4 ms·
so maybe i'm confused, but i thought it worked like this: pre this thing: * before syncing photos to icloud, the device encrypts them with a device local key,
by a-dub 5y ago
so maybe i'm confused, but i thought it worked like this:
pre this thing:
* before syncing photos to icloud, the device encrypts them with a device local key, so they sit on apple's servers encrypted at rest and apple cannot look at them unless they push an update to your device that sends them your key or uploads your photos unencrypted somewhere else
after this thing:
* before syncing photos to icloud, the device encrypts them, but there are essentially two keys. one on your device, and one that can be derived on their servers under special circumstances. the device also hashes the image, but using one of these fancy hashes that are invariant to crops, rotations, translations and noise (like shazam, but for pictures)
* the encrypted photo is uploaded along with the hash (in a special crypto container)
* their service scans all the hashes, but uses crypto magic that does the following:
1) it does some homeomorphic encryption thing where they don't actually see the hash, but they get something like a zero knowledge proof if the image's hash (uploaded along with the image in the "special crypto container") is in their list of bad stuff
2) if enough of these hit, then there's a key that pops out of this process that lets them decrypt the images that were hits
3) the images get added to a list where a room full of unfortunate human beings look at them and confirm that there's nothing good going on in those photos
4) they alert law enforcement
a couple points of confusion to me:
1) i'm assuming they get their "one in a trillion" thing based on two factors. one being the known false positive rate of their perceptual hashing method and the other being their tunable number of hits necessary to trigger decryption. so they regularly benchmark their perceptual hash thing and compute a false positive rate, and then adjust the threshold to keep their overall system false positive probability where they want it?
2) all the user's photos are stored encrypted at rest, it seems that this thing isn't client side scanning, but rather assistance for server side scanning of end to end encrypted content put on their servers.
first off i think it's actually pretty cool that a consumer products company offers end to end encrypted cloud backup of your photos. i don't think google does this, or anyone else. they can just scan images on the server. second off, this is some pretty cool engineering (if i understand it correctly). they're providing more privacy than their competition aaand they've given themselves a way to ensure that they're not in violation of the law by hosting CSAM for their customers.
but i guess the big question is, if people don't like this, can't they just disable icloud?
- upbeat_general 5y agoThey do not provide e2e for iCloud photos (or iCloud backups, etc.) and did not specify that they were planning to. They do not provide more privacy than the competition (in this regard at least), and yes, people can just disable iCloud services. That argument that "you can just not use it" is a pretty weak one for defending poor privacy practices. The same could be said for any website or server with poor privacy practices and given the prevalence of iCloud photos, this change affects lots of people.
- a-dub 5y agoi dunno, reading this thing sure does make it sound like it's only for photos stored in icloud. and the existence of such an elaborate system sure does seem to indicate that photos are end to end encrypted on their servers, otherwise they could just hash them there. it does the matching client side, so the hashes never leave the client. what it does send with the photos is this threshold thing where if enough of them are hits, it reveals them, otherwise they see nothing. it seems pretty unequivocal that this is all a system to support scanning of e2e encrypted images on icloud. there would be no need for such an elaborate system if they could scan the images on the server. as far as i know, google does not e2e encrypt images, and for all we know, they actually do engage in server side scanning. to quote their statement https://www.apple.com/child-safety/ https://www.apple.com/child-safety/: > To help address this, new technology in iOS and iPadOS* will allow Apple to detect known CSAM images stored in iCloud Photos. This will enable Apple to report these instances to the National Center for Missing and Exploited Children (NCMEC). NCMEC acts as a comprehensive reporting center for CSAM and works in collaboration with law enforcement agencies across the United States. > Apple’s method of detecting known CSAM is designed with user privacy in mind. Instead of scanning images in the cloud, the system performs on-device matching using a database of known CSAM image hashes provided by NCMEC and other child safety organizations. Apple further transforms this database into an unreadable set of hashes that is securely stored on users’ devices. > Before an image is stored in iCloud Photos, an on-device matching process is performed for that image against the known CSAM hashes. This matching process is powered by a cryptographic technology called private set intersection, which determines if there is a match without revealing the result. The device creates a cryptographic safety voucher that encodes the match result along with additional encrypted data about the image. This voucher is uploaded to iCloud Photos along with the image. > Using another technology called threshold secret sharing, the system ensures the contents of the safety vouchers cannot be interpreted by Apple unless the iCloud Photos account crosses a threshold of known CSAM content. The threshold is set to provide an extremely high level of accuracy and ensures less than a one in one trillion chance per year of incorrectly flagging a given account. > Only when the threshold is exceeded does the cryptographic technology allow Apple to interpret the contents of the safety vouchers associated with the matching CSAM images. Apple then manually reviews each report to confirm there is a match, disables the user’s account, and sends a report to NCMEC. If a user feels their account has been mistakenly flagged they can file an appeal to have their account reinstated.