I don’t really like Apple, but once in a while they do the right thing. This comes from the App store’s new labels on apps.
Signal just has “Contact info” under the “Data NOT linked to you” category. This is just the phone number + contact discovery.
UPDATE
There’s another post adding telegram here. This is what it looks like:
that’s not actually the case, i read in the signal blog (if i find it i’ll link it) that no metadata travels unencrypted and no metadata is stored on the servers. even in groups, there is no database storing the list of members, as the exchange of keys happens only between devices with zero-knowledge. if all the members of the group reset their phones the group is non-existing anymore as it never was anywhere in the first place.
The signal back end isn’t open source, so the source for that is “trust me bro”. XMPP and matrix back end is fully open source and self-hostable.
They have to store phone numbers, its their primary identifier and routing system.
Its also a single server / cluster all hosted in the US so by definition isn’t secure.
Signal backend is open source.
Edit: you might be thinking of telegram.
this is against the social graph discovery: https://signal.org/blog/private-contact-discovery/ we are talking about a gem in the privacy landscape, there is no software dedicated like this to privacy at this time
this is about metadata: there are no timestamps https://signal.org/blog/looking-back-as-the-world-moves-forward/
more on metadata: https://signal.org/blog/sealed-sender/
That gets linked all the time, even though its just a “proposal”. You don’t know if it works, because the signal back-end is closed source.
this is about the groups: https://signal.org/blog/signal-private-group-system/