[elvisloops]

Historically as long as everything remained "in the app," it was secure. It's an easy assumption to make and communicate to others. Now it's more complicated: there are things that people can unwittingly do "in the app" that make it less secure.

[jonathanstrange]

AFAIK, it has the same security as before. Perfect forward secrecy means that if someone starts recording encrypted messages in transit and two years later obtains an encryption key, they cannot use that key to decrypt the messages they recorded earlier (because of re-keying).

On the other hand, if an adversary captures one of the group participants' phone and breaks device security, and the chat was recorded on that device, then they can access all recorded chats. By the same token, no cryptography can protect against a malicious group participant who records messages.

In the same scenario, cloud backups seem to merely imply that the same adversary can obtain the cloud backup key and therefore decipher the cloud backups if they get their hands on it. They won't need that, however, since the group chat history is already stored on the device. If no chats were recorded on the device at all the situation would be different.