Re: Peer-to-peer SMS text encryption
Key/message size issues is why the comment talked about stenography. One could use a series of SMS message bodies to perform Diffie–Hellman key exchange, as well as for smaller messages. For large messages, encrypt the message & stenographically hide it in a photo. The photo could be sent by SMS or email. If email, the destination email address can be sent by the (already) encrypted SMS channel. The purpose of the App is of course, to do all of this automatically for the users.