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> Also, Apple is quite vague about whether the claim "Apple manually reviews all reports" means Apple employees can view all triggering content, even if it was
by sirn 5y ago
> Also, Apple is quite vague about whether the claim "Apple manually reviews all reports" means Apple employees can view all triggering content, even if it was never uploaded to Apple servers. (How would that work?)
From a quick read of the PDF, my impression is this Safety Voucher is more of an additional metadata generated before a photo is uploaded to iCloud Photo (as oppose to scan photo library to see if it matches and notify Apple narrative). The Safety Voucher is designed in a way that they could decide on the server-side if a photo is CSAM without requiring iCloud to have an ability to decrypt all photos. Safety Voucher comes in three layers:
- The first layer is generating a NeuralHash for a photo and derive a cryptographic header from it based on blind hash provisioned by Apple from a known CSAM NeuralHash (e.g. Apple pre-compute this table for each device). First layer is decryptable if the generated NeuralHash is a known CSAM. (Is it going to be a random data if NeuralHash doesn't exists in the table?)
- The second layer uses Secret Sharing[1] scheme (where a content is decryptable only if >n keys are present) using NeuralHash of a visual derivative as a secret share. Multiple visual derivative is generated for each photo, and certain amount of shares is required to decrypt the layer.
- After the second layer is decrypted, the file is readable.
One thing that doesn't makes sense is iCloud Photos are not currently End-to-End Encrypted, so Apple already have access to all photos. This scheme would only makes sense if they plan to introduce End-to-End Encryption for iCloud Photos (& iCloud Backups?) in the future.
In my opinion, if they were to enable E2EE for photos, this scheme would be slightly better for privacy than Apple having an ability to scan all photos on the cloud (e.g. they can't retroactively decrypt a photo that already has Safety Voucher uploaded to iCloud if the matching NeuralHash doesn't exists at the time of blind hash table generation), but significantly less than having E2EE and not having this system in the first place.
(As someone from a country that is known to abuse this sort of power, I'm not happy about this system existing.)
[1]: https://en.wikipedia.org/wiki/Secret_sharing https://en.wikipedia.org/wiki/Secret_sharing
- gojomo 5y agoYour interpretation might be right, but Apple's vagueness might be strategically hiding things they know would be even more unpopular, if revealed. Still, there's the question of what additional assurance this "manual review" is going to provide, if the Apple personnel only have exactly the same metadata/scrambled-representations as the software has, and not the raw images. They couldn't detect a false positive with that info. Perhaps it's just an option for Apple executives to exercise an extra level of courtesy (or leverage) over the famous & powerful, after seeing the alarm plus the account identity? "We've got a Senator here, we better consult Tim before reporting this through normal channels."
- ec109685 5y agoThe voucher has a low resolution version of the image included with it encrypted within two layers, so Apple, once the threshold has been breached, can view that version of the photo for false positives.
- sirn 5y agoI can't edit anymore but I misunderstood the second layer. The second layer contains a single Secret Share (that can be used to reconstruct a decryption key after certain threshold) and an encrypted image information. User must upload multiple photos matching the initial CSAM database past certain threshold for any of them to be decryptable at all.
- ec109685 5y agoAre you sure the photos themselves aren’t encrypted when uploading? I think only the iCloud backup has the decryption keys. Regardless, it’s way different building a system that has online access to decryption keys with the requirement to scan every photo uploaded versus reserving that access to more locked down servers that have the decryption keys and are only accessed during law enforcement actions.
- sirn 5y agoYou're right, the iCloud Photo is actually encrypted on transit and on the server, but the server has the decryption key[1]. However they've been doing server-side scan for iCloud since quite some times ago[2], which is why I think this technique in the article plus End-to-End Encryption could be better for privacy than the status quo. [1]: https://support.apple.com/en-us/HT202303 https://support.apple.com/en-us/HT202303 [2]: https://www.forbes.com/sites/thomasbrewster/2020/02/11/how-apple-intercepts-and-reads-emails-when-it-finds-child-abuse/?sh=71125dc31c29 https://www.forbes.com/sites/thomasbrewster/2020/02/11/how-a...