At Open Whisper Systems we help develop TextSecure, an encrypted chat application for Android. TextSecure was designed as a general purpose SMS/MMS client which would also automatically encrypt conversations when communicating with other TextSecure users. For those encrypted sessions, TextSecure uses a compact derivative of the well-known OTR protocol.
We’re currently in the process of transitioning TextSecure to use a device’s data channel as a transport for communication with other TextSecure users whenever possible. This enables communication with the upcoming TextSecure for iOS, helps users avoid SMS fees, and obscures conversation metadata from telcos.
The transition to a new transport is also a good opportunity for us to evaluate and introduce additional cryptographic protocol changes. Below is one cryptographic protocol change we’re thinking of making that we’d welcome feedback on.