From: daleh@ix.netcom.com (Dale Harrison (AEGIS))
Date: Thu, 2 Feb 95 20:44:04 PST
To: cypherpunks@toad.com
Subject: Re: The FIREWALL CHIP. U're phone always offhook?
Message-ID: <199502030441.UAA17143@ix2.ix.netcom.com>
MIME-Version: 1.0
Content-Type: text/plain


You wrote: 

>>    We have potential bugging devices in all our houses - The telephone!
>> Of course, when it is onhook, the microphone does not transmit... or
>> does it? 
>
>The Mondo article ("Total Surveillance" by Charles Ostman) was a joke.
>
>From the innacuracies in what I do know about, I assume all of the
>phone-paranoia is also unjustified.

Actually, there is a germ of truth in this.  On older phones (don't know if 
this works on newer electronic phones) when the handset is 'on-hook' a 
switch opens and breaks the voice circuit.  This of course only works for 
DC circuits.  If you drive that same circuit with an AC signal (from 
further up the line) then that 'open' switch becomes a capacitor and acts 
as a band-pass filter.  Signals from the mic will then modulate that AC 
current and can be extracted and reconstructed.  Supposedly the Dutch 
police have perfected this and use it in investigations to circumvent legal 
restrictions on physically bugging suspects homes; or so was alleged a 
couple of years ago during a narcotics trial in Amsterdam.

Dale H.