i am Fabio Pietrosanti of KHAMSA, a partner company of Philip Zimmermann and we recently created a new extension of ZRTP called ZRTP/S that let to do secure communication over 2G/3G CSD 9600bps communication!
In the upcoming weeks we will release the source code on zfoneproject.com website.
It would be *very nice* to start a project to make it working on OpenMoko, i am confident that there should be a lot of request of a fully opensource phone to be used for voice encryption.
Please contact me to email@example.com if you are interested in discussing/brainstorming the project!
I think that using RTP (SRTP) will add too much overhead eventhough it would make seamless handover of VoIP calls from CSD to Wifi straightforward.
Yes I think there are two main issues too hand.
Making VOIP calls over the CSD line will not be possible for the specified reason. Therefore, it may be worthwhile examining the cryptofone implementation as a possible means to implementing a solution over the CSD. As far as I am aware they have appropriate measures to tackle the latency and low bandwidth constraints.
Making VOIP calls over EDGE or GPRS may be more suited?
GPRS and EDGE provide enough bandwidth but the really big issue is latency. That's the main advantage of CSD over GPRS/EDGE.
It is certainly possible to implement the protocols and mechanisms of the cryptophone. In some of their documents it's stated that they developed those protocols to create somewhat of a "standard" but other parts of the documentation mention that the usage of the code etc. is restricted for the purpouse of peer-review of the cryptographic soundness and robustness of their technology. Maybe we should inquire their detailed point of view on this issue and if they would take defensive action if the community were to implement their concepts on top of openmoko...