How come https://riot.im (Matrix) manages to sync between devices AND have E2E, while also being federated?

That's not what you get when you have a secure system, that's what you get when you design a system that can collect (and possibly monetize) the data of millions of users.

I believe that making sync work with E2E bring either security issues or more burden on the user; I would like them in telegram, but I also like the "if you send this message you exactly the device it go to, not an old laptop i forgot in the office, just my phone". it is meant to be secure on a device level.

Yeah, you'd have to manually put your private keys onto your computer from your phone.

Or you do what Matrix does, and given every device its own keypair, and let users track whether they are talking to a trusted decice or not.

That's not what I meant. Yes you can have sync and E2E. With different trade-offs.

Telegram is secure from device to device, not from account to account. If I send you a secure message from my iPad I don't have to worry about the web session I opened a week ago on someone else laptop.

People down voting this comment, care to explain what's wrong with it?

Signal and WhatsApp both support multi-device encrypted chats. Signal is better than WhatsApp in this respect as your primary device doesn't need to be online for it to work.

Signal multi-device support is very limited. Doesn't support multiple mobile devices. Primary device must be a phone, all others must be desktop computers with Chrome.

This. I would love for Signal to have multiple mobile device support.

True, though that's a gap in UI rather than a constraint of the protocol.

If you want to use a standalone app then you can achieve it using NW.js

https://timtaubert.de/blog/2016/01/build-your-own-signal-des...

Huh? How is exchanging encryption keys between your devices and syncing history insecure? Because that's how Wire solves it.

My guess would be concerns about spreading around the keys being too easy, so you might end up with a compromised end point.