Double Ratchet Algorithm

Full ratchet step in the double ratchet algorithm. The Key Derivation Function (KDF) provides the ratcheting mechanism. The first "ratchet" is applied to the symmetric root key, the second ratchet to the asymmetric Diffie–Hellman (DH) key.

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.

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