Using signal-cli to cancel your Signal account

1. blog

For obscure reasons, I have found myself with a phone number registered with Signal but without any device associated with it.

This is the I lost my phone section in Signal support, which rather unhelpfully tell you that, literally:

Until you have access to your phone number, there is nothing that can be done with Signal.

To be fair, I guess that sort of makes sense: Signal relies heavily on phone numbers for identity. It's how you register to the service and how you recover after losing your phone number. If you have your PIN ready, you don't even change safety numbers!

But my case is different: this phone number was a test number, associated with my tablet, because you can't link multiple Android device to the same phone number. And now that I brilliantly bricked that tablet, I just need to tell people to stop trying to contact me over that thing (which wasn't really working in the first place anyway because I wasn't using the tablet that much, but I digress).

So. What do you do? You could follow the above "lost my phone" guide and get a new Android or iOS phone to register on Signal again, but that's pretty dumb: I don't want another phone, I already have one.

Lo and behold, signal-cli to the rescue!

Disclaimer: no warranty or liability

Before following this guide, make sure you remember the license of this website, which specifically has a Section 5 – Disclaimer of Warranties and Limitation of Liability.

If you follow this guide literally, you might actually get into trouble.

You have been warned. All Cats Are Beautiful.

Installing in Docker

Because signal-cli is not packaged in Debian (but really should be), I need to bend over backwards to install it. The installation instructions suggest building from source (what is this, GentooBSD?) or installing binary files (what is this, Debiandows?), that's all so last millennium. I want something fresh and fancy, so I went with the extremely legit Docker registry ran by the not-shady-at-all gitlab.com/packaging group which is suspiciously not owned by any GitLab.com person I know of.

This is surely perfectly safe.

(Insert long digression on supply chain security here and how Podman is so much superior to Docker. Feel free to dive deep into how RedHat sold out to the nazis or how this is just me ranting about something I don't understand, again. I'm not going to do all the work for you.)

Anyway.

The magic command is:

mkdir .config/signal-cli
podman pull registry.gitlab.com/packaging/signal-cli/signal-cli-jre:latest
# lightly hit computer with magic supply chain verification wand
alias signal-cli="podman run --rm --publish 7583:7583 --volume .config/signal-cli:/var/lib/signal-cli --tmpfs /tmp:exec   registry.gitlab.com/packaging/signal-cli/signal-cli-jre:latest --config /var/lib/signal-cli"

At this point, you have a signal-cli alias that should more or less behave as per upstream documentation. Note that it sets up a network service on port 7583 which is unnecessary because you likely won't be using signal-cli's "daemon mode" here, this is a one-shot thing. But I'll probably be reusing those instructions later on, so I figured it might be a safe addition. Besides, it's what the instructions told me to do so I'm blindly slamming my head in the bash pipe, as trained.

Also, you're going to have the signal-cli configuration persist in ~/.config/signal-cli there. Again, totally unnecessary.

Re-registering the number

Back to our original plan of canceling our Signal account. The next step is, of course, to register with Signal.

Yes, this is a little counter-intuitive and you'd think there would be a "I want off this boat" button on https://signal.org that would do this for you, but hey, I guess that's only reserved for elite hackers who want to screw people over, I mean close their accounts. Mere mortals don't get access to such beauties.

Update: a friend reminded me there used to be such a page at https://signal.org/signal/unregister/ but it's mysteriously gone from the web, but still available on the wayback machine although surely that doesn't work anymore. Untested.

To register an account with signal-cli, you first need to pass a CAPTCHA. Those are the funky images generated by deep neural networks that try to fool humans into thinking other neural networks can't break them, and generally annoy the hell out of people. This will generate a URL that looks like:

signalcaptcha://signal-hcaptcha.$UUID.registration.$THIRTYTWOKILOBYTESOFGARBAGE

Yes, it's a very long URL. Yes, you need the entire thing.

The URL is hidden behind the Open Signal link, you can right-click on the link to copy it or, if you want to feel like it's 1988 again, use view-source: or butterflies or something.

You will also need the phone number you want to unregister here, obviously. We're going to take a not quite random phone number as an example, +18002677468.

Don't do this at home kids! Use the actual number and don't copy-paste examples from random websites!

So the actual command you need to run now is:

signal-cli -a +18002677468 register --captcha signalcaptcha://signal-hcaptcha.$UUID.registration.$THIRTYTWOKILOBYTESOFGARBAGE

To confirm the registration, Signal will send a text message (SMS) to that phone number with a verification code. (Fun fact: it's actually Twilio relaying that message for Signal and that is... not great.)

If you don't have access to SMS on that number, you can try again with the --voice option, which will do the same thing with a actual phone call. I wish it would say "Ok boomer" when it calls, but it doesn't.

If you don't have access to either, you're screwed. You may be able to port your phone number to another provider to gain control of the phone number again that said, but at that point it's a whole different ball game.

With any luck now you've received the verification code. You use it with:

signal-cli -a +18002677468 verify 131213

If you want to make sure this worked, you can try writing to another not random number at all, it should Just Work:

signal-cli -a +18002677468 send -mtest +18005778477

This is almost without any warning on the other end too, which says something amazing about Signal's usability and something horrible about its security.

Unregistering the number

Now we get to the final conclusion, the climax. Can you feel it? I'll try to refrain from further rants, I promise.

It's pretty simple and fast, just call:

signal-cli -a +18002677468 unregister

That's it! Your peers will now see an "Invite to Signal" button instead of a text field to send a text message.

Cleanup

Optionally, cleanup the mess you left on this computer:

rm -r ~/.config/signal-cli
podman image rm registry.gitlab.com/packaging/signal-cli/signal-cli-jre