The lack of sync is a feature. That's what you get when you have a secure system.

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.

And its not even available, at least on the linux desktop version.

It just means that nobody you know sells drugs or does other stuff that forces people to choose privacy over convenience.

How come https://riot.im (Matrix) manages to sync between devices AND have E2E, while also being federated? How come I can use both WhatsApp and Signal on both my computer and phone (and they stay in sync)?

It looks like in [1] that each device registered to a user has a device_key and when an encrypted message is sent, they user's public devices keys are requested and the message is encrypted for each device. New devices can't see old messages.

[1] https://matrix.org/docs/guides/e2e_implementation.html

The message isn't encrypted for each device; the message is encrypted once for the room, as part of a 'session' of messages - and then the key data for that session is shared with the devices who are allowed to read it. Thus you can share old session key data with other devices if you want, meaning that new devices /can/ see old messages, although we're still working through the UX for that. (Currently the only way to do it is by import/export session key data in settings and transferring it between devices).

Thanks—so there's another layer of encryption over the ever changing (Megolm) key that encrypts the room, if I understand this. Looks like I simplified too much.

Sort of. Just to clerify: the first layer (Olm) provides a secure channel between pairs of devices, used mainly to share Megolm encryption state between them.

The second layer (Megolm) encrypts each sent message once per room, using a ratchet described by session key data. The session key data is shared 1:1 between the appropriate devices (past and future) over Olm.

WhatsApp on your computer uses your phone as proxy, so it's kind of cheating (you never get the data in 2 devices).