As the issue stands, it should be (or could be) possible, to encrypt voice, that's being passed through GSM voice call in such way, so that it's end-to-end encrypted.
Ideally in form similar to what Signal/Redphone does, but through sms-transport/gsm-voice-call-transport
In past, there were some efforts, to achieve this, such as:
- http://jackpair.com/ (actually funded project)
Currently I've found this, which looks plausible
So the questions to this topic are:
- Is it possible to use modern crypto (TLS1.1+, ECC, ...) through GSM voice channel, to achieve full handshake, dhkey exchange, and forward-secrecy?
- What voice codec would be best to provide plausible voice quality, over such tight channel (ie. 1.2-1.7 kbps) ?
- Is it possible to achieve this fully by software, or would the root/jailbreak requirement be too strict, and hardware device would be better wide-usable solution ?
- What should be used to detect support of encryption (and maybe level/version of support) of the other calling party ? Could that be achieved by DTMF ?
- Is there any difference between applying these concepts to standard GSM calls and VoLTE (Voice over LTE) ?