Double Ratchet

A pure Rust implementation of the Double Ratchet, as specified by Trevor Perrin and Moxie Marlinspike.

The Double Ratchet allows two users to communicate securely: it provides its users with a confidential and authentic channel, which includes forward secrecy and future secrecy. After initialization with a shared secret key and an authenticated public key, the Double Ratchet will automatically handle all key management required to support this channel, which includes handling the decryption of messages that arrive out-of-order.

The Double Ratchet itself requires a public key crypto system that can perform Diffie-Hellman (DH) operations, a secret key crypto system that provides authenticated encryption with associated data (AEAD) and two key derivation functions (KDF). This crate aims to be agnostic towards the implementation of these functions: users of the crate implement the CryptoProvider trait and the DoubleRatchet struct should take care of the rest.¹

Examples

The following example corresponds to the way the Double Ratchet is used in the Signal protocol. For more details about the implementation, see tests/signal.rs, which also supplies SignalCryptoProvider. We assume that Alice and Bob share a secret key SK and Alice knows Bob's public key.

use double_ratchet::{DoubleRatchet};
use rand_os::RandOs;
let mut rng = OsRng::new().unwrap();

type DR = DoubleRatchet<SignalCryptoProvider>;

// Alice intializes and sends the first message
let mut alice = DR::new_alice(&SK, bobs_public_prekey, None, &mut rng);
let pt0 = b"Hello Bob";
let (h0, ct0) = alice.ratchet_encrypt(pt0, b"A2B", &mut rng);

// Bob initializes and receives the first message
let mut bob = DR::new_bob(&SK, bobs_prekey_pair, None);
assert_eq!(
    Ok(Vec::from(&pt0[..])),
    bob.ratchet_decrypt(&h0, &ct0, b"A2B")
);

// After receiving the first message, Bob can send his replies
let pt1 = b"Hi Alice";
let (h1, ct1) = alice.ratchet_encrypt(pt1, b"B2A", &mut rng);
let pt2 = b"How are you?";
let (h2, ct2) = bob.ratchet_encrypt(pt2, b"B2A", &mut rng);
assert_eq!(
    Ok(Vec::from(&pt_b_0[..])),
    alice.ratchet_decrypt(&h_b_0, &ct_b_0, b"B2A")
);

// Note that Alice has not yet received Bob's first message...
let pt3 = b"Good and you?";
let (h3, ct3) = alice.ratchet_encrypt(pt3, b"A2B", &mut rng);
assert_eq!(
    Ok(Vec::from(&pt3[..])),
    bob.ratchet_decrypt(&h3, &ct3, b"A2B")
);
// ...but when she does get it she will be able to decrypt
assert_eq!(
    Ok(Vec::from(&pt1[..])),
    alice.ratchet_decrypt(&h1, &ct1, b"B2A")
);

Installation

The Double Ratchet crate is distributed through crates.io: install it by adding the following to your Cargo.toml:

[dependencies]
double-ratchet = "0.1"

Documentation

The documentation is available here.

Future plans

This isn't even my final form! I intend to add at least the following features and am open for suggestions for more features.

• a Header Encrypted variant of the Double Ratchet

• generalize the KeyStore to allow automatic deletion of very old keys

• provide a way for saving/restoring a DoubleRatchet to storage