An important part of the ruling seems to be that neither Google nor the police had the original image or any information about it, so the police viewing the image gave them more information than Google matching the hash gave Google: for example, consider how the suspect being in the image would have changed the case, or what might happen if the image turned out not to be CSAM, but showed the suspect storing drugs somewhere, or was even, somehow, something entirely legal but embarrassing to the suspect. This isn't changed by the type of hash.
Now you’ve got me interested in what’s going on under the hood, lol. It’s probably like any other statistical model: you can decrease your false negatives (images people have cropped or added watermarks/text to), but at the cost of increased false positives.
Rather simple methods are surprisingly effective [1]. There's sure to be more NN fanciness nowadays (like Apple's proposed NeuralHash), but I've used the algorithms described by [1] to great effect in the not-too-distant past. The HN discussion linked in that article is also worth a read.
[1] https://www.hackerfactor.com/blog/index.php?/archives/432-Lo...
These hashes are not collision-resistant.
I am not at all opposed to any of this "get a damn warrant" pushback from judges.
I am also not at all opposed to Google searching it's cloud storage for this kind of content. There are a lot of things I would mind a cloud provider going on fishing expeditions to find potentially illegal activity, but this I am fine with.
I do strongly object to companies searching content for illegal activity on devices in my possession absent probable cause and a warrant (that they would have to get in a way other than searching my device). Likewise I object to the pervasive and mostly invisible delivery to the cloud of nearly everything I do on devices I possess.
In other words, I want custody of my stuff and for the physical possession of my stuff to be protected by the 4th amendment and not subject to corporate search either. Things that I willingly give to cloud providers that they have custody of I am fine with the cloud provider doing limited searches and the necessary reporting to authorities. The line is who actually has the bits present on a thing they hold.
If it was a cryptographic hash (apparently not), this mathematical near-certainty is necessary but not sufficient. Like cryptography used for confidentiality or integrity, the math doesn't at all guarantee the outcome; the implementation is the most important factor.
Each entry in the illegal hash database, for example, relies on some person characterizing the original image as illegal - there is no mathematical formula for defining illegal images - and that characterization could be inaccurate. It also relies on the database's integrity, the user's application and its implementation, even the hash calculator. People on HN can imagine lots of things that could go wrong.
If I were a judge, I'd just want to know if someone witnessed CP or not. It might be unpleasant but we're talking about arresting someone for CP, which even sans conviction can be highly traumatic (including time in jail, waiting for bail or trial, as a ~child molestor) and destroy people's lives and reputations. Do you fancy appearing at a bail hearing about your CP charge, even if you are innocent? 'Kids, I have something to tell you ...'; 'Boss, I can't work for a couple weeks because ...'.
I think it would obviously be less than ideal for Google to require an employee visually inspect child pornography identified by image hash before informing a legal authority like the police. So it seems more likely that the remedy to this situation would be for the police to obtain a warrant after getting the tip but before requesting the raw data from Google.
Would the image hash match qualify as probable cause enough for a warrant? On page 4 the judge stops short of setting precedence on whether it would have or not. Seems likely that it would be a solid probable cause to me, but sometimes judges or courts have a unique interpretation of technology that I don't always share, and leaving it open to individual interpretation can lead to conflicting results.