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{{short description|Cryptographic key management algorithm}}
{{Redirect|Double ratchet|the hand tool|Wrench}}
[[File:Double Ratchet Algorithm.png|350px|thumb|right|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.<ref>Trevor Perrin (editor), Moxie Marlinspike, "[https://signal.org/docs/specifications/doubleratchet/ The Double Ratchet Algorithm]. Revision 1, 2016-11-20</ref>]]
In [[cryptography]], the '''Double Ratchet Algorithm''' (previously referred to as the '''Axolotl Ratchet'''<ref name="Perrin-2016-03-30">{{cite web|last1=Perrin|first1=Trevor|title=Compare Revisions|url=https://github.com/trevp/double_ratchet/wiki/Home/_compare/6fa4a516b01327d736df1f52014d8b561a18189a...ab41721f9ed7ca0bdac3e24ce9fc573750e0614d|website=GitHub|access-date=9 April 2016|date=30 March 2016}}</ref><ref name="signal-inside-and-out">{{cite web|last1=Marlinspike|first1=Moxie|title=Signal on the outside, Signal on the inside|url=https://whispersystems.org/blog/signal-inside-and-out/|publisher=Open Whisper Systems|access-date=31 March 2016|date=30 March 2016}}</ref>) is a [[Key (cryptography)|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-agreement protocol|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.
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