8 ms·

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

by nullc 5y ago

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-d6b2c633-c5d3-4b3e-bc12-d3b2f3104ec6.png https://user-images.githubusercontent.com/858454/130310372-d... https://user-images.githubusercontent.com/858454/130310383-9fcc80ba-117e-4383-9177-0df24bc99f9a.png 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.

simondotau 5y ago
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.
- joe_the_user 5y ago
  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).
  - simondotau 5y ago
    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.
    - ClumsyPilot 5y ago
      If both false positives and false negatives are irrelevant, then the whole system and argument is irrelevant.
      - simondotau 5y ago
        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.
        DeathArrow 5y ago
        >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.
        emodendroket 5y ago
        There is pretty much no enforcement mechanism against any crime which we can expect to catch everyone. False positives get bounced out by the safeguards, false negatives wouldn't have been caught in the first place... that still presumably leaves a number of true positives who wouldn't have otherwise been caught.
- nullc 5y ago
  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.
  - simondotau 5y ago
    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.
- chmod775 5y ago
  > 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.
  - simondotau 5y ago
    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.
    - tsimionescu 5y ago
      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.
- ddevault 5y ago
  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?
  - pfortuny 5y ago
    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.
  - simondotau 5y ago
    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.

by nullc 5y ago

simondotau 5y ago

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.

joe_the_user 5y ago
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).
- simondotau 5y ago
  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.
  - ClumsyPilot 5y ago
    If both false positives and false negatives are irrelevant, then the whole system and argument is irrelevant.
    - simondotau 5y ago
      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.
      - DeathArrow 5y ago
        >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.
        emodendroket 5y ago
        There is pretty much no enforcement mechanism against any crime which we can expect to catch everyone. False positives get bounced out by the safeguards, false negatives wouldn't have been caught in the first place... that still presumably leaves a number of true positives who wouldn't have otherwise been caught.
nullc 5y ago
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.
- simondotau 5y ago
  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.
chmod775 5y ago
> 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.
- simondotau 5y ago
  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.
  - tsimionescu 5y ago
    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.
ddevault 5y ago
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?
- pfortuny 5y ago
  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.
- simondotau 5y ago
  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.