signal-rs

A Rust implementation of the Signal Protocol. Quite rough around the edges, and no security guarantees given. Just for curiosity.

• Overview
• Quick Start
• Components
• Road Map
• More Information

Overview

The Signal Protocol is at the heart of all mainstream end-to-end encrypted messaging these days: in addition to Signal, both Whats App and Facebook Messenger use it (along with a lot of other messaging apps). It is composed of two main parts: the X3DH key exchange protocol, and the Double Ratchet algorithm.

Quick Start

Run cargo test --all. Among others, this runs the test in crates/signal/src/lib.rs, which is a mockup of a complete end-to-end conversation including both key exchange and several iterations of the ratchet, however, it is entirely in-process.

Then try make server, followed by (in another tab) make. This runs the multi-process example. The first command starts the server, which provides key & message relay. The second command starts the two client processes, which communicate with one another via the server.

Components

The two key crates are x3dh, which implements the key exchange algorithm, and double-ratchet, which implements the session key ratcheting algorithm.

x3dh

The main entity is the Participant, which manages generating and storing the various keys in play. In addition, you can create a local Keyserver to simulate the key relay functions in-process.

double-ratchet

The main entity is the Session, which can be created with a SessionBuilder. Initialize it with the shared session key, begin a connection, and then use the Session to encrypt and decrypt messages.

Road Map

• Clean up x3dh and double-ratchet public APIs & docs.
• Get rid of any unwrap or panic!
• Use the log crate for logging.
• Use a more curated set of crypto implementations.
• Implement the encrypted headers extension.
• Complete client and server example implementations.
• Make a little interactive chat client example.
• Make Keyserver trait asyncable so we can use it with a remote server?
• Make a Relayserver trait for double-ratchet that corresponds to the Keyserver trait for x3dh?

More Information

• The Signal Protocol Specifications, specifically:
  • X3DH, and
  • Double Ratchet.
• Curve25519 math by Dalek Cryptography
  • curve25519-dalek
  • ed25519-dalek
  • x25519-dalek
• Some primitives from RustCrypto: aes and sha2.
• HMAC and HKDF implementations from orion.