Why do whatsapp web application needs phone connection all the time?

Question

WhatsApp just announced a new web application see here.
For some reason, the interface requires the phone to be connected all the time. Is it for performance reasons (not to create additional load on their current servers)? Is there any other constraint that cause that?

Answer 1

The official explanation:

Your session on WhatsApp Web is an extension of WhatsApp on your phone. WhatsApp Web connects to your phone to sync messages, thus you can see all messages on both devices. Thus, the first requirement to being able to use WhatsApp Web is an active WhatsApp account on your smartphone.

Source: https://www.whatsapp.com/faq/en/web/28080002

As you may know your Whatsapp history is only being stored in a database on the phone itself. To see that history in your web browser, it needs to get it from the phone. Whatsapp could have redesigned it, so that everything is stored in the cloud (as many competing messaging apps do). But that seems to be against their philosophy. They keep it tighly coupled to a (one) phone. As you may know you cannot install Whatsapp on multiple phones using the same account. The web interface is just a remote for Whatsapp running on your phone.

And even though I don't know for sure, I think it's more secure too. It wouldn't surprise me if the data that's sent between the web app and the phone is encrypted in a way that even Whatsapp themselves cannot decrypt. Maybe the QR code is generated client-side (in the browser) and by scanning it using the app there is no need to exchange the keys through Whatsapp's servers. That way they don't ever get the encryption keys and will not be able to inspect the data that gets routed through their servers.

Note: Of course Whatsapp could at any time change their implementation of both the app or the web app and enable eavesdropping.