Telephone Eavesdropping
Jeff Johnston
I recall reading a while back [I forget the source, of course... :( ]
about the possibilities for eavesdropping on conversations in a room over
the telephone wires, without having planted any bugs and _without_ having
the receiver off the hook...
The gist was that even when a phone is hung up, a small current still
flows through the phone's circuits (for what reason I can't remember)
_including_ the microphone... The result being that conversations in the
room with the telephone would show up as an _extremely_ small signal that
could be measured from, say, the switching box down the road. I don't
even recall if it was suggested that this might work in practice, maybe
the signals would be undetectable through the line noise...
Anyone heard anything like this? Anyone with more than a passing
knowledge of telephone electronics care to comment?
Feel free to be technical...
--
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Jeff Johnston -- ellipsis advocate jjoh...@unixg.ubc.ca
"How broad and long my poodle waxes!" -Johann Wolfgang von Goethe
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Daniel B Case
>Didn't see this in the FAQ so I thought I'd bring it up...
>
>I recall reading a while back [I forget the source, of course... :( ]
>about the possibilities for eavesdropping on conversations in a room over
>the telephone wires, without having planted any bugs and _without_ having
>the receiver off the hook...
>
>The gist was that even when a phone is hung up, a small current still
>flows through the phone's circuits (for what reason I can't remember)
>_including_ the microphone... The result being that conversations in the
>room with the telephone would show up as an _extremely_ small signal that
>could be measured from, say, the switching box down the road. I don't
>even recall if it was suggested that this might work in practice, maybe
>the signals would be undetectable through the line noise...
>
>Anyone heard anything like this? Anyone with more than a passing
>knowledge of telephone electronics care to comment?
William Poundstone mentions this in his chapter on phone-phreaking in "Bigger
Secrets". It's called an "infinity microphone" and it works exactly the way
you suggested. The question is whether or not you would need to rig the phone
to get a usable signal.
"There's no BODY-CHECKING in golf!!!"-Charlie Brown, to Lucy.
V140...@ubvms.cc.buffalo.edu
Daniel Case Prodigy: WDNS15D
State University of New York at Buffalo GEnie:DCASE.10
Daniel Burstein
it's -kind-of true, and was even the subject of an article in New
Scientist about a year ago.
Basically, a Dutch Hacker named Ron something noticed that the way most
phones are designed, it -is- possible to monitor stuff in the room with
the phone on the hook.
technical stuff warning ahead!!! >>>>>
in a standard phone ,the switchhook disconencts the phone by physicaly
breaking ("opening") the circuit. basically it moves two wires apart and
leaves an air gap between them.
this is quite effective at blocking the (normal) DC voltages that go
through phone lines, BUT !!!! it can be bridged by using a high frequency
alternating current.
In effect, Ron made a capicitance jump across the air-gap.
He demonstarted this to the New Scientist reporter by inserting the high
frequency current 30 feet (or maybe meters) down the line from the
phone and was able to listen to the phone microphone, EVEN THOUGH IT WAS
on-hook.
He figured that alpha-bet agency types could hook into the line somewhat
farther down the loop.....
--------------
btw, there are many other techniques for using a telephone to monitor the
room. there were certain things you could do with the older "1A2" (the
standrad multi-line pickup and hold, lights, etc.) phones which were
widespread from more or less 1955 to 1985 in the Bell System. but that's
more a followup to alt.dcom.telecom.
------------
Speaking of which: If you live in an apartment building, chances are you
have an intercom door buzzer on your wall. have you checked -its- microphone?
take care
bill nelson
:
: William Poundstone mentions this in his chapter on phone-phreaking in "Bigger
: Secrets". It's called an "infinity microphone" and it works exactly the way
: you suggested. The question is whether or not you would need to rig the phone
: to get a usable signal.
Only one problem, you have to rig the microphone in the phone line for it to
work. You cannot simply call up the person's number and listen in. It also
works on higher current - not any leakage currents.
Bill
bill nelson
: in a standard phone ,the switchhook disconencts the phone by physicaly
: breaking ("opening") the circuit. basically it moves two wires apart and
: leaves an air gap between them.
:
: this is quite effective at blocking the (normal) DC voltages that go
: through phone lines, BUT !!!! it can be bridged by using a high frequency
: alternating current.
:
: In effect, Ron made a capicitance jump across the air-gap.
