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SaltyRTC WebRTC Task for Java

Build status Java Version License Chat on Gitter

This is a SaltyRTC WebRTC task implementation for Java 8+.

For an application example, please see our demo application.


The package is available on Bintray.


compile 'org.saltyrtc.tasks.webrtc:saltyrtc-task-webrtc:0.18.0'




To create the task instance, you need to use the WebRTCTaskBuilder instance which can be used to configure the task before creating it.

The below configuration represents the default values chosen by the builder as if you had not configured the builder and just called .build() directly.

final WebRTCTask task = new WebRTCTaskBuilder()

To send offers, answers and candidates, use the following task methods:

  • task.sendOffer(offer: @NonNull Offer): void
  • task.sendAnswer(answer: @NonNull Answer): void
  • task.sendCandidates(candidate: @NonNull Candidate[]): void

You can register an event handler in the following way:

task.setMessageHandler(new MessageHandler() {
    public void onOffer(@NonNull Offer offer) {
        // Handle offer

    public void onAnswer(@NonNull Answer answer) {
        // Handle answer

    public void onCandidates(@NonNull Candidate[] candidates) {
        // Handle candidates

Data Channel Crypto Context

The task provides another security layer for data channels which can be leveraged by usage of a DataChannelCryptoContext instance. To retrieve such an instance, call:

final DataChannelCryptoContext context = task.createCryptoContext(;

You can encrypt messages on the sending end in the following way:

final Box box = context.encrypt(yourData);

On the receiving end, decrypt the message by the use of the crypto context:

final Box box = new Box(message);
final byte[] yourData = context.decrypt(
    box, DataChannelCryptoContext.NONCE_LENGTH);

Note, that you should not use a crypto context for a data channel that is being used for handover. The task will take care of encryption and decryption itself.


Before initiating the handover, the application needs to fetch the SignalingTransportLink instance which contains the necessary information to create a data channel.

final SignalingTransportLink link = task.getTransportLink();

final DataChannel.Init parameters = new DataChannel.Init(); = link.getId();
parameters.negotiated = true;
parameters.ordered = true;
parameters.protocol = link.getProtocol();

final DataChannel dataChannel = peerConnection.createDataChannel(
    link.getLabel(), parameters);

Note that the data channel used for handover must be created with the label and parameters as shown in the above code snippet.

Now that you have created the channel, you need to implement the ISignalingTransportHandler interface. Below is a minimal handler that forwards the necessary events and messages to the created data channel.

final ISignalingTransportHandler handler = new ISignalingTransportHandler() {
    public long getMaxMessageSize() {
        return peerConnection.sctp().getMaxMessageSize();

    public void close() {

    public void send(@NonNull final ByteBuffer message) {
        // Note: Always send binary
        dataChannel.send(new DataChannel.Buffer(message, true));

Furthermore, you have to bind all necessary events in order to connect the data channel to the SignalingTransportLink.

dataChannel.registerObserver(new DataChannel.Observer() {
    // [...]

    public void onStateChange() {
        switch (dataChannel.state()) {
            case OPEN:
            case CLOSING:
            case CLOSED:

    public void onMessage(@NonNull final DataChannel.Buffer buffer) {
        if (!buffer.binary) {
            // Note: This should be handled as a protocol error
        } else {

    // [...]

The above setup will forward the CLOSING/CLOSED state and all messages to the task by the use of the SignalingTransportLink. On OPEN, the handover will be initiated.

To be signalled once the handover is finished, you need to register the handover event on the SaltyRTC client instance.


The library uses the slf4j logging API. Configure a logger (e.g. slf4j-simple) to see the log output.

Manual Testing

To try a development version of the library, you can build a local version to the maven repository at /tmp/maven:

./gradlew uploadArchives

Include it in your project like this:

repositories {
    maven { url "/tmp/maven" }

Coding Guidelines

Unfortunately we cannot use all Java 8 features, in order to be compatible with Android API <24. Please avoid using the following APIs:

  • java.lang.annotation.Repeatable
  • AnnotatedElement.getAnnotationsByType(Class)
  • java.lang.FunctionalInterface
  • java.lang.reflect.Method.isDefault()
  • java.util.function

The CI tests contains a script to ensure that these APIs aren't being called. You can also run it manually:

bash .circleci/

Automated Testing

1. Preparing the Server

First, clone the saltyrtc-server-python repository.

git clone
cd saltyrtc-server-python

Then create a test certificate for localhost, valid for 5 years.

openssl req -new -newkey rsa:1024 -nodes -sha256 \
    -out saltyrtc.csr -keyout saltyrtc.key \
    -subj '/C=CH/O=SaltyRTC/CN=localhost/'
openssl x509 -req -days 1825 \
    -in saltyrtc.csr \
    -signkey saltyrtc.key -out saltyrtc.crt

