Concrete example of threat modeling: if someone found out I was using Signal, for any reason at all, would that cause problems for me?
If yes, then Signal is not a good option. If no, then Signal may be appropriate. Why? Because in their documentation, they explicitly state that while messages are confidential, the fact that you’re using Signal cannot be hidden, and so they don’t make that guarantee.
Tbf, can’t the other party mess it up with signal too?
Yes, but this is where threat modeling comes into play. Grossly simplified, developing a threat model means to assess what sort of attackers you reasonably expect to make an attempt on you. For some people, their greatest concern is their conservative parents finding out that they’re on birth control. For others, they might be a journalist trying to maintain confidentiality of an informant from a rogue sheriff’s department in rural America. Yet others face the risk of a nation-state’s intelligence service trying to find their location while in exile.
For each of these users, they have different potential attackers. And Signal is well suited for the first two, and only alright against the third. After all, if the CIA or Mossad is following someone around IRL, there are other ways to crack their communications.
What Signal specifically offers is confidentiality in transit, meaning that all ISPs, WiFi networks, CDNs, VPNs, script skiddies with Wireshark, and network admins in the path of a Signal convo cannot see the contents of those messages.
Can the messages be captured at the endpoints? Yes! Someone could be standing right behind you, taking photos of your screen. Can the size or metadata of each message reveal the type of message (eg text, photo, video)? Yes, but that’s akin to feeling the shape of an envelope. Only through additional context can the contents be known (eg a parcel in the shape of a guitar case).
Signal also benefits from the network effect, because someone trying to get away from an abusive SO has plausible deniability if they download Signal on their phone (“all my friends are on Signal” or “the doctor said it’s more secure than email”). Or a whistleblower can send a message to a journalist that included their Signal username in a printed newspaper. The best place to hide a tree is in a forest. We protect us.
My main issue for signal is (mostly iPhone users) download it “just for protests” (ffs) and then delete it, but don’t relinquish their acct, so when I text them using signal it dies in limbo as they either deleted the app or never check it and don’t allow notifs
Alas, this is an issue with all messaging apps, if people delete the app without closing their account. I’m not sure if there’s anything Signal can do about this, but the base guarantees still hold: either the message is securely delivered to their app, or it never gets seen. But the confidentiality should always be maintained.
I’m glossing over a lot of cryptographic guarantees, but for one-to-one or small-group private messaging, Signal is the best mainstream app at the moment. For secure group messaging, like organizing hundreds of people for a protest, that is still up for grabs, because even if an app was 100% secure, any one of those persons can leak the message to an attacker. More participants means more potential for leaks.
When I see E2EE and XMPP mentioned, I think of this blog post by Soatok, outlining some very odd cryptographic choices in XMPP + OMEMO: https://soatok.blog/2024/08/04/against-xmppomemo/
I would very much like to see a richer playing field than just Signal for private messaging, but it’s a tough nut to crack. For exactly which aspect that turns me away from XMPP for E2EE, I think this nails it down:
you only need check whether OMEMO is on by default (it isn’t), or whether OMEMO can be turned off even if your client supports it (it can).
When the competition is Signal, these sorts of details matter a lot.
If you’re using SLAAC for auto IP assignment, then the resulting EUI-64-based address would be essentially static, based on the premise that your MAC address and local subnet prefix don’t change. Privacy extensions night get in the way, as well as Android’s randomized MAC feature, but those are adjustable.