Double Ratchet Algorithm

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 to provide end-to-end encryption for instant messaging. After an initial it manages the ongoing renewal and maintenance of short-lived session keys. It combines a cryptographic ratchet based on the Diffie–Hellman key exchange (DH) and a ratchet based on a key derivation function (KDF) like e.g. a hash function and is therefore called a double ratchet.

The developers refer to the algorithm as self-healing because it automatically disables an attacker from accessing the cleartext of later messages after having compromised a session key.

The Double Ratchet Algorithm was developed by Trevor Perrin and Moxie Marlinspike (Open Whisper Systems) in 2013 and introduced as part of the in February 2014. The Double Ratchet Algorithm's design is based on the DH ratchet that was introduced by Off-the-Record Messaging and combines it with a symmetric-key ratchet modeled after the (SCIMP). The ratchet was initially named after the critically endangered aquatic salamander axolotl, which has extraordinary self-healing capabilities. In March 2016, the developers renamed the Axolotl Ratchet as the Double Ratchet Algorithm to better differentiate between the ratchet and the full protocol, because some had used the name Axolotl when referring to the Signal Protocol.

The Double Ratchet Algorithm features properties that have been commonly available in end-to-end encryption systems for a long time: encryption of contents on the entire way of transport as well as authentication of the remote peer and protection against manipulation of messages. As a hybrid of DH and KDF ratchets, it combines several desired features of both principles. From OTR messaging it takes the properties of forward secrecy and automatically reestablishing secrecy in case of compromise of a session key, forward secrecy with a compromise of the secret persistent main key, and plausible deniability for the authorship of messages. Additionally, it enables session key renewal without interaction with the remote peer by using secondary KDF ratchets. An additional key-derivation step is taken to enable retaining session keys for out-of-order messages without endangering the following keys.

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