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Considering how expensive are the data in some parts of Europe, I completely expect, that people will be ripping it off and selling it as a cellular modem with unlimited data on eBay for 99EUR.

Could be but seems like it would be expensive. IIRC the content scanners send 1 image per second.

Surely they just send an image hash.

would that work with different resolutions etc?

On device or in the cloud, the advertisers need some method to match images. Might as well fully exploit your advertisees and have the client device calculate the hashes.

As to the actual matching strategy, I would guess a perceptual hash that is robust to some amount of noise and varying resolutions. Since you are comparing repeated hashes, say at 1Hz, you have multiple chances over the observation window to match a fingerprint to the lookup database. 30 minute show x 60 frames per minute = 1800 hashes to make the id

Or they will have agreements to jump through passing phone networks or via the ISP's "CoxWiFi" network that they resell as a backhaul