I heard this on Today R4 last week being reported as looking for matches against known child porn images, and seen other reports since saying the same thing using a "Neural Hash" generated using a neural network.
A cryptographic hash is a many to one transformation - you input a file / image and out comes a single value such as "ABCD12345", which makes it possible to do an efficient comparison.
Sometimes it may be possible for two different files / images to generate the same hash or "collision". Presumably Apple have a very good hashing algorithm / neural net which will not generate collisions!
And of course if supposed matches are then also checked by human eyes as a failsafe that sounds like a good system from the point of view of avoiding false positives - though some may still have privacy concerns.
So will it be only exact matches to the database? Hardly likely to be useful since those are all "known" images i.e. already found and incorporated into the database. So "matching" is just presuming it's got some similarity to a digital measure of a small human showing too much flesh with possible "abuse" of certain identifiable body parts.
So look at what we get when we use the various image search engines and all the "near misses" for various ladies in different poses, wearing (or not) skimpy clothing...
How many images are going to get referred for human review, look at the problem Google et al are having with their content moderators.
This all smacks of serious PR BS or an extremely intrusive breach of people's privacy. Where's the opt-in for Apple customers to say I don't mind you scanning all my private photos or work photos?
They say the scanning will be done on the device and then "possibly offending" images posted up to the cloud for review. Will unknown pedos just start working for Apple as content moderators?
So every iPhone will be regularly updating its database of "neural hashes" to include new images that have been found that add to the "precision" of the AI scanning? Will this memory be separate to what you pay for as usable by you the purchaser? Will Apple have another cost to charge for in the increasingly bloated price of their devices?
The only agencies who should be scanning unpublished material are government authorities charged with upholding their jurisdictions law, how is it right for a greedy corporate to be intruding even further on the private materials of its customers? No crime has been committed in taking a photo, only if that photo is known to be of illegal material.