Double Ratchet Algorithm

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In cryptography, the Double Ratchet Algorithm (previously referred to as the Axolotl Ratchet) is a key management algorithm that was developed by Trevor Perrin and Moxie Marlinspike in 2013. It can be used as part of a cryptographic protocol to provide 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 cryptographic so-called "ratchet" based on the Diffie–Hellman key exchange (DH) and a ratchet based on a key derivation function (KDF), such as a hash function, and is therefore called a double ratchet. The algorithm provides forward secrecy for messages, and implicit renegotiation of forward keys; properties for which the protocol is named. This has been extended as of October 2025 to the Sparse Post Quantum Ratchet (SPQR) with a CRYSTALS-Kyber quantum ratchet to provide post-quantum Forward Secrecy and Post-Compromise Security guarantees in what the Signal authors call a Triple Ratchet.