The Double Ratchet Algorithm is a protocol (similar to OTR) that provides for
perfect forward secrecy in (a)synchronous communications. It uses triple
Diffie-Hellman for authentication and ECDHE for perfect forward secrecy. The
protocol is lighter and more robust than the OTR protocol - providing better
forward and future secrecy, as well as deniability.

The protocol was developed by Trevor Perrin and Moxie Marlinspike. Its chief
use currently is in the Open Whisper Systems Signal package.

WWW: https://github.com/rxcomm/pyaxo

PR: 222040
Submitted by: Yuri Victorovich (maintainer)
Reviewed by: lifanov (mentor), matthew (mentor), koobs, sunpoet
Approved by: lifanov (mentor), matthew (mentor), koobs
Differential Revision: https://reviews.freebsd.org/D12310