No, matches are not verified on the phone. On the phone, your image hash is used to look up an encrypted/blinded (via the server's secret key) CSAM hash. Then your image data (the hash and visual derivative) is encrypted with that encrypted/blinded hash. This encrypted payload, along with a part of your image's hash, is sent to Apple. Then on the server, Apple uses that part of your image's hash and their secret key to create a decryption key for the payload. If your image hash matches the CSAM hash, the decryption key would unlock the payload.

In addition, they payload is protected at another layer by your user key. Only with enough mash matches can Apple put together the user decryption key and open the very innards of your image's payload containing the full hash and visual derivative.

To quote a sibling comment, who looked into the horses' mouth:

> Apple’s method of detecting known CSAM is designed with user privacy in mind. Instead of scanning images in the cloud, the system performs on-device matching using a database of known CSAM image hashes provided by NCMEC and other child-safety organizations. Apple further transforms this database into an unreadable set of hashes, which is securely stored on users’ devices.

https://www.apple.com/child-safety/pdf/CSAM_Detection_Techni...

I believe the hash comparisons are made on Apple's end. Then the only way to get hashes will be a data breach on Apple's end (unlikely but not impossible) or generating it from known CSAM material.

That's not what Apple's plans state. The comparisons are done on phone, and are only escalated to Apple if there are more than N hash matches, at which point they are supposedly reviewed by Apple employees/contractors.

Otherwise, they'd just keep doing it on the material that's actually uploaded.

Ah, never mind, you're right:

> Apple’s method of detecting known CSAM is designed with user privacy in mind. Instead of scanning images in the cloud, the system performs on-device matching using a database of known CSAM image hashes provided by NCMEC and other child-safety organizations. Apple further transforms this database into an unreadable set of hashes, which is securely stored on users’ devices.

https://www.apple.com/child-safety/pdf/CSAM_Detection_Techni...

He is not right, though. The system used will not reveal matches to the device, only to the server and only if the threshold is reached.

> That's not what Apple's plans state. The comparisons are done on phone

Yes but as stated in the technical description, this match is against a blinded table, so the device doesn’t learn if it’s a match or not.