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Comment by gorjusborg | Hacker News Reader

> Out of curiosity, what is false positive rate of a hash match?

No way to know without knowledge of the 'proprietary hashing technology'. Theoretically though, a hash can have infinitely many inputs that produce the same output.

Mismatching hash values from the same hashing algorithm can prove mismatching inputs, but matching hash values don't ensure matching inputs.

> I wonder if (or why) human verification is actually necessary here

It's not about frequency, it's about criticality of getting it right. If you are going to make a negatively life-altering report on someone, you'd better make sure the accusation is legitimate.

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I'd say the focus on hashing is a bit of a red herring.

Most anyone would agree that the hash matching should probably form probable cause for a warrant, allowing a judge to sign off on the police searching (i.e., viewing) the image. So, if it's a collision, the cops get a warrant and open up your linux ISO or cat meme, and it's all good. Probably the ideal case is that they get a warrant to search the specific image, and are only able to obtain a warrant to search your home and effects, etc. if the image does appear to be CSAM.

At issue here is the fact that no such warrant was obtained.

> Most anyone would agree that the hash matching should probably form probable cause for a warrant

I disagree with this. Yes, if we were talking MD5, SHA, or some similar true hash algo, then the probability of a natural collision is small enough that I agree in principle.

But if the hash algo is of some other kind then I do not know enough about it to assert that it can justify probable cause. Anyone who agrees without knowing more about it is a fool.

That's fair. I came away from reading the opinion that this was not a perceptual hash, but I don't think it is explicitly stated anywhere. I would have similar misgivings if indeed it is a perceptual hash.

I think it'll prove far more likely that the government creates incentives to lead Google/other providers to fully do the search on their behalf.

The entire appeal seems to hinge on the fact that Google didn't actually view the image before passing it to NCMEC. Had Google policy been that all perceptual hash hits were reviewed by employees first, this would've likely been a one page denial.

If the hash algorithm were CRC8, then obviously it should not be probable cause for anything. If it were SHA-3, then it's basically proof beyond reasonable doubt of what the file is. It seems reasonable to question how collisions behave.

I don't agree that it would be proof beyond reasonable doubt, especially because neither google nor law enforcement can produce the original image that got tagged.

By original do you mean the one in the database or the one on the device?

If the device spit out the same SHA3, then either it had the exact same image, or the SHA3 was planted somehow. The idea that it's actually a different file is not a reasonable doubt. It's too unlikely.

By the original, I mean the image that was used to produce the initial hash, which Google (rightly) claimed to be CSAM. Without some proof that an illicit image that has the same hash exists, I wouldn't accept a claim based on hash alone.

Oh definitely you need someone to examine the image that was put in the database to show it's CSAM, if the legal argument depends on that. But that's an entirely different question from whether the image on the device is that image.

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