archutil download: http://download.openpma.org/archutil/archutil.tbz
Well, since everything has been active on the Archos hacking front, I think I will take this opportunity to release archutil. I have been working on this utility for a while and I consider it to be mostly bug free. Please leave me a note if you find anything that doesn't work as advertised.
This will allow you to decrypt nearly all AOS2 files. There is no documentation besides the code and the built-in help. If you have any questions, I'd be happy to answer them. It also verifies different signatures and will tell you what key was used to verify it. Code is also included to sign files with a built-in private key, or with a private key passed on the command-line. And finally, functionality is included to sign a firmware update file with your own private key.
Most of this has been initially tested. None of it has been thoroughly tested. Again, feel free to let me know of any bugs that you find.
And let me just get the first question out of the way: it will not work for A5IT files. It probably would if somebody could find its keys.
Now let me get the second question out of the way: this is not useful for hacking your Archos unless you know their private key. And let's not waste time talking about brute-forcing the key, shall we? It's just not feasible at this time.