Using Signal for webhook notifications

Signal is (in my opinion) a fantastic messaging app. It is cross platform, so you don't need to worry about having an Android or iOS device. It is accessible on Windows, Mac and Linux and it is end to end encrypted.

Signal is also incredibly lightweight and utilitarian. There are no ads, analytics, or any other form of data collection. As Signal is built and operated by a non-profit, they have no reason to slow their app down with anything that isn't necessary.

Because of these features, I've been looking to see if there is a way I can leverage Signal to forward notifications to myself in a timely manner.

Aren't Signal accounts tied to a phone number?

Well yes, but we don't need an additional phone number.

During the initial registration flow on Signal, you need to verify and claim your phone number. This is essentially Signal's anti-spam process. Validation is done by receiving an SMS message from the Signal registration service with a six digit code.

Normally this would be a bit of a problem. I'm not going to enter into a mobile phone contract for a seperate phone number simply to register it and use it with Signal just to have my software update notifications be more secure and cross platform.

Luckily, Signal allows you to provide access to your account to different devices via the Linked Devices feature. Simply scan a QR code and provide your approval, and the new device has account access.

Introducing signal-cli-rest-api

signal-cli-rest-api is exactly what you'd think it is. An open source REST API for the Signal CLI, entirely dockerized.

Getting this up and running with a docker compose file is incredibly easy, as you don't need any environment variables to get setup. All you need is a directory to provide that the signal cli can use to store the cryptographic key used during the Linked Device stage.

signal:
    image: bbernhard/signal-cli-rest-api:latest
    restart: unless-stopped
    ports:
      - "8080:8080"
    volumes:
      - "./signal:/home/.local/share/signal-cli"

Once you run the api for the first time, navigate to http://localhost:8080/v1/qrcodelink?device_name=signal-api, open Signal on your mobile phone, go to Settings > Linked devices and scan the QR code shown.

From here, you just need to format your notifications appropriately, as shown with this CURL example

$ curl -X POST -H "Content-Type: application/json" 'http://localhost:8080/v2/send' -d '{"message": "Test via Signal API!", "number": "+4412345", "recipients": [ "+44987654" ]}'

Because we're registering it using our own account, we simply replace number with our registered number and the recipients array simple needs a single string containing our own number.

Now simply provide the details of your signal api to be used in Webhooks, and you're getting notifications via Signal!

This is just a single example of what the signal-cli-rest-api can do, check out the Swagger docs for the full list of endpoints.