Would be interesting to deploy an app that can send encrypted/signed SMS. It would likely spike your messaging bill thought, as fitting all that in 140 would be hard, resulting in multiple SMSs going out pr message sent.
It was done before. See Signal's predecessor, Textsecure (before they added Signal protocol, pre-2.7.0).
If I recall it limited encrypted messages to 60 characters before splitting. It was also annoying in that you needed to initiate an OTR handshake to start messaging.
That handshake, or the initial exchange of public keys, is the one thing that seems to stymie general adoption of encryption for personal communication.
Current Signal fortunately doesn't make this apparent to the end user, it's install and go message whomever advertises support.
I'm ditching Pidgin-OTR today actually because of the OTR usability issues needing to handshake on every laptop restart. Will be going over to bitlbee terminating OTR on a server and then accessing my ZNC supporting push through TLS. Much less jarring.
OMEMO would be better than OTR for this, but not much supports it yet.
