🚀 go-pugleaf

In article <1992Nov9.181433.12025@shearson.com> pmetzger@snark.shearson.com (Perry E. Metzger) writes:
>gilchr@ee.ualberta.ca (Andrew Gilchrist) writes:
>joke. No need for you to contemplate "a lot of funding" -- you can
>just go out and buy the equipment any time you like. AT&T is the only
>vendor I know of that packages this all with a cellular transmitter,
>but you can buy similar phones from Cylink, Motorola and other
>vendors. And this is just the list of DOMESTIC manufacturers of such
>things -- doubtless there are folks outside the country that make
>them, too.
>
>Its all really easy. CELP compression can be done with cheap DSPs --
>you can buy the code if you don't feel like writing it yourself. The
>encryption can be done an order of magnitude faster than you need even
>on conventional processors.  V.32bis modem modules are a dime a dozen.
>You don't need to be AT&T to do this -- any talented EE with a few
>weeks of work could put a system to do this together.

Can someone tell me how these digitally encrypted phones provide
reasonable security in light of the following problem.  Phones lines
can tolerate raw data rates of 9.6 kbps whereas voice requires 
64 kbps.  Let's say the CELP compression reduces the voice
requirement down to 9.6 kbps (voice becomes noticibly choppy thereafter).
Where do they allocate the bandwidth needed for sycronization and error
correction.  Assuming the phone uses a strong block cipher such as DES
in code book mode (stream ciphers would have error propogation problems)
this would imply blocks of duration 6 seconds.  An uncorrected channal bit
error would result in white noise for 6 seconds.  Voice is redudant but
NOT that redudant.  From this discussion, either you increase the signalling
bandwidth through use of multiple phone lines or ISDN, or you improve
compression technology or you use a weaker cipher.  Perhaps someone can
post the specs for one of these digital encryption phones which I've
never come across.

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| Luis Toshio Yu                | "Now, if only I have something clever to  |
% lcmyu@crypto2@uwaterloo.ca      % say about that..." :-)                  %
% Electrical Engineering          % Opinions expressed here are mine, so    %
| University of Waterloo, Ontario | credit me for your happiness.           |
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