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 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 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. New session keys are exchanged after a few rounds of communication. 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. This property was later named Future Secrecy, or Post-Compromise Security. (Wikipedia).
