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Notably, none of the the algorithms tested in the cited study are Apple's NeuralHash, or comparable algorithms. They look at aHash, pHash (plus a variant thereof), dHash, and and PDQ (used at Facebook for similar applications, apparently). The first 4 date to between 2004 and 2010; the last is more recent, but conceptually similar - the citation [0] for PDQ puts it in the same bucket of 'shallower, stricter, cheaper, faster' algorithms as the first four.

No one has proposed any of those as 'illegal image detectors'. Apple's NeuralHash may or may not be robust to the same or different perturbations but the cited study provides basically no new information to inform the conversation its press release wants to be a part of.

[0]: https://github.com/facebook/ThreatExchange/blob/main/hashing...

1 comments

Neuralhash is so easy to trick that not only can I make small changes that change the hash, I can make small (visible but non-annoying) changes to make an image entirely match an essentially arbitrary hash.

E.g. here is a Lena I modified to match Barbra:

https://user-images.githubusercontent.com/858454/130310372-d...

https://user-images.githubusercontent.com/858454/130310383-9...

Simply making the hash not match in any single bit is so trivial that it's hardly worth trying. If you give me a 360x360 image of your choice though I'll make one for you if you still have any doubts.

I don't think Apple has ever claimed that NeuralHash was hardened against intentional false positives. And when properly considered as just one component of their CSAM scanning architecture, NeuralHash being hardened against intentional false positives isn't necessary for it to serve its purpose.

> being hardened against intentional false positives isn't necessary for it to serve its purpose.

Except that every time they get a positive, the device will upload the image to Apple, where at some point an actual human will have to check it.

Queue people (4chan) essentially DDOS'ing that infrastructure by turning harmless memes into false positives and distributing them widely. They'll probably automate it too. The chance they won't do such a thing is about 0.01% - if they haven't already. This is just a slight twist on the common prank of making people Google terms related to CSAM.

Not only will that cause Apple to violate the privacy of innocent people who were just saving cat pictures from Facebook, it will also require a lot more manpower than Apple is probably anticipating to keep up with the amount of false positives that will come in. And that isn't even considering unintentional false positives.

I don't think Apple is planning to start sending out "oopsie we violated your privacy and looked at your memes/family photos" mails to affected people. But it might come to that too once this planned practice is tested against current or future European privacy laws (and such a notification would be the bare minimum in my opinion).

That won't go over well once the first people share such an experience.

> Queue people (4chan) essentially DDOS'ing that infrastructure by turning harmless memes into false positives and distributing them widely. They'll probably automate it too. The chance they won't do such a thing is about 0.01% - if they haven't already.

This might actually be brilliant and we might end up thanking them for it. It might pretty much destroy this filter.

> This is just a slight twist on the common prank of making people Google terms related to CSAM.

And here is the problem: there is one failure mode of this and that is if a bunch of ordinary people get jailed for innocent memes before the public gets whiff of whats happening.

So if they do they should probably be targeted as a waterhole "attack" at Apple execs and pro censorship politicians.

Apple employees reading this: think very closely before downloading wallpapers the next few years.

This is false.

1. An image is only scanned if it's in your Photo Gallery and about to be uploaded to Apple's servers.

2. The image would need also to match an unspecified second hash algorithm which runs on Apple servers. If it doesn't match this hash as well, it's not considered a match and won't be a candidate for human review.

3. Twenty nine more dual-hash collisions would also need to be imported into their iCloud photo gallery. Human review can only occur if a total of thirty images have matched both hash algorithms. This requirement is cryptographically enforced.

4. Apple human reviewers would only ever see these hash matched images. Which means they'd only ever see some manipulated memes/cat pictures, never any of your personal photos.

>The image would need also to match an unspecified second hash algorithm which runs on Apple servers.

Security by obscurity in 2021?

It’s not security by obscurity, it’s a privately held and operated secret.

Your argument works just as well if for any x, NeuralHash(x)=1.

The fact is that a 'first line of defense' that is trivial to fool is useless. The fact that there are other lines of defense doesn't mean that the one they would make me run on my own hardware has nay merit whatsoever.

> Your argument works just as well if for any x, NeuralHash(x)=1.

That's good, because they weren't arguing that NeuralHash is infallible, but that its fallibility does not lead to the consequences outlined by the GP.

> The fact is that a 'first line of defense' that is trivial to fool is useless.

Useless to you, but not to apple. It reduces the set of pictures to process from "everything" to "those identified by the algorithm as designed, plus artificial false positives". It's impossible for the second set to be larger than the first, so it's by definition not useless.

This is why it's strange how people are focusing so hard on the weaknesses of NeuralHash when what we should care about is how well the system as a whole works.

