I was going to write something about iCloud backup and encryption but I realize that I’ve written it all. So here are a bunch of old posts: blog.cryptographyengineering.com/2012/04/05/icl…
And here’s one on iCloud Keychain from 2016, which ends with the following question. blog.cryptographyengineering.com/2016/08/13/is-… 
And a slightly more recent one about Apple moving its encryption keys to China. blog.cryptographyengineering.com/2018/01/16/icl…
iCloud Keychain is an interesting product, only because it shows how much trouble Apple went to in order to ensure that people could back up their passwords to iCloud without getting them all stolen.
What I mean: it contrasts with an anti-encryption narrative (that plaintext is good enough for most people, e2e is gratuitous for most users), in that the need to seriously protect passwords is intuitively obvious.
With messaging you could picture the Apple execs saying “how can we push out more warrantproof encryption to mess with the FBI” or whatever, if you’re inclined. With KeyChain you picture them saying “crap, how screwed would we be if all our customers’ passwords got stolen.”
TL;DR iCloud Keychain is the *most* powerful security system Apple has on its server side, and it’s also the one that’s *least* likely to be driven by privacy ideals or the desire for hip marketing. Apple seems legitimately scared of getting its keystore hacked.
