From: cme@ellisun.sw.stratus.com (Carl Ellison)
Newsgroups: sci.crypt
Subject: Re: A Copper Balloon
Date: 9 Nov 92 16:52:28 GMT
Article-I.D.: transfer.1dm50cINNcse
Organization: Stratus Computer, Software Engineering
NNTP-Posting-Host: ellisun.sw.stratus.com

In article <1992Nov7.142220.1683@guvax.acc.georgetown.edu> denning@guvax.acc.georgetown.edu writes:
>I'd like to suggest another possibility, which for want of a
>better name I'll call the "copper balloon".  It's quite clear that key 
>registration goes over like a "lead balloon".  My question is: is this 
>any better?  worse?  I would also like to suggest that we keep this
>thread focused on this proposal and not on the merits/demerits of
>wiretapping in general.


I concur and would advocate that we keep this subject line free from the
wiretapping discussion.  It's going on quite nicely over in
comp.org.eff.talk.


One way to separate this conversation from privacy in general is to view
the copper balloon as a proposal for a nationwide, encrypted voice and data
service.  In this plan, the phone companies would provide encrypting wired
phones, cellular phones and modems using the 3-way DH key exchange which
Dorothy proposes.  These phones (due to volume production) would be cheap
enough that all business and many personal phones would use encryption.
The encryption, meanwhile, would be high quality: 1000+ bit DH, 3-fold
DES-CBC (with a full 360 bits of key).

This has the advantage of cutting down on industrial espionage.

This undercuts companies like Cylink or Racal-Guardata -- doesn't put them
out of business because you would need their current products to provide
privacy even from the Gov't -- but you would presumably take away from them
the business of people who just want to avoid non-gov't espionage.


So:  what if the entire national communications infrastructure were
well encrypted?  People could add their own encryption, of course,
but most wouldn't.  Most are lazy or don't see the need.



This gains us control over industrial espionage and the tapping of cellular
phones by private citizens.

It also gains us some civil liberty.  Without getting a key from the
intermediary, the gov't (presumably even the NSA) wouldn't be able to do
its tapping.  That request can be logged and preserved for later FOIA
retrieval.

Meanwhile, this mechanism could presumably be extended from 3 to more than
3 agents, along the lines of Silvio's proposal, with each one getting a
clear record of each tap request and each one having to rule on the
validity of the order.

I claim, therefore, that this mechanism would save us from Nixon-style
abuses while still permitting unrestricted private encryption and leaving
open to wiretapping a large number of calls (probably just the same calls
that are open today).


However, I would like Dorothy to post details of the algorithm she
proposes.  Based on her description of its behavior, I assume it's like the
following -- but this differs from Silvio's proposal:

	agree on common prime, p, and ranno a, (1< a < p).

	each party, i, choose random, secret x[i] (1 < x[i] < (p-1))

	1 <= i <= n

	each party compute y[i,1] = a^x[i] mod p and pass it to (i+1)

	each party compute y[i,j] = y[i-1,j-1]^x[i] mod p

	the values y[i,n] will all be the same

-- 
-- <<Disclaimer: All opinions expressed are my own, of course.>>
-- Carl Ellison						cme@sw.stratus.com
-- Stratus Computer Inc.	M3-2-BKW		TEL: (508)460-2783
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Newsgroups: sci.crypt