Subject: Re: requesting proposals -- change detection
From: kragen@pobox.com (Kragen Sitaker)
Date: Sat, 26 Jun 1999 23:15:51 -0400 (EDT)

Jonathan Shapiro writes:
> How might one go about "stamping" a distribution is such a way that you can
> reliably *detect* when a customer has altered it?  I have a
> recollection of somesort of signing proposal, but I don't recall how it
> actually worked.
> 
> This would largely eliminate the liability issue in my mind.
> 
> Any pointers or suggestions?

Well, let's take the simplest case.  You have some passive storage
device (a disk, or an EPROM, or whatever).  You want to determine
whether its contents are identical to what you originally put on it.

For n = 0 to the number of bits in your original distribution, compare
bit n of the storage device's contents to bit n of the original
distribution.  If it is different, the customer has altered it.  If no
bits are different, the customer has not altered it.

I assume this was obvious to you when you wrote the above question, so
I assume the problem you actually want to solve is slightly more
difficult.  How, exactly, does it differ from the problem I described
above?

-- 
<kragen@pobox.com>       Kragen Sitaker     <http://www.pobox.com/~kragen/>
According to my medieval text in the seventh century a finalizer raised a
dead object named Gorth who infected every computer in Cappidocia ending
Roman rule in the region.  -- Charles Fiterman on gclist@iecc.com