Client dictates what ads are shown. Fb knows what ads are shown to who. Fb now can deduce what topics people are talking about. Technically convo info has leaked. If someone is getting served ads for Trump, they probably like Trump. If they are getting ads for Biden they probably like Biden. Etc…..
Yes, so that would violate the end to end principle. If the client downloaded all of the possible ads and the selection was totally local, and interaction with any of them was a user choice I think that could still be fairly described as E2E though. Or ads were fetched by private information retrieval.
