Here’s a good look at what the Signal protocol is all about.
“WhatsApp recently announced that client communications are now end-to-end encrypted using Open Whisper System’s “Signal Protocol” (previously Axolotl). This has received quite a bit of press lately due to WhatsApp’s massive user base, along with the controversial going dark debates. Less importantly, the crypto-nerd in me loves Signal. Because of all of this, I thought I would write a blog series on some of Signal’s design decisions that I feel are well-designed.”
Source: End-to-End WhatsApp: An Opinionated Series on Why Signal Protocol is Well-Designed