Technology

I do, yes. Signal’s not perfect, but it’s very good, especially for everyday conversations. Compared to comparable secure alternatives, it’s more user friendly and in many cases it’s more consistently secured (specifically thinking of Matrix here).

There are keys for your conversations on every device you have linked to Signal, but not on the Signal servers. Each device gets its own set of keys, which are discarded and updated with new derived keys as you send and receives messages. Your phone is the one in charge of handing the initial keys out to other devices and in charge of revoking them. The server has a message queue for each device, but not the keys. (The messages are deleted from the queue after being delivered to the device or after 30 days, at which point that device is unlinked.)

I also found this article that discusses the data that ends up getting stored on Signal’s servers.

One big vulnerability for Signal is that it trusts your local machine too much, especially on Desktop but also, to an extent, on mobile. If you have malware on your system, if you don’t encrypt your drives, or if someone is able to guess your password, then someone can get full access to your conversation history. On desktop, the database with all your conversations is stored in an encrypted database, but the key to decrypt the database is stored in plaintext. That key could be secured in the system keychain, which would make it resistant to being read by malware, but it isn’t. It could be encrypted locally with a user-provided PIN or password… but it isn’t. And I have seen enough of people struggling to contribute code to Signal that I suspect if someone contributed a PR to fix that, it wouldn’t get merged.

That all said, if someone stole your laptop, even if they got access to past conversations you can cut them off from future conversations by unlinking the device from your phone. And if someone cloned your Signal data and tried to connect as you, then it would be pretty obvious that something was going on when you logged in on your own laptop, since it would get very glitchy, and as soon as you unlinked and re-linked it would cut off the attacker’s access.

This whole vulnerability makes Signal’s resistance to a proper, securely encrypted, user-managed cross-platform backup system even more frustrating. It feels, honestly, like security theater. But even so, I prefer the current state over having the backup in Signal’s cloud, regardless of whether SGX is involved; I want to manage my own backup. Like other proponents of a proper backup system, I believe that the user should be in charge of determining if a conversation should be ephemeral, and Signal provides a tool to do that - disappearing messages.

The other big vulnerability Signal has is not being anonymous, largely thanks to being linked to a phone number. Anyone you chat with has your number, and anyone who has your number can check to see if you’re on Signal (though you can at least disable the latter option). Things like Sealed Sender help protect your identity and the identities of your contacts from outside observers, but they aren’t perfect. This lack of anonymity is the main reason given for not trusting Signal when your threat model involves state surveillance, but as a counterpoint to that - as recently as 2021, Edward Snowden endorsed Signal and said he uses it every day. If Snowden trusts it to secure his conversations, I’m not worried about it for my own.

Here's a point in favour:

https://signal.org/blog/cellebrite-vulnerabilities/

Per that link, the encrypted master key is stored.