How hard is it to reprogram the DSP that comes with a cellular phone right now? I've never opened one up. Can you just unsolder a rom, read it, insert your own code for DH key exchange, add some encryption, burn a new ROM and have a secure phone?
Also, it is not sufficient to incorporate encryption merely into the cellular phone itself. You need the cooperation of the base station, at least if you want to interoperate with an ordinary telephone on the land side of your connection. Now it would be possible to provide your own encryption on an end-to-end basis using a data (as opposed to voice) bearer service from the carrier, but this would require the person you call to have compatible equipment (vocoder, modem, encryption routines, keys, etc). Phil