Create a Java keystore containing this certificate.

keytool -import -trustcacerts -alias root \
    -file saltyrtc.crt -keystore saltyrtc.jks \
    -storetype JKS -storepass saltyrtc -noprompt

Create a Python virtualenv with dependencies:

python3 -m virtualenv venv
venv/bin/pip install .[logging]

Finally, start the server with the following test permanent key:

export SALTYRTC_SERVER_PERMANENT_KEY=0919b266ce1855419e4066fc076b39855e728768e3afa773105edd2e37037c20 # Public: 09a59a5fa6b45cb07638a3a6e347ce563a948b756fd22f9527465f7c79c2a864
venv/bin/saltyrtc-server -v 5 serve -p 8765 \
    -sc saltyrtc.crt -sk saltyrtc.key \

2. Running Tests

Make sure that the certificate keystore from the server is copied or symlinked to this repository:

ln -s path/to/saltyrtc-server-python/saltyrtc.jks

With the server started in the background and the saltyrtc.jks file in the current directory, run the tests:

./gradlew test


Release Checksums

These are the SHA256 hashes for the published releases of this project:

  • v0.18.0: be01a55f674d3c8ebc5b785d4ea06ddff0a8f7960aaf9d947fc2f155443d95a1
  • v0.17.0: 97b052618be3ac970d359a99ebc4134e78dad9e5f87be4939394b271aca501ce
  • v0.16.1: af6878a27761624962ab6bc45eedf7b2f5196036d1cf5dcd69fe5724737a2d59
  • v0.16.0: 0bffd6f38b348ae051c9ced7a7a11f2777eba427b1329eb5500166d1884af8f5
  • v0.15.0: f50d9e22e4bc7059cd2bc5547678aae8e1f2304fe05901bcb0dc0cd1d3d13991
  • v0.14.1: 1c4c8ec94c3aab12d29bf842f23b7598286290d46e4aba76302e002cd96072c9
  • v0.14.0: f8cc8cc51dab11f263d07a1b23ad18e6f1b718387b646feebb447be3b0fee8e4
  • v0.13.0: 122360a8586526a6293099995b813c96af0c422376f37f1227f36a3415b0d49e
  • v0.12.0: a6803909f39712b753b475b5b199e9807c767f78790ec3a59b625f80179a43c8
  • v0.11.0: b4161390b63a0e9166e9991357e57c4a28a1f452f378af0fca571ca19f4aa99d
  • v0.10.0: fd8602252a130836d0be309e0fdd2bb72741cc92e4346d2ce0e0a95980ae9361
  • v0.9.2: 4142aead0eb700cfe8871e1a0c16e20fdea0249599574ffc0cff3624bd2c778f
  • v0.9.1: 73694f9a4912344457f1c6ae19aaa788131e551c9781554663c3411ce86da6bc
  • v0.9.0: f9dd33fa19b3be5245ac96c5464b2d1fb6506f87696e272a8264ae6726f09900
  • v0.6.1: b928cd705501e7be8cf4be0f54fb890f21a5abfd33899d603604bbb7681a7ef1
  • v0.6.0: 7487a1c6e0be4dce7627b62dc1cc9eb17d4d8da38ec8ec88e928163596e50795
  • v0.5.1: ec74be6551dd4c886f4f4c4928b5645249b9dd203836a5808aed9d6a602c251c
  • v0.5.0: 6949efa248dcbf22b9c2ffec1063ce54557cfce4be11ef6f4d9fcb0549bca150
  • v0.4.1: 105423ed377e8c149cb6a4fe67be63e43bd83102191740edc4e3c4b0f3a9cbfe
  • v0.4.0: a71ec1a7af32c104179569d037fea249a9c399d2eb27b07456d9ae578170dc2d
  • v0.3.3: 73e9a067c849836a5634e8d61f4bc9e046bb91feebc85ebc6a01de35866ee27e
  • v0.3.2: 4c36304a207fff8030a1b21b883c97f30101b44963817107a8d6034d433ca27f
  • v0.3.1: 27856a600db2fe1e1a8d8449123c4a110a481ca828cfa43ee618a65f86ca83d0
  • v0.3.0: 66a92915c1936a8065aa2cbb824ee6ba91d0c2667c22aa0f0bc88a79a4f710fe

You can use tools like gradle witness to make sure that your application always gets the correct version of this library.

Responsible Disclosure / Reporting Security Issues

Please report security issues directly to one or both of the following contacts:


Copyright (c) 2016-2019 Threema GmbH

Licensed under the Apache License, Version 2.0, <see LICENSE-APACHE file>
or the MIT license <see LICENSE-MIT file>, at your option. This file may not be
copied, modified, or distributed except according to those terms.


WebRTC Task for saltyrtc-client-java.





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