They can't read them in transit, but if you use Facebook's app to decrypt the message (which is the only option) then obviously Facebook can read your messages if it wants to.

While this is true, the app is a client whose code and behaviour can be analysed by end users to validate that it is not behaving nefariously. The same cannot be said for FB's servers or the data in transit, which is solved by E2EE.

The Facebook app can read the messages just as you do when the app decrypts the messages locally.

Sure, but the facebook app doesn't do that (or at least no security researchers have noticed it doing that). The point facebook is making is that if they had a backdoor that let them read the messages that it could be used for other purposes.

Consider that right now to intercept end2end encrypted messages (and lets just assume facebook has that) a malicious actor has to get physical possession of your device, which implies that you have to be specifically targeted. With a backdoor malicious actors would then be able to target many other things, including for example facebook's servers, to access those messages (and all of them at once). I think simply avoiding the bad PR of "hackers downloaded everyone's messages between 2017 and now" is most of why they want this stuff end to end encrypted.

But there already is a backdoor that lets them read the messages. Facebook actually implementing the functionality to take advantage of that backdoor doesn't change anything. The security of your message relies on the security of Facebook's infrastructure either way.

If Facebook implements a switch in the app that would send your decrypted messages back to them, then a malicious actor would have to compromise Facebook to obtain those message. Right now, a malicious actor can obtain your messages by compromising Facebook and implementing that switch themselves. From a security standpoint that seems identical to me.

Absence of evidence is not evidence of absence.

The app can bundle the message telemetry with the rest of the encrypted app telemetry when it phones home and even the best security researchers would not detect it. It can also process it locally for profiling and other advertising means.

> even the best security researchers would not detect it

By examining only the network traffic, sure, but what about by examining the code?

Is the facebook app open source?

Nope!

That hasn't necessarily stopped people from learning something about how it works, though.

true, but does someone do it? I remember that auditing Truecrypt was already a substantial deliberate undertaking.

And that' not even taking into account that it might be possible to push something different to each user.

They read the plaintext from the keyboard. Then the app can send it to them. The problem is how to prevent people to look into the network traffic (encryption?) and to decompile the app.