I'm experienced with Apple products - but there was one time that I actually got stuck in an E2EE loop and was forced to reset all E2EE data on iCloud. I don't know what I did wrong - but if someone in tech, like myself, can get stuck in an E2EE lockout, I can't imagine other people.*
*This was not Advanced Data Protection. This was stuff that was E2EE for all accounts - like passwords and health information. As such there's no recovery contact.
doesn't that stop iCloud syncing, at least on my end? I understand I can't control what happens on the other end of the conversation but that is all I need to do on my end, right?
If you choose to have some of your data in iCloud, it is transported and stored encrypted. However, one of the keys is escrowed in a HSM cluster for an audited recovery process.
This is how you go request access be restored via technical support with Apple. This is also how surviving family members can get access to photos and the like (requiring a court order, at least in the US). Since they have the key, government entities can request access within the extent of their respective local laws.
If you turn on the Advanced Data Protection feature, Apple no longer has that key escrowed, cannot help with account recovery, and can no longer give out a key they don't have.
Apple turns over customer data on over 70,000 customers per year without a warrant under FISA/702 (prism) and NSLs. The number gets bigger every year. This isn’t a theoretical threat. The number is even bigger if you include all the search warrants, too.
EDIT: Even if you enable their optional e2ee for backups (which nobody does), iMessage the platform is still vulnerable because the conversations you have with others are insecure because the other end of the conversation is escrowing their keys to Apple via non-e2ee backups. If you enable ADP iMessage only becomes secure for the case where you are only iMessaging yourself.
It’s simply not private or secure. You can’t be “slightly encrypted” or “mostly private”.
Read the parts about Messages in iCloud, the service used to sync messages between devices. Those keys are included in the non-e2ee iCloud Backup. Both are enabled by default.