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The Problem with Perceptual Hashes

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Re: The Problem with Perceptual Hashes

#221

Post by Klonoar »

bigiain wrote: post #187
Klonoar wrote: post #114

Earlier quoted context omitted.

I would really like people to start answering this: what exactly do you think has changed? e.g, >That’s very different from authorities taking a sneak peek into my stuff. To be very blunt: - The opt out of this is to not use iCloud Photos. - If you _currently_ use iCloud Photos, your photos are _already_ hash compared. - Thus the existing opt out is to... not use iCloud Photos. The exact same outcome can happen regar…

> The opt out of this is to not use iCloud Photos. Wasn’t yesterday’s version of this sorry about how Apple is implementing this as a client side service on iPhones? https://news.ycombinator.com/item?id=28068741 I don’t know if the implication there is “don’t use the stock Apple camera app and photo albums”, or “don’t store any images on yours Phone any more” if they are scanning files from other apps for perceptual…

...yes, and the client-side check is only run before syncing to iCloud Photos, which is basically just shifting the hash check from before upload (client side) to after upload (server side).

Re: The Problem with Perceptual Hashes

#222

Post by Klonoar »

visarga wrote: post #203
Klonoar wrote: post #120

Earlier quoted context omitted.

>What exactly do you think is the same as before? The same checking when you synced things to iCloud. As has been repeated over and over again, this check happens for iCloud Photos. It's not running arbitrarily. Your photos were compared before and they're being compared now... if you're using iCloud Photos.

It's not the same because before the hashing was done in the cloud, but now the model is accessible locally, you just need to take pictures. This means it's easier to hack. If someone discovers a way to reliably generate adversarial images they can send such images to someone else to iSWAT them.

If your definition of "hack" is "get bob to accept bad file", no, this model is not easier - it's just different.

You could literally piggyback on the directories that Macs use to sync to iCloud Drive, get an image in there, and then it gets scanned by iCloud. This is not some new theoretical attack - and in fact, this would be the "hack" for the new one as well since it requires iCloud sync to trigger anyway.

Re: The Problem with Perceptual Hashes

#223

Post by trynumber9 »

altitudinous wrote: post #219
trynumber9 wrote: post #214

Earlier quoted context omitted.

Does it scan files within archives? If it does, you could download the wrong zip and instantaneously be over their threshold.

The scanning is to take place within iCloud Photos, which handles images / videos etc on an individual basis. It would be a pretty easy thing to do for Apple to calculate hashes on these. I'm not sure how iOS handles archives, but it doesn't matter - remember it isn't 100% or 0% with these things - say only 50% of those people store images in iCloud Photo, catching out only 50% of those folk is still a good result.

Yeah, I'm not sure. Just is a bit worrying to me. On my device iCloud Drive synchronizes anything in my downloads folder. If images contained within zips are treated as individual images, then I'm always just one wrong click from triggering their threshold.

Re: The Problem with Perceptual Hashes

#224

Post by kelnos »

visarga wrote: post #211
fogof wrote: post #155

Earlier quoted context omitted.

Well, presumably at that point, someone in that position would just reveal their own files with the hash an prove to the public that they weren't illegal. Sure, it would be shitty to be forced to reveal your private information that way, but you would expose a government agency as fabricating evidence and lying about the contents of the picture in question to falsely accuse someone. It seems like that would be a scan…

You can reveal your files and people can accuse you you deleted the incriminating ones.

Not if you show the file that matches the perceptual hash that "caught" you.

Re: The Problem with Perceptual Hashes

#225

Post by kelnos »

runlevel1 wrote: post #111
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.

Gigs of software updates and podcast episodes are regularly downloaded to phones without being noticed. How frequently do most people look at their camera roll? I'd be surprised if it's more than a few times a week on average. Does an attacker even need access to the phone? If iCloud is syncing your photos, your phone will eventually see all your pictures. Unless I've misunderstood how this works, the attacker only n…

> I'd be surprised if it's more than a few times a week on average.

For me it's probably 5-7 times per day, but I also take a lot of photos.

I think a few times a week is probably low-balling it, even for an average.

Re: The Problem with Perceptual Hashes

#226

Post by Klonoar »

Renaud wrote: post #144
Klonoar wrote: post #114

Earlier quoted context omitted.

