This is a ratcheting forward secrecy protocol that works in synchronous and asynchronous messaging environments. The protocol overview is available here, and the details of the wire format are available here.

PreKeys

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.

Sessions

The axolotl 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 three ways:

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.
PreKeyWhisperMessages. A client can receive a PreKeyWhisperMessage from a recipient and use it to establish a session.
KeyExchangeMessages. Two clients can exchange KeyExchange messages to establish a session.

State

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.

Using libaxolotl

Install time

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

IdentityKeyPair    identityKeyPair = KeyHelper.generateIdentityKeyPair();
int                registrationId  = KeyHelper.generateRegistrationId();
List<PreKeyRecord> preKeys         = KeyHelper.generatePreKeys(startId, 100);
PreKeyRecord       lastResortKey   = KeyHelper.generateLastResortKey();
SignedPreKeyRecord signedPreKey    = KeyHelper.generateSignedPreKey(identityKeyPair, 5);

// Store identityKeyPair somewhere durable and safe.
// Store registrationId somewhere durable and safe.

// Store preKeys in PreKeyStore.
// Store signed prekey in SignedPreKeyStore.

Building a session

A libaxolotl client needs to implement four interfaces: IdentityKeyStore, PreKeyStore, SignedPreKeyStore, and SessionStore. These will manage loading and storing of identity, prekeys, signed prekeys, and session state.

Once those are implemented, building a session is fairly straightforward:

SessionStore      sessionStore      = new MySessionStore();
PreKeyStore       preKeyStore       = new MyPreKeyStore();
SignedPreKeyStore signedPreKeyStore = new MySignedPreKeyStore();
IdentityKeyStore  identityStore     = new MyIdentityKeyStore();

// Instantiate a SessionBuilder for a remote recipientId + deviceId tuple.
SessionBuilder sessionBuilder = new SessionBuilder(sessionStore, preKeyStore, signedPreKeyStore,
                                                   identityStore, recipientId, deviceId);

// Build a session with a PreKey retrieved from the server.
sessionBuilder.process(retrievedPreKey);

SessionCipher     sessionCipher = new SessionCipher(sessionStore, recipientId, deviceId);
CiphertextMessage message      = sessionCipher.encrypt("Hello world!".getBytes("UTF-8"));

deliver(message.serialize());