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Signal Protocol implementation for the browser based on libsignal-protocol-java.

/dist       # Distributables
/build      # Intermediate build files
/src        # JS source files
/native     # C source files for curve25519
/protos     # Protobuf definitions
/test       # Tests


A ratcheting forward secrecy protocol that works in synchronous and asynchronous messaging environments.


This protocol uses a concept called 'PreKeys'. A PreKey is an ECPublicKey and an associated unique ID which are stored together by a server. PreKeys can also be signed.

At install time, clients generate a single signed PreKey, as well as a large list of unsigned PreKeys, and transmit all of them to the server.


Signal Protocol is session-oriented. Clients establish a "session," which is then used for all subsequent encrypt/decrypt operations. There is no need to ever tear down a session once one has been established.

Sessions are established in one of two ways:

  1. PreKeyBundles. A client that wishes to send a message to a recipient can establish a session by retrieving a PreKeyBundle for that recipient from the server.
  2. PreKeySignalMessages. A client can receive a PreKeySignalMessage from a recipient and use it to establish a session.


An established session encapsulates a lot of state between two clients. That state is maintained in durable records which need to be kept for the life of the session.

State is kept in the following places:

  • Identity State. Clients will need to maintain the state of their own identity key pair, as well as identity keys received from other clients.
  • PreKey State. Clients will need to maintain the state of their generated PreKeys.
  • Signed PreKey States. Clients will need to maintain the state of their signed PreKeys.
  • Session State. Clients will need to maintain the state of the sessions they have established.


This implementation currently depends on the presence of the following types/interfaces, which are available in most modern browsers.


Include dist/libsignal-protocol.js in your webpage.

Install time

At install time, a libsignal client needs to generate its identity keys, registration id, and prekeys.

var KeyHelper = libsignal.KeyHelper;

var registrationId = KeyHelper.generateRegistrationId();
// Store registrationId somewhere durable and safe.

KeyHelper.generateIdentityKeyPair().then(function(identityKeyPair) {
    // keyPair -> { pubKey: ArrayBuffer, privKey: ArrayBuffer }
    // Store identityKeyPair somewhere durable and safe.

KeyHelper.generatePreKey(keyId).then(function(preKey) {
    store.storePreKey(preKey.keyId, preKey.keyPair);

KeyHelper.generateSignedPreKey(identityKeyPair, keyId).then(function(signedPreKey) {
    store.storeSignedPreKey(signedPreKey.keyId, signedPreKey.keyPair);

// Register preKeys and signedPreKey with the server

Building a session

A libsignal client needs to implement a storage interface that will manage loading and storing of identity, prekeys, signed prekeys, and session state. See test/InMemorySignalProtocolStore.js for an example.

Once this is implemented, building a session is fairly straightforward:

var store   = new MySignalProtocolStore();
var address = new libsignal.SignalProtocolAddress(recipientId, deviceId);

// Instantiate a SessionBuilder for a remote recipientId + deviceId tuple.
var sessionBuilder = new libsignal.SessionBuilder(store, address);

// Process a prekey fetched from the server. Returns a promise that resolves
// once a session is created and saved in the store, or rejects if the
// identityKey differs from a previously seen identity for this address.
var promise = sessionBuilder.processPreKey({
    registrationId: <Number>,
    identityKey: <ArrayBuffer>,
    signedPreKey: {
        keyId     : <Number>,
        publicKey : <ArrayBuffer>,
        signature : <ArrayBuffer>
    preKey: {
        keyId     : <Number>,
        publicKey : <ArrayBuffer>

promise.then(function onsuccess() {
  // encrypt messages

promise.catch(function onerror(error) {
  // handle identity key conflict


Once you have a session established with an address, you can encrypt messages using SessionCipher.

var plaintext = "Hello world";
var sessionCipher = new libsignal.SessionCipher(store, address);
sessionCipher.encrypt(plaintext).then(function(ciphertext) {
    // ciphertext -> { type: <Number>, body: <string> }
    handle(ciphertext.type, ciphertext.body);


Ciphertexts come in two flavors: WhisperMessage and PreKeyWhisperMessage.

var address = new SignalProtocolAddress(recipientId, deviceId);
var sessionCipher = new SessionCipher(store, address);

// Decrypt a PreKeyWhisperMessage by first establishing a new session.
// Returns a promise that resolves when the message is decrypted or
// rejects if the identityKey differs from a previously seen identity for this
// address.
sessionCipher.decryptPreKeyWhisperMessage(ciphertext).then(function(plaintext) {
    // handle plaintext ArrayBuffer
}).catch(function(error) {
    // handle identity key conflict

// Decrypt a normal message using an existing session
var sessionCipher = new SessionCipher(store, address);
sessionCipher.decryptWhisperMessage(ciphertext).then(function(plaintext) {
    // handle plaintext ArrayBuffer


To compile curve25519 from C souce files in /native, install emscripten.

grunt compile


Copyright 2015-2018 Open Whisper Systems

Licensed under the GPLv3:

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