The Cryptobox
04/08/03
by zmaster

Well the hey-days of boxing are pretty much over, but here is an idea
that is still effective and can be useful if you're mildly paranoid.  I
am currently jobless and bored, so I have a lot of free time to concoct
devious new inventions.  I was browsing the cal.phonelosers.org forum
one day when I came up with an idea to thwart the tap-happy authorities.
I do some experimenting with radio projects, so I am no stranger to the
concepts of modulation.  So I thought to myself, why not modulate the
phone signal in a way that unmodified phones wouldn't be able to decode?
Time to do some research.  I did some checking around online and the
experts agree: standard phones use current modulation to transmit the
sound of your voice to the other party(s) on the line.  The current
fluctuations are very small which made me think: a small circuit could
take these fluctuations and convert them to another form.  The
Indubitable Forest M. Mimms III held the answer out to me.  Anyone who
frequents Radio Phrack will be familiar with his engineer's notebooks.
Digging around in one of these I came upon a voltage to frequency
converter.  You're supposed to use this to take measurements from a
sensor circuit (digital thermometer, etc.), and record them to audio
tape to be played back through a frequency to voltage converter to a
multimeter.  Sorry Forest, I don't own a tape recorder.  OK so how to
change the current to voltage?  A standard 741 op-amp inverter.  This
same inverter will turn voltage back into current as well.  OK so I made
a trip to the Shack and got all the stuff (and some weird looks from
those assholes who can never remember my address).  Hooked it all up in
about 2 hours and put it in a shiny blue box.  Then I called my sister
who lives across town and let it scream at her for a couple minutes.
She *69'ed me and bitched me out saying she didn't have a fax machine
and to stop trying to hack her phone.  I got my friend to build one that
weekend and we tested it out that evening.  With a little tweaking we
had it going and sounding good.  Just to be sure I picked up another
extension and sure enough, it screamed at me.

Are you begging to know how to build this yet?  Well Myst3r1i0us Ph00l
broke it down and made some shitty schematics for me.  Basically you
need to build the V/F and F/V converters seperately, along with two
inverters.  Attach one inverter between pin 5 (input) of the V/F
converter and a DPTT (double pole, triple throw) switch.  Connect pin 5
(input) to the switch terminal across from pin 3.  Connect the ring line
wire from the wall jack to the center terminal on the same pole as pin
3.  Connect the ring line wire from your phone to the terminal/pole
opposite the wall line.  These two wires should be on the terminals that
are going to be connected to the others when the switch is moved.  The
other inverter goes between pin 3 (output) of the F/V converter and the
switch.  This should be on the same pole as the other phone-side
connections, but obviously on its own terminal.  Pin 2 of the F/V
converter should be connected to the terminal opposite pin 3.  The third
throw of the switch should have a wire between the two terminals.  This
returns the phone to normal use when the box is not needed.  The common
line wire should be connected directly from the wall to your phone.  I
have included a schematic and wiring diagram here, but the wiring
diagram should be disregarded.
       
OK now that you've built it you're gonna want to know how to use it.
There should be three potentiometer knobs sticking out of your neat
little box.  One controls the frequency range that the box outputs, the
other two control the inverter ranges.  Once you set these last two for
optimum sound quality on your phone, leave them alone.  I suggest using
those little ones that you have to use a screwdriver to change.  The
other one can be changed at will as long as the receiving box is tuned
accordingly.  Now the downfall to this is only one person can talk at a
time.  The box can operate only in send OR recieve mode.  I don't have
the patience or the inclination to mess around with the duplexer, so I
will leave that up to you to modify it so it sends and receives at the
same time.  The box only draws about 25+/- mA so in theory you could run
it off the line current.  However, I wouldn't suggest this as it might
interfere with the reception quality.  You might also want to add a
power switch and a couple LED's to indicate whether you are in send or
recieve mode.
       
I did most of the testing of this circuit with a software simulator, so
there may still be some kinks to work out.  However I have built and
used the box and it works fine as far as I can tell, and no telco goons
have come knocking on my door.  Anyone else who eavesdrops on the line
will think you are online happily downloading porn or whatever it is you
like to do in your spare time.  If you have any questions about this
project, email me at zmasterjwb@netscape.net and I'll try to help you
out.  Have fun and remember, nothing is illegal until you get caught.
         
Legal Stuff:  The author of this article assumes no resposibility for
any damage incurred to the person or property of anyone who attempts to
assemble and/or use this project.  Find someone else to sue for your
mistakes.

--The Z Master