Signald spectrum2 backend

What is this?

This is a way to connect to the signal instant messaging network using other messaging networks such as XMPP.

It can act as a linked device to your main signal device (your phone), or as a standalone replacement for official signal clients. The latter is my personal use case so this is what is most tested.

Alternatively, you might be interested in the signald python interface in spectrum2_signald/signald.py.

What do I need?

• signald
• spectrum2
• qrencode to link an existing signal device

Could you be a little more specific?

Python stuff

python3 -m venv signald-venv
source signald-venv/bin/activate
pip install spectrum2-signald

XMPP Server

Configure a specific component in your XMPP server, for instance with prosody, add something like this to /etc/prosody/prosody.cfg.lua:

modules_enabled = {
  -- [...]
  "privilege";
}

 -- [...]

Component "signal.example.com"
    component_secret = "something-random"
    modules_enabled = {"privilege"}

-- [...]

VirtualHost "example.com"
    privileged_entities = {
        ["signal.example.com"] = {
            roster = "both";
            message = "outgoing";
        }
    }

The privilege thing is not mandatory, but will allow carbons for messages sent from official signal clients to be sent to XMPP clients.

From my experience, reloading configuration and components while prosody is running is not enough to get the privilege thing working, so you might need to systemctl restart prosody at this point.

Signald

Have the /var/lib/signald/attachments directory readable for the user that runs spectrum2 if you want attachments to work.

Spectrum2

Modify the signal.cfg file to match the component secret, path to the right python interpreter and main.py script, and http upload stuff for attachments. Copy it to /etc/spectrum2/transports. You should now be able to use spectrum2_manager signal.example.com start.

XMPP client

• Discover services on your server.
• Subscribe to the gateway signal.example.com, using your phone number as a username.
• If you already used signald with this phone number, your roster should populate with your signal contacts and you should be able to send receive/contacts.
• If you did not configure signald for this phone number, you should receive a message from a signal@signal.example.com prompting you with instructions to either register your phone number or to link signald to the official signal app.
• To see which groups are available send a message with "groups" as body to this user to get a link of XMPP URIs to join your groups.

How secure is this?

Not as secure as using the official signal client, especially since this is pre-alpha software. You can read more about it in this gitlab issue. As a rule of thumb, if your main concern is privacy and security, you should use the official signal clients. However, if you are running your own XMPP server, this shouldn't be a lot less "secure" than anything else on your XMPP server.

Multi-user considerations

Right now, it is not safe at all to allow public registrations, because of spectrum and signald. This should be improved some day. Right now, my advice is to only use it on your own server, and disable public registrations as soon as you register your user.

What works?

• Send/receive private messages.
• Join and send/receive messages from groups.
• Attachments if the the script has access to a dir that is publicly available via http.
• Carbons for self messages sent from official signal clients.

What doesn't work?

• It crashes quite often, especially this issue seems to be really annoying.
• Groups need to be manually joined via the XMPP client.
• Unsubscribing via XMPP does not delete the signald user.

What is the license?

Something free (libre), yet to be determined. I am no expert in this stuff.