[FabHK]

> Based on their whitepaper, it's a smaller version of the original image,

I seem to recall that the white paper speaks of a "visual derivative" without specifying it further.

[koolhaas]

The Technical Summary uses "visual derivative" without clarification, but their Threat Model PDF clarifies it further as thumbnails:

>The decrypted vouchers allow Apple servers to access a visual derivative – such as a low-resolution version – of each matching image.

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