Using The Java Library

Configuration

On Android:

dependencies {
  compile 'org.whispersystems:signal-protocol-android:(latest version number)'
}

For pure Java apps:

<dependency>
  <groupId>org.whispersystems</groupId>
  <artifactId>signal-protocol-java</artifactId>
  <version>(latest version number)</version>
</dependency>

Install time

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

Java

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 libsignal-protocol client needs to implement the SignalProtocolStore interface. This will manage loading and storing of identity, prekeys, signed prekeys, and session state.

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

Java

SignalProtocolStore signalProtocolStore = new MySignalProtocolStore();

// Instantiate a SessionBuilder for a remote recipientId + deviceId tuple.
SessionBuilder sessionBuilder = new SessionBuilder(signalProtocolStore,
                                                   recipientId, 
                                                   deviceId);

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

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

// Deliver the message using whatever transport you'd like!
deliver(message.serialize());