This is a *very* good question. I would also throw in Nostr DMs into that comparison as well, as I believe one of these 3 have the best chance of being the global communication protocol in the future.
I've looked at all 3 and tbh I think that Nostr is still the best choice, even though some people don't like that in it's current form, it exposes chat metadata.
But Nostr dms are not that secure.. I love Nostr but I think keet or simplex serve a different purpose? It can even be integrated potentially I think for DMs?
Why are Nostr DMs not secure?
first and foremost, every relay and everybody with your public key can see who you are talking to.... which is definitively not secure/private. if your private key is ever leaked, you will never know that people can be spying on you and they will have access to all your history with all your messages and few other things more
The same thing happens if you lose your password, so that's not really any different.
As for the public key issue, just like I mentioned initially, the metadata is being exposed, but again, is that such big of a problem? It's like having your followers list set to public instead of private. I personally don't think that's such a big issue.
Furthermore, there is currently some progress being made in this direction in trying to anonymize this data as well. So even this last hurdle could be fixed.
I still think Nostr DMs are a better solution overall than either Simplex or Keet. Both of which lack the option of sharing the chat between multiple devices (as chats are not long lived on a server) and the overall UX is horrendous (for Simplex you need to start a conversation with the other person while being physically in the same place, since you don't have a long lived unique identifier).
Im not a cryptographer .. just think that from most people that I’ve heard though it’s true that is better than sending plain text you can’t be sure that someone is or not reading your full history of all messages and you can’t change the key or revoke a key access etc
Maybe there would be a point where we use Nostr’s relays to help in the discovery but then once two peers connect, the communication uses other tech
