This repo contains code for an Android app that sends and receives encrypted text messages using a combination of ECDSA and AES256.
When starting a conversation with a person, the app does a lookup on that person's phone number in its internal database, to see if it has a key for that person. If not, it prompts the user to begin a key exchange between them and who they would like to talk to. The other person will receive a notification that someone wants to exchange keys with them, which they can respond or deny. If a key was found for the phone number entered, users can transparently send and receive messages, without any further action.
In order to avoid dealing with 7-bit character encoding issues, which is the default encoding for SMS messages, this app uses Android's sendDataMessage functionality, which sends a standard 8-bit character. As the 8-bit SMS format does not come with the standard SMS protocol features for handling message sent confirmations or multipart messages, a small TCP-like protocol is implemented in the app.
Broadly speaking, the app breaks down into UI, and backing services. The main screen in the UI is the Conversation activity, and sending and receiving messages is handled by the two services SenderSvc and ReceiverSvc.
Implement additional UI to let users know messages are encrypted
Some cell phone carriers block 8-bit PDUs, which are used in this app as a means of sending the encrypted data. Work on the app has thus been discontinued, although a conversion back to 7-bit PDUs should in theory enable the app to work across all carriers.