How about no phone numbers for registration at all?
Then let your phone number receive the spam instead?
It prevents the creation of an unlimited number of signal accounts by a single user with no cost to the user but cost to signal and other signal users.
edit: Your are probably right in that it does not change the risk of spam for a single user, as you could guess the phone number or just iterate over all known phone numbers and try to connect to them.
requiring phone numbers only solves the cost problem for signal(The company/legal entity) and lowers(hopefully) the amount of spam that would get send.
* Gives the servers virtually no control over communications between parties.
* Goes through huge pains to minimize serverside metadata storage.
* Is a sealed system end-to-end; the client and the server are part of a single coherent design that together make promises about privacy and security that apply to every user of the system; Matrix is a protocol ecosystem.
A good example of this is group messaging: Matrix servers control group membership. In Matrix, group membership is key management; a Matrix server decides who can decrypt your group messages. That's not how Signal works! But I don't think anybody seriously thinks Signal is a replacement for a large Slack.
And yet uses AWS: https://news.ycombinator.com/item?id=39414322
The threat model assumes attackers have maximal control of the server environment.
Hence my position remains unmoved.
This continued insistence (widespread - not just you!) on the benevolence and good faith of US intelligence, post-Snowden, doesn't make any sense to me.
Take a step back and note that nobody on HN is going to make an argument premised on "you should trust NSA to follow the rules". You can accept that as an axiom and have easier conversations here.
...and this is the declassified part.
This is not good enough. Signal server is a single point of failure: NSA (and any other attacker, e.g., China) knows that the users can't go elsewhere, so it's very easy to target them all (thanks to the Signals's politics of walled garden). In case of Matrix, there are thousands of servers around the world, which you have to find and get into. They can run completely different software. This is not very scalable or easy.
https://www.nybooks.com/online/2014/05/10/we-kill-people-bas...
The paper you linked to was published before they started collecting and storing sensitive user data in the cloud
I have no idea how to get my extended family on a Matrix homeserver without extensive handholding. I can barely figure it out myself and I was a huge XMPP nerd that ran my own ejabberd server for years.