Axolotl

Axolotl (now Signal Protocol) is the open-source cryptographic protocol providing end-to-end encryption, perfect forward secrecy, and post-compromise security for asynchronous messaging.

This protocol, originally named Axolotl by creators Trevor Perrin and Moxie Marlinspike (Open Whisper Systems), is the gold standard for secure communication. It combines the Double Ratchet Algorithm with the Extended Triple Diffie-Hellman (X3DH) key agreement: this design ensures new session keys are generated after every message, preventing retroactive decryption even if a long-term key is compromised. Deployed across major platforms like Signal, WhatsApp, and Google Messages, it secures the daily communications of billions of users (over a billion on WhatsApp alone), delivering confidentiality, integrity, and cryptographic deniability.

https://signal.org/docs/