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{{#seo:
|title= Double Ratchet Algorithm – zaoniao Wiki
|keywords=double ratchet algorithm,double ratchet algorithm dra,double ratchet algorithm explained,ratcheted encryption,key exchange
|description=Double Ratchet Algorithm is a key management algorithm that was developed by Trevor Perrin and Moxie Marlinspike in 2013
}}
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[[File:Double Ratchet Algorithm.png|350px|right|Double Ratchet Algorithm]]
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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 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 protocol provides confidentiality, integrity, authentication, participant consistency, destination validation, forward secrecy, backward secrecy (aka future secrecy), causality preservation, message unlinkability, message repudiation, participation repudiation, and asynchronicity.

== Functioning ==
{{#ev:youtube|-ofZMnKkp_Y|450|right|Encrypting Matrix Building a universal end-to-end encrypted communication ecosystem with Matrix and Olm
by Matthew Hodgson}}
A client renews session key material in interaction with the remote peer using Diffie–Hellman ratchet whenever possible, otherwise independently by using a hash ratchet. Therefore, with every message a client using the double ratchet advances one of two hash ratchets (one for sending, one receiving) which get seeded with a common secret from a DH ratchet. At the same time it tries to use every opportunity to provide the remote peer with a new public DH value and advance the DH ratchet whenever a new DH value from the remote peer arrives. As soon as a new common secret is established, a new hash ratchet gets initialized.

As cryptographic primitives, the Double Ratchet Algorithm uses
* for the DH ratchet
* Elliptic curve Diffie–Hellman (ECDH) with Curve25519,
* for message authentication codes (MAC, authentication)
* Keyed-Hash Message Authentication Code (HMAC) based on [[SHA-256]],
* for symmetric encryption
* the Advanced Encryption Standard (AES), partially in Cipher Block Chaining mode (CBC) with padding as per PKCS#5 and partially in Counter mode (CTR) without padding,
* for the hash ratchet
* HMAC.

== Applications ==

The following is a list of applications that use the '''Double Ratchet Algorithm''' or a custom implementation of it:
* ChatSecure
* Conversations
* Cryptocat
* Facebook Messenger
* G Data Secure Chat
* Gajim
* Google Allo
* Pond
* Signal
* WhatsApp
* Wire

Other:
* In May 2016, Viber said that their encryption protocol is a custom implementation that "uses the same concepts" as the '''Signal Protocol'''.

==Source==

[http://wikipedia.org/ http://wikipedia.org/]
[[Category:Cryptography]]
[[Category:Cryptographic algorithms]]

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