[AnthonyMouse]

End-to-end encryption is where the client device encrypts the message and then the other end's client device decrypts the message. It doesn't matter how many servers it goes through, none of them can read it, that's the entire point of end-to-end encryption.

The hard part is associating some identity with the user's keys, but when the ID is your phone number or email address, the entity doing that is inherently your phone company or email provider. You can standardize a way to do that, i.e. to sign up you get an SMS or email with a code and have to enter the code. The client can automate that if it has access to read your SMS or email, or otherwise you enter it manually.

[naravara]

If the person on the other end is using a non-Apple client you cannot verify independently that their client isn’t peeping. It’s the client, not the servers.

But also when it comes to managing the keys and syncing across devices it’s also the servers.