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The Problem with Perceptual Hashes

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Re: The Problem with Perceptual Hashes

#101

Post by asimpletune »

“ Even at a Hamming Distance threshold of 0, that is, when both hashes are identical, I don’t see how Apple can avoid tons of collisions, given the large number of pictures taken every year (1.4 trillion in 2021, now break this down by iPhone market share and country, the number for US iPhone users will still be extremely big).”

Is this true? I’d imagine you could generate billions a second without having a collision, although I don’t know much about how these hashes are produced.

It would be cool for an expert to weigh in here.

Re: The Problem with Perceptual Hashes

#102

Post by mrtksn »

munk-a wrote: post #99
mrtksn wrote: post #80

Earlier quoted context omitted.

IMHO, Unless everything being E2E encrypted becomes the law we can’t do anything about it because that’s not Apple’s initiative but comes from people whose job is to know things and they cannot resist keeping their hands out of these data collecting devices. They promise politicians that all the troubles will go away if we do that. Child pornography, Terrorism? Solve it the old way. I don’t know why citizens are obli…

Unless I missed a big part of this story Apple isn't being compelled by a court to comply - so if the presence of this tech causes a larger PR stink than publicly backing out of the rollout then Apple will read the writing on the wall. Public and political pressure is definitely an issue - but it's still soft-pressure so applying more pressure in the other direction will be compelling to Apple.

Apple was supposed to E2E encrypt everything. Then reports surfaced that FBI complained and they stopped.

The are speculations about this being Apple’s solution to government demands so that they can continue migrating to E2E.

They are trying a solution where the device reports you to the authorities so that Apple gets out of the business of knowing your data.

Re: The Problem with Perceptual Hashes

#103

Post by tornato7 »

danachow wrote: post #69
emodendroket wrote: post #43

Earlier quoted context omitted.

A couple gigabytes is a lot of photos... and they'd all be showing up in your camera roll. Maybe possible but stretching the bounds of plausibility.

A couple gigabytes is enough to ruin someone’s day but not a lot to surreptitiously transfer, it’s literally seconds. Just backdate them and they may very well go unnoticed.

It's also possible to 'hide' photos from the reel in the photos app. Many people are unaware of that feature so an attacker could hide as many photos they want in your iCloud.

Re: The Problem with Perceptual Hashes

#104

Post by nanidin »

yellow_lead wrote: post #61

Regarding false positives re:Apple, the Ars Technica article claims > Apple offers technical details, claims 1-in-1 trillion chance of false positives. There are two ways to read this, but I'm assuming it means, for each scan, there is a 1-in-1 trillion chance of a false positive. Apple has over 1 billion devices. Assuming ten scans per device per day, you would reach one trillion scans in ~100 days. Okay, but not al…

> Apple has over 1 billion devices. Assuming ten scans per device per day, you would reach one trillion scans in ~100 days.

People like to complain about the energy wasted mining cryptocurrencies - I wonder how this works out in terms of energy waste? How many people will be caught and arrested by this? Hundreds or thousands? Does it make economic sense for the rest of us to pay an electric tax in the name of scanning other people's phones for this? Can we claim it as a deductible against other taxes?

Re: The Problem with Perceptual Hashes

#105

Post by asimpletune »

mrtksn wrote: post #72

The technical challenges aside, I’m very disturbed that my device will be reporting me to the authorities. That’s very different from authorities taking a sneak peek into my stuff. That’s like the theological concept of always being watched. It starts with child pornography but the technology is indifferent towards it, it can be anything. It’s always about the children because we all want to save the children. Soon t…

I have been a big Apple fan ever since my first computer. This is the first time I legitimately thought I need to start thinking about something else. It’s kind of sad.

Re: The Problem with Perceptual Hashes

#106

Post by dogma1138 »

starkd wrote: post #59

The method Apple is using looks more like a cryptographic hash. That's entirely different (and more secure) than a perceptual hash. From https://www.apple.com/child-safety/ "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…

The cryptography is most likely done at a higher level than the perception comparison and is quite likely done to protect the CSAM hashes than your privacy.

My interpretation of this is that they still use some sort of a perception based matching algorithm they just encrypt the hashes and then use some “zero knowledge proof” when comparing the locally generated hashes against the list, the result of which would be just that X hashes marched but not which X.

This way there would be no way to reverse engineer the CSAM hash list or bypass the process by altering key regions of the image.

Re: The Problem with Perceptual Hashes

#108

Post by 7373737373 »

stickfigure wrote: post #10

I've also implemented perceptual hashing algorithms for use in the real world. Article is correct, there really is no way to eliminate false positives while still catching minor changes (say, resizing, cropping, or watermarking). I'm sure I'm not the only person with naked pictures of my wife. Do you really want a false positive to result in your intimate moments getting shared around some outsourced boiler room for…

I, too, have worked on similar detection technology using state of the art neural networks. There is no way there won't be false positives, I suspect many, many more than true positives.

It is very likely that as a result of this, thousands of innocent people will have their most private of images viewed by unaccountable strangers, will be wrongly suspected or even tried and sentenced. This includes children, teenagers, transsexuals, parents and other groups this is allegedly supposed to protect.

The willful ignorance and even pride by the politicians and managers who directed and voted for these measures to be taken disgusts me to the core. They have no idea what they are doing and if they do they are simply plain evil.

It's a (in my mind entirely unconstitutional) slippery slope that can lead to further telecommunications privacy and human rights abuses and limits freedom of expression by its chilling effect.

Devices should exclusively act in the interest of their owners.

Re: The Problem with Perceptual Hashes

#109

Post by burnished »

KarlKemp wrote: post #82
yellow_lead wrote: post #61

Regarding false positives re:Apple, the Ars Technica article claims > Apple offers technical details, claims 1-in-1 trillion chance of false positives. There are two ways to read this, but I'm assuming it means, for each scan, there is a 1-in-1 trillion chance of a false positive. Apple has over 1 billion devices. Assuming ten scans per device per day, you would reach one trillion scans in ~100 days. Okay, but not al…

Do you really believe that if they scan your photo library at 10am and don't get any false positives, another scan five hours later, with no changes to the library, has the same chance of getting false positives as the first one, independent of that result?

If you take photos, then yes?

Re: The Problem with Perceptual Hashes

#110

Post by ezoe »

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 that match the hash to the authority. The authority use that report for a convenient reason to false-arrest you.

Now what happens if you sue the authority for the intentional false-arrest? Demand the original intended file for the hash? "No. We won't reveal the original file because it's child abusing image, also we don't keep the original file for moral reason"

But come to think of it, we already have tons of such bogus pseudo-science technology like the dogs which conveniently bark at police's secret hand sign, polygraph, and the drug test kit which detect illegal drugs from thin air.

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