How does using Matrix compare with using Signal, privacy-wise?
I'm not sure if it is entirely accurate to compare them in this way, as "Matrix" refers to simply the protocol, whereas "Signal" could refer to the applications, server, and protocol. That being said, is there any fundamental difference in how the Matrix ecosystem of federated servers, and independently developed applications compares to that of Signal that would make it less secure, overall, to use?
The most obvious security vulnerability that I can think of is that the person you are communicating with (or, conceivably, oneself, as well) is using an insecure/compromised application that may be leaking information. I would assume that the underlying encryption of the data is rather trustworthy, and the added censorship resistance of federating the servers is a big plus. However, I do wonder if there are any issues with extra metadata generation, or usage tracking that could be seen as an opsec vulnerability for an individual. Signal, somewhat famously, when subpoenaed to hand over data, can only hand over the date that the account was created, and the last time it was used. What would happen if the authorities go after a Matrix user? What information about that user would they be able to gather?
Signal, somewhat famously, when subpoenaed to hand over data, can only hand over the date that the account was created, and the last time it was used.
That's old news really. Signal is subject to the laws of whatever country they operate in. As such, a country can compel them to start logging information, and they must comply.
This is what happened with ProtonMail with a protestor in France/Switzerland. I forget which way round it was, I think they were protesting in France but went back to Switzerland, then because of an agreement between France and Switzerland and because it was a crime in both countries the Swizz government was able to compel ProtonMail to start tracking their IP addresses and used this to locate and arrest the protestor.
They most likely can't compel Signal to give them the messaging data. If they did, this would require a new key exchange, which they could MITM, but this should issue a notification to both users. It's possible a government could compel them to do this silently, however this would require app updates and I'd hope this would be where Signal draw the line. However ultimately there's no way of telling if the US haven't already done this.
Open Source ftw. Matrix is far better than Signal in this regard, in fact I rate Threema over Signal, although it's only slightly better.
Threema is very security focused at the expense of user experience. You lost your private key? Tough luck, here's a new account and everyone will have to verify your new account again. I have had conversations with friends over five or six of their accounts because they use threema just like WhatsApp. I stopped recommending threema to folks without a basic technical understanding. Signal is more than good enough for their needs.