purple-signal

A libpurple/Pidgin plugin for signal (formerly textsecure). Using modified parts of signal-cli which in turn is employing a fork of the official implementation.

State

This plug-in is no longer being developed. The author writes:

This plug-in essentially was a C/C++ wrapper around the (official) Java implementation libsignal-service-java. However, Signal itself is moving away from their own reference implementation. They now favour libsignal-client, which implements the same functionality in Rust. This plug-in ended up as a C wrapper around a Java compatibility layer around a Rust binary which is hard to build. Everything is moving in different directions and nothing is stable. Even nodejs seems to be involved at some point.

Moving away from the platform-independent Java implementation, Signal has pulled support win32 so hard not even third parties can maintain it. Maybe purple-signald will be revived so at least Linux users can have Signal in Pidgin again.

Download

Binaries once were availiable at https://buildbot.hehoe.de/purple-signal/builds/.

See INSTALL on how to install this plug-in.
If you want to build the plug-in yourself see BUILD.

Concept

libpurple plug-ins are written in C.
The signal client library is written in Java.
This project uses JNI to create a Java VM instance within the C part of the plug-in. An instance of the Java Class implementing the signal client is created. Upon receival of a message, a static method is called. This call-back method is a native method implemented in C. It forwards the data to libpurple.
This may be considered dark magic by some.

Please note this is the third purple plugin I have ever written. I still have no idea what I am doing.

Alternative Backends

These back-ends were considered, but not used:

• libsignal-service-java cannot link to existing accounts
• SignalProtocolKit deprecated, recommends libsignal-protocol-c
• libsignal-protocol-c development seems to have stalled in 2018
• textsecure "The API (…) is in flux"
• libsignal-client "use outside Signal not yet recommended"

Features

This plug-in is a proof-of-concept with very little features:

• Register a new account (without captcha only)
• Link to an existing account
  Use the provided code text with signal-cli
  or
  Scan the provided QR code with with the official app
• Simple Messaging
  • one-to-one conversation
  • group conversations (linked only)
  • receiving attachments
• Contact list download

Please note that, as of now, you have to add your contacts manually and they appear "offline" in the buddy list.

For more functionality, check out purple-signald. It offers more features, but it depends on a third-party service and thus is Linux only.

Missing Features

• Retry failed messages
• Sending attachments
• Proper group chats
• Group chat management (explicit join, leave, administration, …)
• everything else

Known Problems

• May require new link or registration after a crash. Your number can be banned temporarily.
• If the plug-in crashes while handling a message, the message is lost.
• The message cache is not implemented and I have no idea what that may entrail. Probably the above.
• First message from purple-signal to official signal is not sent properly.
• Avatars are downloaded into working directory.
• It takes two minutes for the internal JVM to shut down. Or it crashes when terminating Pidgin. This issue disappears and reappears erratically.