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They don't need to hold the pictures anywhere to use the hash. 1. Police somehow finds child porn. 2. They generate the hash[1]. 3. They distribute the hash to
by sz4kerto 12y ago
They don't need to hold the pictures anywhere to use the hash. 1. Police somehow finds child porn. 2. They generate the hash[1]. 3. They distribute the hash to large storage providers. 4. Storage providers compare the hash with the data they store. 5. Profit.
[1]:hash might not necessarily be something like MD5, it might be a more sophisticated photo fingerprint.
Edit:
Anyway, I have no idea why is this on HN again, nth time. It's old news and also automated photo fingerprinting is not exactly a huge issue for cloud storages where you store unencrypted data on the first place. It's just another machine reading the bytes of your data, and those bytes have been read by many other machines as well. They don't even use this for advertising or something that could make you a product. They don't even check for pirated software or stuff like this, so this is not exactly groundbreaking news.
- Karunamon 12y agoThat leaves a question of collisions open. The chances are really low, but not zero - does a hash match automatically generate events that lead to your front door being kicked open and your pets shot, or is there an actual human that needs to open the file and verify that yeah, it is a child abuse image?
- sz4kerto 12y agoHey, they _have_ that picture. If there is a hash collision, then a human will look at it. (Not that hash collisions are _that_ common.) Why would they come to your door for the stuff you store at them? :) Edit: you mentioned lottery below. Well, waiting for a collision in the case of a good hash function is a lottery where nobody's winning for a few billion years :).
- Paul-ish 12y agoWith any good hash function there is pretty much zero risk of this happening.
- Karunamon 12y agoI'm not willing to ruin someone's life over the equivalent of winning a particularly bad lottery.
- Scaevolus 12y agoYour chances of having your life ruined by any other cause is at least 2^64 times more likely than because your uploaded cute_cat.jpg has the same hash as a flagged child_porn.jpg.
- dagw 12y agoI'm assuming we're not talking about SHA1 hashes here (which would be trivial to get around), but image matching hashes (probably something like http://phash.org/ http://phash.org/) which are much more fuzzy. If that is the case then collisions will be far more common.
- eli 12y agoDepending on the exact hash, the odds might be more like winning 10,000 lotteries.
- Ded7xSEoPKYNsDd 12y agoNote that we're talking about fuzzy image-matching hashs that attempt to match an image to its source even after it was cropped at the edges, maybe got a new watermark somewhere and was recompressed with JPEG a few times. I don't know the false positive rate of these algorithms but I imagine they're much bigger than that of a cryptographic hash function (that's what analogies like yours usually refer to).
- Retra 12y agoA hash collision is not proof of anything. You just look at the picture if the hash matches. It is not illegal to own an image that hashes to the same thing as an illegal image would.