[kevinchen]

"checksums of file and photo data are used to help Apple de-duplicate and optimize your iCloud and device storage"

This is likely describing content-addressable storage. It is the underpinning of many iCloud services that store user files / blobs. It is also a commonly used pattern in backend services generally.

https://en.wikipedia.org/wiki/Content-addressable_storage

[drbawb]

The problem is that a stream cipher is going to have some per-object uniqueness (a salt, IV, etc.), so by design even if you feed it related input blocks you will get different output blocks. This is, of course, antithetical to deduplication: so you need to check/store the hash of the input before it goes through the cipher.

The presentation about ZFS' native encryption[1] covers many of these sorts of trade-offs necessary to do full-disk encryption at scale.

[1]: https://www.youtube.com/watch?v=frnLiXclAMo