android application, Google play store no, it's for personal use, family, friends...
Each app is installed on the smartphone to speak, for example.
1- phone1 (install app) talking to contact <-RSA + AES-> phone2 (install app) talking to contact
2- Key exchange, check !! Now if you can speak with confidence !!
3- phone1 (install app) talk contact <-RSA + AES-> phone2 (x) talk contact
You can not talk, do not have the app installed.
To install the application on your smartphone, install key question, this key and against key can be random, according IMEI for example, or the unique identification.
Key1: 1234567890123456 ; this comes in the application installation
Key2: ???????????????? ; here is enter the correct code.
If the code is correct, the app works
It would take a key generator, Android, c ++, Java, VC ++ VBasic eg the whatsoever.
Could be used OfficeSIP PC Server, it is open source.
In P2P streaming or whatever.