And 5. The “bad” hashes are secret, so it’s not possible to create a false positive that would trigger Apple’s human review.

They aren't all secret. Finding a site with pictures in that database is a single google search away.

The pictures aren't secret. The hashes are. We don't know what hash algorithm is employed on Apple's server side, so we can't feed that algorithm with these non-secret images and get the hashes.

That's a very optimistic view.

In reality, they need to automate something while they never get any feedback whatsoever if their efforts are successful, let alone worthwhile.

Crooks that attack Paypal at least know when they succeed - they see the money. In this case, not only they won't see the money, they won't know if what they're doing is "overwhelming" the system or not.

What also seems to be missed is that it’s trivial to foil such attacks in an automated way. The work required on Apple’s end would be smaller than that of the attackers.

Complaints about the proposal seem to overly focus on the reliability of singular aspects of the automated detection system when this has already been planned for - additionally those comments seem entirely unaware of the threshold technology in play.

If they implement should we all start intentionally spamming 'hot hash hits' to overwhelm the system? I guess someone would either need the CP hash already or break the law and morals to get one.

> I guess someone would either need the CP hash already or break the law and morals to get one.

I presume Apple didn't break the law and morals to construct the hashes.

Not everyone's morals are the same. There are a lot of people who don't think it's immoral to simply download an image. Especially if you didn't pay for it with money / ads.

And some people don't think it's immoral to stare at CSAM all day long if it's what you're paid to do in order to confirm that it is indeed CSAM especially since this knowledge will be somehow used to prevent further creation of CSAM... somehow? Probably?

I am being facetious, I know, but my point is that humans' morals and ethics are incredibly widely diverse ― especially if we're talking about the "effective" instead of "proclaimed" ethics (i.e., the values people actually follow instead of values they believe/claim to follow).

Uh, the gp points both intentional false positives and intentional false negatives (IE, anyone intentionally keeping CSAM can easily hide it while anyone can create an image that triggers the hash and gets human to scan your image and etc).

Intentional false negatives are mostly irrelevant. If you want to store galleries of illegal material on your phone, there are much better ways to do it which don't involve them being visually mangled, even if only slightly.

Intentional false positives require knowledge of a hash. And even that isn't enough to get a human to scan your image. You would need to acquire a library of A1 CSAM images/hashes. You would need to generate images which match both NeuralHash and the unspecified secondary hash algorithm which runs on Apple's cloud servers. And then you'd need to repeat this trick a sufficient number of times to trigger human review, at which point a few people (probably some minions in a developing country) get to look at the images for 1 second before pressing the "not CSAM" button and having the images sent to a technical team in Cupertino for analysis.

If both false positives and false negatives are irrelevant, then the whole system and argument is irrelevant.

Huh? False negatives and false positives are irrelevant with respect to the utility of the overall system. It means they don't render the system irrelevant.

Intentional false negatives are irrelevant because catching 100% of CSAM was never stated as a goal of the system. And because it's just not realistic to expect to see any real-world use by CSAM aficionados. Anyone savvy enough to understand the principles behind hashing (and trust some command line tool downloaded from Github) is probably smart enough to have better places to store their child porn than an iCloud gallery.

Intentional false positives are irrelevant because there are, depending how you count them, between two and six other layers which would need to be thwarted or exploited in order for intentional false positives in NeuralHash to result in any adverse outcome.

>Intentional false negatives are irrelevant because catching 100% of CSAM was never stated as a goal of the system. And because it's just not realistic to expect to see any real-world use by CSAM aficionados. Anyone savvy enough to understand the principles behind hashing (and trust some command line tool downloaded from Github) is probably smart enough to have better places to store their child porn than an iCloud gallery.

So if the purpose of the system is not to catch persons dealing with CP, what is the purpose? Breaking user's privacy?

We have a history of evil intentions masked by fighting against CP, drugs, money laundry and crime in general.

To the contrary.

A system that detects Covid-19 in infected people and doesn't detect it in healthy people, where the errors of both types (false positives and false negatives) are irrelevant, is not only relevant, it's a very well designed system.

> Intentional false positives require knowledge of a hash.

The code/network detecting this will be run on the client. So yes, a 'hash' or an equivalent will be known.

You would have to hope that the secondary system is robust enough to not fall victim to the same manipulated image. And it really is on Apple to first prove it isn't.

> The code/network detecting this will be run on the client. So yes, a 'hash' or an equivalent will be known.

They use cryptography to conceal the hash database from the users to guard it against accountability.

Because of this, if they stuff the database with political or ethnically targeted images no one will be able to tell, you just have to trust that they haven't done that.

