Hello! Newbie to this group, excited to contribute. Let me know if I am doing anything wrong.

One central tension in deciding on the number of topics is user privacy vs. relevance. The more topics, the more content can be personalized / targeted, but simultaneously, the more a topic could itself be sensitive or deanonymizing.

What if we fetched content privately, using private information retrieval? We could have the browser itself perform private retrievals for content, and be sure that the individual topics never leave the browser unencrypted. This could increase privacy and relevance simultaneously.

The main concern is computation/communication overhead, though recent work makes it seem very practical. I think we could make this kind of 'progressive', in that in addition to the existing browsingTopics() call, we could also have something like sendDetailedBrowsingTopicsPrivately(<url>) (it's a bad name, I know). This would POST an encrypted PIR query for some more fine-grained topics to the supplied URL. Personalization code could choose to use one or the other, depending on network conditions etc.

No information about this more fine-grained topic would be readable by the server, since the query stays encrypted. This would make it safer to use finer-grained topics, and increase the relevance of personalized content.

Is this an exciting idea to folks? I am happy to write up some more detailed thoughts, or even make a demo doing PIR with the existing experimental Topics API.