These are called in-line encryption systems. They're generally not apps, but separate devices with automated ciphertext transmission. I've been working on somet

These are called in-line encryption systems. They're generally not apps, but separate devices with automated ciphertext transmission. I've been working on something that does this http://github.com/maqp/tfc http://github.com/maqp/tfc and that can be plugged to almost any transport system with relative ease. The current design is using v3 onion services for each endpoint.