:
: He demonstarted this to the New Scientist reporter by inserting the high
: frequency current 30 feet (or maybe meters) down the line from the
: phone and was able to listen to the phone microphone, EVEN THOUGH IT WAS
: on-hook.
This claim does not make sense. First, the capacitance across that switch
would be too small to pass anything except an rf signal. Second, there is
still no dc to power the microphone - it is still an open circuit.
: --------------
:
: btw, there are many other techniques for using a telephone to monitor the
: room. there were certain things you could do with the older "1A2" (the
: standrad multi-line pickup and hold, lights, etc.) phones which were
: widespread from more or less 1955 to 1985 in the Bell System. but that's
: more a followup to alt.dcom.telecom.
All of them require the phone to actually be "off hook", however.
Bill
Lennart Benschop
: dan...@panix.com (Daniel Burstein) writes:
: : frequency current 30 feet (or maybe meters) down the line from the
: : phone and was able to listen to the phone microphone, EVEN THOUGH IT WAS
: : on-hook.
: This claim does not make sense. First, the capacitance across that switch
: would be too small to pass anything except an rf signal. Second, there is
: still no dc to power the microphone - it is still an open circuit.
Well, the claim could make sense. Old telephones contain a carbon
microphone. This microphone contains fine grains of carbon that are pressed
together by the sound pressure, making a varying resistance. If rf passes
through the hook contact and the microphone, it gets amplitude modulated by
the sound the microphonme picks up. (carbon microphones were actually
used in the very first AM transmitters.)
In normal operation DC is modulated by the microphone, but RF would do it as
well. You need a diode detector to demodulate the RF modulation that you
pick up.
--
Lennart Benschop --- len...@stack.urc.tue.nl
"Real programmers do it in hacks"
52 65 61 6C 20 70 72 6F 67 72 61 6D 6D 65 72 73 20 64 6F 20 69 74 20
69 6E 20 68 61 63 6B 73 2E Forth/C/6809/Linux/ZX-Spectrum/Z80/80x86
Charles Lasner
>
>Well, the claim could make sense. Old telephones contain a carbon
>microphone. This microphone contains fine grains of carbon that are pressed
>together by the sound pressure, making a varying resistance. If rf passes
>through the hook contact and the microphone, it gets amplitude modulated by
>the sound the microphonme picks up. (carbon microphones were actually
>used in the very first AM transmitters.)
Not really. The carbon microphones work by changing their resistance, but
only over a limited resistance range. The reason this is adequate is because
there is a significent current flow through the microphone, i.e., it is a
quite low impedance device. (Try hooking a carbon microphone, a speaker, and
a battery all in series, and note the fact that you have just constructed the
guts of a megaphone. Some real megaphones use primitive amplifiers but some
don't, and megaphone speakers are more efficient than most so the need for
an amp is actually marginal, etc. In fact, face the speaker at the mike
and you get acoustical howl, proving that the carbon mike has a positive
amplification gain when used this way, i.e., the sound pressure energy creates
a larger electric energy change, etc.)
Additionally, the carbon microphone has a lot of shunt capacitance, thus
severely limiting its RF usage. In short, you would need tremendous RF
energy to even notice any change in the RF's amplitude, so any of these
claims are off by about 5 orders of magnitude, etc.
Carbon microphones were used in the early days of AM transmitters though.
Back then, tubes ran at fairly low voltages with high currents. A carbon
mike could be used as a cathode return variable resistor to modulate a
transmitter (in the days before high-powered plate modulators which are
essentially high-powered audio amps transformer-coupled into the power
supply of the transmitter; the microphone aspect of such an amp is determined
by what drives the amp, and could be almost any suitable mike, etc.).
Eventually, modulation became a low-powered issue, since the transmitters
are actually linear amplifiers driven by itty-bitty transmitters, etc.
cj "Will tune transmitters for food" l
YuNoHoo
Didn't get the entire thread here I'm afraid. But you can
look up SF (special facility) in "The Spycatcher's
Encyclopedia of Espionage" by Peter Wright. Of course,
a lot of the stuff in this book may be nonsense.
Btw., some time ago some guy working in a European
company making ISDN stuff reported that they found
a simple way of implementing a SF for their ISDN
phone. Don't know if it worked with other brands
of ISDN phones.
---
YuNoHoo
"Nobody can talk Swedish except the Swedes,
and it's silly to try."
Peter Wright
Paul Monty Ashley
>
>Of course, a lot of the stuff in this book may be nonsense.
>
Mmmmmmmotto.
-"Monty"