Nokia voice encryption device project

Hi there guys ! Yesterday, I had a project ideea. I thought about a gsm encryption device. The

A5 algorythm is safe enough for a lame hacker who want to tap a conversation over the air, but

the guys from the phone company can listern to you quite easy. So , why not building a few

crypting devices, and double encoding the communications between a few phones ...
I thought about two methods of doing this.
1. The encoding device can be inside the phone, inserted between the A/D D/A converter and

the main processor, where the digital signal is A5 encrypted.I studied Nokia 3310 schematic,

and found out quite interesting things.
COBBA asIC provides an interface between the baseband and the RF circuitry . COBBA performs

A/D conversion of the analog signal comming from the microphone, and fr the transmit path

COBBA performs D/A conversion.COBBA is connected with the main processor MAD2WD1 with two

serial interfaces, one for transferring rx/tx signals between MAD2WD1 and COBBA, and one for

transferringcodec Rx/Tx samples.I thought about cutting the PCM samples lines ( rx and Tx )

and inserting a microcontroller programmed to crypt theese samples. As you can see in the

pictures, on theese lines there are four fumpers ... so I think that can access these lines,

somewhere on the surface of PCB, without desoldering MAD2 or COBBA. If someone knows where

theese jumpers are located on the PCB please mail me.


2.The encoding device can be done outside the phone, conected at Rx/Tx on the external

accessory connector, and the call will be in fact a data type call. But this in far more

complicated.I have an ideea doing this, but I would prefer the first way.


If someone is interested in this project and can help somehow, please feel free topost here .

Why do this in hardware, when it can be done in software?
You'd have to find the piece of DSP code responsible for doing voice, yes it's a lot of work but so is this approach