Knowledge of a matching hash isn't a big barrier for another reason: Matching images are widely available. NCMEC reports that they had over 20 million matching images reported to them last year. It's a very reasonable assumption that some 4channer will helpfully troll the internet for likely illegal images and put up a large database of likely matches.

False positives can also go the other way: Modify illegal images to match existing lawful images which are already in circulation. One could likely take legal hardcore pornography, crop it to make it impossible to identify the ages of the talent, add captions to falsely claim that they are under-age, tweak it to match targeted images, and submit it to the databases.

That's all just baseless conspiracy theorising.

No, it is not. The hash is computed on the client, but hash comparison happens on the server, using some cryptographic magic that only reveals a “visual derivative” if enough images match. It’s fairly complex and I don’t recall the details, but the library of “bad” hashes is not accessible.

No, the entire premise of NeuralHash is that no information about your pictures leaves your system unless it has been flagged as 'potentially CSAM'. NeuralHash itself is basically a DB of CSAM hashes encoded as an artificial neural network.

In your example, each picture on your phone (that you are trying to upload to iCloud) would be hashed, and the hash would be sent to Apple. This is exactly the opposite of what they are promising with NeuralHash.

Edit: I am almost entirely wrong, see the comment reply from krisoft. Apologies!

The other reason why false negatives are mostly irrelevant is because the hash DB can be updated to include them, so the risk of storing them in iCloud remains the same.

That doesn't fly.

The encrypted list is stored on the devices for security and performance reasons: otherwise it would be too easy to ship people customized lists with arbitrary images. You can't just rapidly revise the list without undermining the privacy protection here.

Also the space of modifications is gigantic, for any given image there should be millions of differently hash variations which can be reached without major visual damage. (at least for neuralhash the figure is billions)... so you can't just stick all of them in the database.

You could stick widely circulating variations in the database, however... sure.

I'm honestly not sure what you're refuting here.

Are you saying the hash DB can't be updated or that already uploaded photos won't be compared to hash updates?

So, you're accepting that Apple's approach is useless for anyone who's "determined" but you still seem to be arguing there's some merit in this complex "architecture" of surveillance.

I disagree that it's useless. In fact I wouldn't be surprised if it's already been effective despite not being implemented yet. The announcement of an impending threat of scanning probably managed to get most CSAM afficinados to swear off iCloud already. And if so, it would be no small irony that the noisy criticism would have ensured that Apple was heard loud and clear by the CSAM community.

So the idea is that mass surveillance is great even if it's effect is scaring criminals while it actually only catches people at random?

We've arrived at the opposite of the common reprise of law. No longer "It is Better that Ten Guilty Persons Go Free Than that One Innocent Person be Convicted" but "It is better that Ten Innocent Persons Be Dragooned Than that One Guilty Person Go Free"

But is getting them off iCloud the goal? As a society, are we better off?

The article and kristjansson's post are about intentional false negatives.

My comment was pointing out that the well known fact that the neuralhash is highly vulnerable to preimage attacks (intentional false positives) also means that is is highly vulnerable to intentional false negatives. Changing the 96-bit hash to a specific value while minimizing the visible image change is a LOT harder than just changing any single bit, but the harder task has been amply demonstrated.

Neuralhash is one component, yes, but it is a limiting component. If a slightly tweaked image has a different NeuralHash it will never be detected by their scheme.

I don't think Apple is concerned about the <0.1% of CSAM aficionados who read about Apple's plans to scan for CSAM and thinks "hey, let me blindly trust some tool I found on the internet to slightly mangle my highly illegal images so that I can continue co-mingling them with personal photos in iCloud, accessible to anyone who picks up my phone while unlocked. Or any other device synchronised to the same iCloud account."

The App Store is presumably full of easy to use apps which can store photo galleries in a separately encrypted vault. This is where people will go after being shooed away from iCloud, not image manglers.

Say I'm a cop who wants to get into your phone but can't get a warrant. I happen to have an image on my (Android) phone which looks benign to a human, but triggers Apple's CSAM detectors when scanned. I text it to you, the CSAM detector goes off, and suddenly I have probable cause.

Cops plant illegal drugs on suspects all the time. Why not this?

Because it wouldn't work.

1. You would need at least 30 such images;

2. You'd have to somehow convince your target to import all 30 images into their photo gallery;

3. The chances of the images accidentally passing the human review step is infinitesimal;

4. Unlike a bag of drugs which actually do contain drugs, these fake images would be like police finding a bag of sugar with "FAKE COCAINE FOR MANUFACTURING PROBABLE CAUSE" written on it.

Imagine a politician receiving dozens of photos which trigger this CSAM algorithm because he has iCloud photos as defaul, and then somehow someone gets wind of the fact…

BANG: one less. Try to explain to the public that it is only the first line of defense against child sex and that it does not imply anything at all.