The Double Ratchet Algorithm's design is based on the DH ratchet that was introduced by Off-the-Record Messaging (OTR) and combines it with a symmetric-key ratchet modeled after the Silent Circle Instant Messaging Protocol (SCIMP). The Double Ratchet Algorithm was developed by Trevor Perrin and Moxie Marlinspike ( Open Whisper Systems) in 2013 and introduced as part of the Signal Protocol in February 2014. Unlike a mechanical ratchet, however, each state is unique. In the mechanical sense, a ratchet only allows advancement in one direction a cryptographic ratchet only allows keys to be generated from the previous key. The term "ratchet" in cryptography is used in analogy to a mechanical ratchet. " Axolotl" was in reference to the salamander's self-healing properties. This property was later named Future Secrecy, or Post-Compromise Security. This effectively forces the attacker to intercept all communication between the honest parties, since they lose access as soon as a key exchange occurs that is not intercepted. New session keys are exchanged after a few rounds of communication. The algorithm is considered self-healing because under certain conditions it prevents an attacker from accessing the cleartext of future messages after having compromised one of the user's 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. After an initial key exchange it manages the ongoing renewal and maintenance of short-lived session keys. It can be used as part of a cryptographic protocol to provide end-to-end encryption for instant messaging. 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.
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