Re: Clever ways to hide a compare

jonathon <spamtrap@xxxxxxxxxx> wrote:
> Suppose you are writing a protection scheme, and need
> to compare 2 values. Is there a clever way to do this
> without an outright 'cmp' instruction? Of course it's always
> possible to use the value in a calculation that is needed for
> correct functionality of the program, but I was wondering
> if there is a way to have 2 values compared in a sneaky
> way where it is not immediately evident what is happening.

Well, a `cmp` is nothing but a `sub` with the resultant
thrown away. So you could us that.

But if I were of a mind to write a software licencing/
protection scheme (and I'm not, but I don't think such things
are automatically evil), then I'd proceed very differently:

I'd provide users with a longish (40 char) serial number
that hashed down to the same ~32 bits. These bits would be
a crypto key to correctly decrypt and decompress a critical
code section. This can be defeated by some techniques,
and there are further countermeasures.

-- Robert



.

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