Lots of important gotchas here. Don’t get me wrong, this is great, but there are definitely drawbacks to be aware of. The main one being their use of convergent encryption for file deduplication:
Some metadata and usage information stored in iCloud remains under standard data protection, even when Advanced Data Protection is enabled. For example, dates and times when a file or object was modified are used to sort your information, and checksums of file and photo data are used to help Apple de-duplicate and optimize your iCloud and device storage — all without having access to the files and photos themselves.
“Standard data protection” here means that the data is protected with a key Apple controls, not end-to-end encrypted. For example, the following metadata is not end-to-end encrypted with files in iCloud Drive:
- The raw byte checksums of the file content and the file name
- Type of file, when it was created, last modified, or last opened
- Whether the file has been marked as a favorite
- Size of the file
- Signature of any app installers (.pkg signature) and bundle signature
- Whether a synced file is an executable
This is actually extremely significant metadata. With checksums and file names available, an attacker only needs the original file to determine whether you have a copy. Sure, if your files are unique they wouldn’t be readable, but this is something to keep in mind if it’s in your threat model.