From: Eric Blossom <eb@sr.hp.com>
Date: Thu, 3 Mar 94 15:46:41 PST
To: wex@media.mit.edu
Subject: YA semi-novice question
In-Reply-To: <9403031930.AA19033@media.mit.edu>
Message-ID: <9403032345.AA12469@mwmax.sr.hp.com>
MIME-Version: 1.0
Content-Type: text/plain


Alan Wexelblat writes:
   Can someone please tell me how 3-way calling would work under Clipper (or
   other encrypted phone system)?  That is, I'm Alan talking to Betty.  During
   the conversation Clive calls me and I want to "add" him so that we have a
   3-way conversation.

   How do the key/session exchanges happen?

Basically it doesn't, unless you have extra hardware.  This is the
same problem that you have with ISDN phones.  No extensions.

If you were going to do it, somebody acts as a hub, call it H, and the
other two users A and B connect to the hub.   Note that A and B are
not directly connected.

	A <----> H <----> B

A and H negotiate a session key.
B and H negotiate a session key.

H acts as a digital mixer.
H decrypts input from A using the A/H key and forwards it to B
encrypted under B/H.  H decrypts input from B using the B/H key and
forwards it to A encrypted with the A/H session key.

In the middle of all this (H) all components of the conversation are
in the clear.  They are mixed appropriately and sent out to the other
participants encrypted under the respective session keys.

Eric Blossom