[runnerup]

Why does it need to tell everyone I have a Signal account?

Adobe doesn't tell everyone that I own Photoshop. Gmail doesn't tell everyone that I have a Gmail email. PornHub doesn't tell everyone that I subscribe to their premium account.

Why the fuck does Signal need to? Broadcast should be off by default, on by opt-in.

[frabcus]

This page gives some details and the technical reason can be inferred:

https://support.signal.org/hc/en-us/articles/360007061452-Do...

It doesn’t send the number to them - Signal don’t get the contact list from your phone. It uses a very clever private contact discovery protocol.

The reason their phone has to know is so that they can then message you. Otherwise there would be no way to message people - a phone has to know who is on Signal to be able to do that.

While the notification could be off by default, since the phone necessarily has to know when your contacts are on Signal for the app to function, it is being transparent about the privacy situation.

Details of the private contact discovery system:

https://signal.org/blog/private-contact-discovery/

[pmontra]

It could be as little as "everybody does it, let's do it too" or they actually thought about it. By signaling to contacts that somebody installed Signal they make it more likely that those contacts use Signal to communicate instead of any other system. It helps the growth of Signal.

Adobe and PornHub don't have the contacts list. Google likely does but maybe they are restrained by privacy laws.