Double Ratchet Algorithm

The Double Ratchet Algorithm, previously called the Axolotl Ratchet, is a key management algorithm developed by Trevor Perrin and Moxie Marlinspike in 2013. It can serve as part of a cryptographic protocol that provides end-to-end encryption for instant messaging. After an initial key exchange, it manages the ongoing renewal and maintenance of short-lived session keys. It combines a ratchet based on the Diffie-Hellman key exchange with a ratchet based on a key derivation function, such as a hash function, which is why it is called a double ratchet. The algorithm provides forward secrecy for messages and implicit renegotiation of forward keys, the two properties for which the protocol is named. As of October 2025 it has been extended to the Sparse Post Quantum Ratchet, which adds a CRYSTALS-Kyber quantum ratchet to give post-quantum forward secrecy and post-compromise security in what the Signal authors call a Triple Ratchet.