[Cryptography] Crypto Trick Makes Software Nearly Impossible to Reverse-Engineer

[Peter Gutmann]

Henry Baker <hbaker1 at pipeline.com> writes:

>Torrey's method encrypts software code such that it's only decrypted by the
>computer's processor at the last possible moment before the code is executed.

This looks like a modern update of an old trick from the late 1908s/early
1990s, you set the trap bit on the CPU and decrypt each instruction before
it's executed, then re-encrypt it afterwards.  Unfortunately this only worked
under DOS, as soon as protected-mode anything came along (e.g. a DOS box under
Windows 3.1), it didn't work any more (or at least I couldn't be bothered
trying to figure out what was required).  The downside was that it made
execution really, really slow, but if you ran just a few core bits of code
that way then it was manageable.

Peter.