At the time of writing there is a Android and an iOS version. It is developed and maintained by Open Whisper Systems. Signal App – is the only visible part to the end user. If you do not need voice/video calls this component can be omitted. For this component you can use a standard On-premises or Cloud TURN implementation, such as Linux Coturn () which is available as a standard package on the majority of Linux distributions. It is used for establishing voice and video calls.

STUN/TURN Server – is a VoIP media traffic NAT traversal server and gateway. Without the push service the App has to be permanently connected to the Signal-Server in order to receive the messages, thus reducing the battery life of the mobile device or the user hast manually check for new messages by opening the App. It is entirely written in Java and the configuration is sored in a yml file which must be passed as an command line argument to the java binaries. Push-Server – acts as a gateway between Signal-Server and Google Cloud Messaging / Apple Push Notification Service. Before running Signal-Server the Push-Server must be up and running. The configuration is stored in a yml file (at a first glance it seems quite complex) and is passed as a command line argument to the java binaries. An additional PostgreSQL database and Redis NoSQL object store are needed in order to run the Signal-Server. Signal-Server is entirely written in Java source code is available at the following URL. It manages the registration of new accounts, stores and distributes the public keys of the clients, relays and temporarily stores the end-to end encrypted messages, enforces rate limits in order to avoid denial of service attacks and provides pre-signed URLs to the mobile client for uploading files. Signal-Server – is the central element of the Signal Messenger ecosystem, being constantly developed by Open Whisper Systems. In the following sections I will provide some information about the system components and their interconnections. The source code is available on GitHub () only for audition purposes without providing additional documentation about on premises installations.

The main system component is the Signal-Server which is maintained by Open Whisper Systems. For user identification standard cellular mobile numbers are used. It uses the TCP/IP (Internet) to send one-to-one and group messages, which can include text, files, voice notes, images and videos, and make one-to-one voice and video calls.

Signal is an end-to-end encrypted communications application for Android and iOS similar to WhatsApp but open source.

Signal messenger web client