which it then runs and decrypts the message to get to the next stage... enjoy...