Sample paragraph from http://whispersystems.org/blog...

The result is a ratcheting protocol that combines the best of a DH ratchet’s
“future secrecy” properties with the optimal “forward secrecy” properties from a
hash ratchet. It also simplifies the wire format and eliminates all the key ID
book keeping. Nice properties like cryptographically-enforced message ordering
and replay protection all come for free, without any complex record keeping
required. The code is simpler, and the protocol security is more robust.

We think this represents an improved cryptographic protocol for asynchronous
messaging systems like TextSecure. Another asynchronous messaging system, Pond,
has also incorporated it.
