From: fnerd@smds.com (FutureNerd Steve Witham)
Date: Mon, 22 Mar 93 15:20:11 PST
To: Timothy Newsham <newsham@wiliki.eng.hawaii.edu>
Subject: Re: Encrypted computing questions
Message-ID: <9303222315.AB22500@smds.com>
MIME-Version: 1.0
Content-Type: text/plain


Timothy Newsham <newsham@wiliki.eng.hawaii.edu> asks a lot of questions
about my "crypto goal" of encrypted computing.  Rather than answer his
questions one at a time, I'll try to clarify what I meant.  Also, below
I ask about a related crypto goal of "trans-cryption."

The idea is that you have a program and maybe some initial state information.
The program accepts data in some encrypted form.  It does the equivalent of
decrypting it and processing it.  EXCEPT...
  o  It never turns any part of it into plaintext (unless the output is
     plaintext, in which case it only shows up after the computing is done).
  o  It's impossible by analyzing the code or watching it run to figure out
     what it did, or which bits went where.
  o  In particular, you can't figure out the decrypting key by analyzing
     the program or watching it run.
  o  If there's state information left over, it has to be in some 
     encrypted form.
Working backward from these goals, I figure that the program is somehow 
combined with its private key when it is turned into a crypto-program.
How do you convince users that this is what you've done, that your program
is actually a crypto program that never reveals its private key or 
plaintext of its inputs--that it doesn't just decrypt and then process the
plaintext--and that you've thrown away the private key used to make it?

When it runs, it has to process all its data in encrypted form!  What sort
of encryption could this be?  The data has to be shuffled all around
continuously, and it would seem you'd have to have lots of "if" statements
to make sure it behaved very differently with small input changes, so
that no one could tell what any input bit affected.

If the program were an interpreter for a general-purpose language, 
you could send it encrypted programs to run.

A related idea is trans-cryption.  Is it possible to do the equivalent
of two RSA encryption steps (e.g., decrypt with one key, then encrypt
with another), without storing information that would let you deduce either 
of the keys or the data between the two steps?

-fnerd
fnerd@smds.com (FutureNerd Steve Witham)