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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:

Currently I've found this, which looks plausible

So the questions to this topic are:

  1. Is it possible to use modern crypto (TLS1.1+, ECC, ...) through GSM voice channel, to achieve full handshake, dhkey exchange, and forward-secrecy?
  2. What voice codec would be best to provide plausible voice quality, over such tight channel (ie. 1.2-1.7 kbps) ?
  3. 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 ?
  4. 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 ?
  5. Is there any difference between applying these concepts to standard GSM calls and VoLTE (Voice over LTE) ?
  • Root/jailbreak would be necessary in any case since phone OSes don't allow third-party applications to control the call audio stream. For VoLTE first of all you'd get much better bandwidth assuming both devices use VoLTE, but besides that you can't do much more because although it's an IP-based protocol, it is only between the client running on the phone and the IMS server. From there the IMS server could do anything with the stream, so you still need to consider it as a dumb audio stream - you can't just put encrypted data in the RTP packets since it may get transcoded down the line. – André Borie Aug 1 '16 at 19:34
  • you need a "modem", but 1.7kbps should be on the inside edge of capability. – dandavis Aug 1 '16 at 20:23
  • @dandavis see the linked paper, its effective speed after protocol overhead. – Marek Sebera Aug 1 '16 at 21:24
  • <a href="en.wikipedia.org/wiki/ZRTP">ZRTP</a> supports encryption on GSM and other legacy standards. – user119214 Aug 2 '16 at 0:39

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