The Problem with Perceptual Hashes • Hacker News Board

Re: The Problem with Perceptual Hashes

#311

Post by tjoff » Sat, Aug 07, 2021, 1:26 PM UTC

marcellus23 wrote: post #301

tjoff wrote: post #286

Earlier quoted context omitted.

Doesn't need to, the detection is client side at first.

No, like many others commenting on the issue, you seem to only have a vague idea of how it works. Only photos being uploaded to iCloud are being scanned for CSAM.

Then tell us. Because this is what apple says:

The Messages app will use on-device machine learning to warn about sensitive content, while keeping private communications unreadable by Apple.

Next, iOS and iPadOS will use new applications of cryptography to help limit the spread of CSAM online, while designing for user privacy.

https://www.apple.com/child-safety/

There is no ambiguity here. Of course they will scan images in the cloud as well, but they are explicit in saying that it is (also) on the device itself.

Re: The Problem with Perceptual Hashes

#312

Post by ChrisMarshallNY » Sat, Aug 07, 2021, 1:32 PM UTC

That’s a really useful explanation.

Thanks!

Re: The Problem with Perceptual Hashes

#313

Post by gpm » Sat, Aug 07, 2021, 1:45 PM UTC

visarga wrote: post #199

Decker87 wrote: post #195

Earlier quoted context omitted.

He's been completely silenced. If you don't believe me, you can hear it from him next week when he's on the largest news network talking about being silenced.

Yes, because we all trust him at this point /s

I'm pretty sure the person you are replying to was being sarcastic, "he's so silenced that he can tell you about it".

Re: The Problem with Perceptual Hashes

#314

Post by gpm » Sat, Aug 07, 2021, 1:48 PM UTC

wyager wrote: post #173

gpm wrote: post #169

Earlier quoted context omitted.

Canning Parler is Apple choosing not to advertise and send you an app they don't like, i.e. it's Apple exercising it's own right to free speech. Agree or disagree with it, it's categorically different from Apple spying on what the files you have are saying (not even to or via Apple) and reporting it to the government.

Apple also disallows you from installing things without going through them, so “choosing not to advertise and send” has a lot more significance than your wording implies. It’s not like they have a curated App Store for apps they like; there’s literally no other way to add software to the device.

Right, but the fallout that prevents you from installing it is an incidental consequence of Apple choosing not to promote it and Apple choosing to use it's monopoly on app distribution as an income channel.

Speech not happening because Apple didn't go out of it's way for it to create a route for it to happen without Apple being involved, isn't really that shocking or similar to Apple scanning private files. (Apple being allowed to prevent you from installing what you want on your phone is shocking from an anti-trust perspective, but not from a speech perspective).

Re: The Problem with Perceptual Hashes

#315

Post by cvwright » Sat, Aug 07, 2021, 1:52 PM UTC

bastawhiz wrote: post #165

cvwright wrote: post #146

Earlier quoted context omitted.

The crypto here is for the private set intersection, not the hash. So your device has a list of perceptual (non-cryptographic) hashes of its images. Apple has a list of the hashes of known bad images. The protocol lets them learn which of your hashes are in the “bad” set, without you learning any of the other “bad” hashes, and without Apple learning any of the hashes of your other photos.

Well therein lies the problem: perceptual hashes don't produce an exact result. You need to compare something like the hamming distance (as the article mentions) of each hash to decide if it's a match. Is it possible to perform private set intersection where the comparison is inexact? I.e., if you have two cryptographic hashes, private set intersection is well understood. Can you do the same if the hashes are close,…

From what I’ve read, part of the magic here is that Apple’s perceptual hash is an exact hash. Meaning, you don’t have to do the Hamming distance thing.

Admittedly, I haven’t had a chance to read the original source material yet. It’s possible that the person I heard this from was wrong.

Re: The Problem with Perceptual Hashes

#316

Post by [deleted] » Sat, Aug 07, 2021, 1:53 PM UTC

delusional wrote: post #272

ezoe wrote: post #110

The problem of hash or NN based matching is, the authority can avoid explaining the mismatch. Suppose the authority want to false-arrest you. They prepare a hash that matches to an innocent image they knew the target has in his Apple product. They hand that hash to the Apple, claiming it's a hash from a child abuse image and demand privacy-invasive searching for the greater good. Then, Apple report you have a file th…

What about trolling. Assume 4chan figures out apples algorithm. What now happens when they start generating memes that happen to match known child pornography? Will anyone who saves those memes (or repost them to reddit/facebook) be flagged? What will apple do once flagged false positive photos go viral?

[deleted]

Re: The Problem with Perceptual Hashes

#317

Post by fortran77 » Sat, Aug 07, 2021, 2:01 PM UTC

pixl97 wrote: post #305

dagw wrote: post #302

Earlier quoted context omitted.

I've had my bag sniffed at airports at least 50 times, and they've never stopped me. So there must be something else going on as well

The handler didn't think you looked untrustworthy and didn't hint for the dog to bark.

Drug detection dogs sit down near the bag. They don't bark.

Re: The Problem with Perceptual Hashes

#318

Post by fortran77 » Sat, Aug 07, 2021, 2:02 PM UTC

avnigo wrote: post #271

> These cases will be manually reviewed. That is, according to Apple, an Apple employee will then look at your (flagged) pictures. I'm surprised this hasn't gotten enough traction outside of tech news media. Remember the mass celebrity "hacking" of iCloud accounts a few years ago? I wonder how those celebrities would feel knowing that some of their photos may be falsely flagged and shown to other people. And that we…

So we have an Apple employee, the type of person who gets extremely offended over such things as "Chaos Monkeys," deciding if someone is a criminal? No thanks!

Re: The Problem with Perceptual Hashes

#319

Post by emodendroket » Sat, Aug 07, 2021, 2:03 PM UTC

ezoe wrote: post #110

The problem of hash or NN based matching is, the authority can avoid explaining the mismatch. Suppose the authority want to false-arrest you. They prepare a hash that matches to an innocent image they knew the target has in his Apple product. They hand that hash to the Apple, claiming it's a hash from a child abuse image and demand privacy-invasive searching for the greater good. Then, Apple report you have a file th…

Parallel construction is another way this is often pursued.

Re: The Problem with Perceptual Hashes

#320

Post by emodendroket » Sat, Aug 07, 2021, 2:04 PM UTC

dagw wrote: post #302

thaumasiotes wrote: post #210

Earlier quoted context omitted.

https://reason.com/2021/05/13/the-police-dog-who-cried-drugs... > Similar patterns abound nationwide, suggesting that Karma's career was not unusual. Lex, a drug detection dog in Illinois, alerted for narcotics 93 percent of the time during roadside sniffs, but was wrong in more than 40 percent of cases. Sella, a drug detection dog in Florida, gave false alerts 53 percent of the time. Bono, a drug detection dog in Vi…

I've had my bag sniffed at airports at least 50 times, and they've never stopped me. So there must be something else going on as well

The police only call in the dogs when they wish to search and the driver does not agree. The airport doesn't do things this way so the same strategy wouldn't work.