They can do that, but then when you switch devices, you lose your contact list. That's not what happens with the built-in contact list.

This issue has been rehashed dozens of times on HN before (use the search bar below) and has basically nothing to do with the article.

actually, the contact list could include the signal identifiers

It's rather funny to me that I still get notifications that someone I knew ages ago (but are still in my contact list) are now on Signal.

I find it funny rather than scary because I knew the tradeoffs Signal decided on even before I started using it. The fact that you use Signal itself is not a secret. If you need that to be secret, use a burner phone or something else. But I'm pretty sure not everyone else knew that when they joined.

In one case, I got notification that a guy I knew in high school had joined Signal, he was pretty far left then, and googling him I found out he was extremely far left now (splinter of a splinter of an anti-electoral Maoist group). I sent him a friendly note welcoming him, and explaining the basic thing I've explained here, that everyone can see you joining signal.

I never got a reply.

Is there a notification if they leave Signal (delete the app or their account)?

One way would be for Signal to create a hash of all your contacts phone numbers and look up if those hashes exists on the server. No contact details needed on the server, just the hash of the phone number connected to Signals user ID.

Moxie talked about it here: https://signal.org/blog/contact-discovery/.

> The first instinct is often just to hash the contact information before sending it to the server. If the server has the SHA256 hash of every registered user, it can just check to see if those match any of the SHA256 hashes of contacts transmitted by a client.

> Unfortunately, this doesn’t work because the “preimage space” (the set of all possible hash inputs) is small enough to easily calculate a map of all possible hash inputs to hash outputs. There are only roughly 10^10 phone numbers, and while the set of all possible email addresses is less finite, it’s still not terribly great. Inverting these hashes is basically a straightforward dictionary attack. It’s not possible to “salt” the hashes, either (they always have to match), which makes building rainbow tables possible.

Regardless of this, Ricochet and Briar both use Tor hidden services and they are metadata-free. You do not send any metadata to any servers. I have a link to the design of Ricochet that is easily digestible in some of my other comments.