I would really like people to start answering this: what exactly do you think has changed? e.g, >That’s very different from authorities taking a sneak peek into my stuff. To be very blunt: - The opt out of this is to not use iCloud Photos. - If you _currently_ use iCloud Photos, your photos are _already_ hash compared. - Thus the existing opt out is to... not use iCloud Photos. The exact same outcome can happen regar…

What has changed is the inclusion of spyware technology on the device that can be weaponised to basically report on anything. Today it's only geared toward iCloud and CSAM. How many lines of codes do you think it will take before it scans all your local pictures? How hard do you think it will be for an authoritarian regime like China, that Apple bends over backwards to please, to start including other hashes that are…

>What has changed is the inclusion of spyware technology on the device that can be weaponised to basically report on anything.

- You are running a closed source proprietary OS that you cannot verify is not already doing anything.

- This could theoretically already be weaponized (with the existing server-side implementation) by getting someone to download a file to a folder that iCloud automatically syncs from.

>iCloud is opt-out.

Yes, and that's how you opt out of this scanning. It's the same opt-out as before.

>Under the guise of something no-one can genuinely be against (think of the children!) they have now included a pretty generic snitch into your phone and made everyone accept it.

I dunno what to tell you. I think the system as designed is actually pretty smart[1] and more transparent than before.

If you used iCloud before, and you're putting photos up that'd be caught in a hash comparison, you've already got a snitch. Same with any other cloud storage, short of hosting your own.

[1] I reserve the right for actual bona-fide cryptographers to dissect it and set the record straight, mind you.

Re: The Problem with Perceptual Hashes

#227

Post by deanclatworthy »

kelnos wrote: post #224
visarga wrote: post #211

Earlier quoted context omitted.

You can reveal your files and people can accuse you you deleted the incriminating ones.

Not if you show the file that matches the perceptual hash that "caught" you.

Would a court be compelled to provide that hash to your defence? Arguable as it could be used by criminals to clean their collection. And by that time your life is ruined anyway.

Re: The Problem with Perceptual Hashes

#228

Post by kelnos »

wonnage wrote: post #150
Renaud wrote: post #144

Earlier quoted context omitted.

What has changed is the inclusion of spyware technology on the device that can be weaponised to basically report on anything. Today it's only geared toward iCloud and CSAM. How many lines of codes do you think it will take before it scans all your local pictures? How hard do you think it will be for an authoritarian regime like China, that Apple bends over backwards to please, to start including other hashes that are…

We gotta stop with the China bogeyman every time a privacy issue comes up. This is a feature designed by an American company for American government surveillance purposes. China is perfectly capable of doing the same surveillance or worse on its own citizens, with or without Apple. China has nothing to do with why American tech is progressively implementing more authoritarian features in a supposedly democratic count…

I think it's irresponsible to avoid thinking about how bad actors might use a technology you've developed.

And is it really unfathomable that the US government could use this sort of thing for evil? I mean, wind back the clock to something like the Red Scare. If they had iPhones back then, they totally would have pressured Apple to add hashes for communist imagery, and use that to persecute people (or worse).

(Before anyone brings this up: I do categorically reject the notion of "that was in the past; that couldn't happen today". If you truly believe that, I have a bridge you might be interested in purchasing...)

Re: The Problem with Perceptual Hashes

#229

Post by bigiain »

stetrain wrote: post #215
bigiain wrote: post #200

Earlier quoted context omitted.

> The post office scans your mail through various machines in transit. That is a totally bogus comparison. The post office 100% does NOT can the content of every piece mail they handle. Not even close to the same scenario as Apple/governments being able to continually and silently check your phone/photo library for images on their watch list.

I’m pretty sure lots of mail gets x-rayed, perhaps even more looking for malicious packages or substances. I agree that data content scanning is more invasive than physical scanning. It was an intentionally simplistic example not meant to defend Apple.

Parcels, maybe. I’d bet it’s a tiny percentage though.

I doubt the entire world has enough X-ray machines to scan even a vanishingly small percentage of the envelopes the postal service delivers every day.

Re: The Problem with Perceptual Hashes

#230

Post by [deleted] »

vineyardmike wrote: post #31
bryanrasmussen wrote: post #11

Earlier quoted context omitted.

Not knowing anything about it but I suppose various governmental agencies maintain corpora of nasty stuff and that you can say to them - hey we want to roll out anti-nasty stuff functionality in our service therefore we need access to corpora to test at which point there is probably a pretty involved process that requires governmental access also to make sure things work and are not misused otherwise - how does anyon…

Test it against innocent data sets, then in prod swap it for the opaque gov db of nasty stuff and hope the gov was honest about what is in it :) They don't need to train a model to detect the actual data set. They need to train a model to follow a pre-defined algo

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