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Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!cs.utexas.edu!uunet!n3dmc!johnl
From: johnl@n3dmc.UU.NET (John Limpert)
Newsgroups: misc.consumers,misc.misc,sci.misc,sci.electronics
Subject: Re: Telephone privacy gadgets
Keywords: Telephone, wiretap, bug, countermeasures, etc.
Message-ID: <807@n3dmc.UU.NET>
Date: 24 Oct 89 03:07:35 GMT
References: <799@mccall.uucp> <776@ariel.unm.edu> <397@hq.af.mil> <20247@mimsy.umd.edu> <1917@dover.sps.mot.com>
Reply-To: johnl@n3dmc.UUCP (John Limpert)
Followup-To: misc.consumers
Organization: N3DMC, Silver Spring, Maryland
Lines: 23

In article <1917@dover.sps.mot.com> waters@darla.sps.mot.com (Strawberry Jammer) writes:
>Audio scrambling using analog means is very easy to break as a result,
>digital scrambling can be made extremly difficult but requires 3-4X the
>bandwidth of the telephone line (2.5Khz Vs 16Khz). Not something you can use
>from your home or office.

I know that there are NSA approved secure telephone systems for voice
grade lines, I have seen ads in Aviation Week & Space Technology for the
systems.  Unfortunately, the NSA doesn't allow distribution of the
technical manuals to ordinary civilians. 

Are there any low bit rate, commercially available voice encoder/decoder
chips? I have seen variable slope delta modulation chips in
unclassified, commercial equipment at 24/32 kbps.  Decent audio quality
but too fast for a normal voice grade line modem. 

The voice encoder/decoder seems to be the hardest part of a secure
digital phone system.  A relatively inexpensive system could be
assembled with DES encryption and a V.32 modem.

-- 
John A. Limpert			I'm the NRA!
Internet: johnl@n3dmc.UU.NET	UUCP: uunet!n3dmc!johnl