An implementation of Open Whisper System's Double Ratchet Algorithm in Haskell.

The Double Ratchet algorithm is used by two parties to exchange encrypted messages based on a shared secret key. Typically the parties will use some key agreement protocol (such as X3DH) to agree on the shared secret key. Following this, the parties will use the Double Ratchet to send and receive encrypted messages. This gives the cryptosystem the property of perfect forward secrecy in which compromise of long-term keys does not compromise past session keys. Forward secrecy protects past sessions against future compromises of secret keys or passwords.

The parties derive new keys for every Double Ratchet message so that earlier keys cannot be calculated from later ones. The parties also send Diffie-Hellman public values attached to their messages. The results of Diffie-Hellman calculations are mixed into the derived keys so that later keys cannot be calculated from earlier ones. These properties gives some protection to earlier or later encrypted messages in case of a compromise of a party's keys.

The example will use the pre-shared keys bob_private and bob_public and ssk.

To compile the example:

> stack ghc Example.hs --package ratchet 
> ./Example server
> ./Example client

To load in GHCi:

> stack ghci --package network
> :load Example.hs

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 Double Ratchet Algorithm

Signal Protocol protocol combines the Double Ratchet Algorithm, prekeys, and a triple Diffie–Hellman (3-DH) handshake, and uses Curve25519, AES-256 and HMAC-SHA256 as primitives.

• Extended Triple Diffie-Hellman

Copyright 2017-2018 Adjoint Inc

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