Thanks for the link.
Actually, didn't even know that WhatsApp changed their backup mechanism, which previously used a mechanism that allowed e.g. google to obtain the messages.
Overall, the concept seems to be the same: use a secret, only known to the client to secure data stored at a untrusted location.
The major differences include:
- closed vs open source (it's easy to validate the mechanism in the implementation whereas you pretty much need to trust WhatsApp that they don't leak the key)
- WhatsApp uses a third party provider for storing the data whereas with Matrix your Homeserver is responsible for messaging and the backup
- only the main app can access the backup whereas with Matrix any of your clients can independently read and write to the backup (because there is no main client)
- WhatsApp stores everything in the backup, a Matrix client only the keys, because the messages are stored somewhere else
