From: monty.harder@famend.com (MONTY HARDER)
Date: Mon, 4 Sep 95 17:47:53 PDT
To: CYPHERPUNKS@toad.com
Subject: maximizing cryptographic return
Message-ID: <8B072A1.00030003E8.uuout@famend.com>
MIME-Version: 1.0
Content-Type: text/plain


VZ> this list are aware of the idea that good encryption is often used
VZ> to send a low-bandwidth session key, which is then used to encrypt
VZ> that session using a less sophisticated but less computationally-demanding
VZ> algorithm. hence you seem to have good security at a computational
VZ> price that is less than encrypting everything with the secure protocol.

  Why must this process be limited to two levels?

VZ> I wonder if some very cheap algorithms, in terms of computation time,
VZ> could be used for the "on the fly" encryption of the voice using those
VZ> bit. would XOR with the pad be totally out of line?

  The RSA could be used by the caller to precompute the session key to
send to the reciever. The session key (IDEA or whatever) could be used
to send "subsession keys" which are actually parameters for the PRNGs
(use at least two, with different periodic characteristics, and XOR them
together) that create the pad for your XOR.

  The subsession size should be chosen so that very little "clearvoice"
is transmitted in each subsession. Perhaps a bit of randomness is in
order here, as well. Along with the PRNG parms, a length field, within
certain absolute limits. Now the spook doesn't even know where one
subsession ends, and the next begins.

  Add to this the use of a (lossy?) compression engine that can run with
little power, and a simple microcontroller (or several cheaper ones in
parallel-I can see one master for the session and subsession key
management and several slaves to handle the on-the-fly (en)(de)cryption
itself) should be able to do the job, fitting the subsession key
exchange in right along with the cyphervoice.

  Ideallly, we could have a box that could pull its power from the phone
line, and take touch-tone control inputs.


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