[sametmax]

They are, but apple have the keys : https://thehackernews.com/2016/01/apple-icloud-imessages.htm...

So basically, they could be in clear text, it's pretty much the same.

[niij]

Please see this link[1], Apple explains exactly how keys are stored in their datacenters (Hint: it is not in clear text). They use HSM's which destroy the user's key after 10 failed attempts.

[1]: https://www.apple.com/business/docs/iOS_Security_Guide.pdf

[snuxoll]

This is only for Keychain escrow, device backups are not protected by HSM's.