In a previous post, I wrote about a new set of technologies “Privacy Preserving Advertising”, which are intended to allow for advertising without compromising privacy. This post discusses one of those proposals–Federated Learning of Cohorts (FLoC)–which Chrome is currently testing. The idea behind FLoC is to make it possible to target ads based on the interests of users without revealing their browsing history to advertisers. We have conducted a detailed analysis of FLoC privacy. This post provides a summary of our findings.
In the current web, trackers (and hence advertisers) associate a cookie with each user. Whenever a user visits a website that has an embedded tracker, the tracker gets the cookie and can thus build up a list of the sites that a user visits. Advertisers can use the information gained from tracking browsing history to target ads that are potentially relevant to a given user’s interests. The obvious problem here is that it involves advertisers learning everywhere you go.
FLoC replaces this cookie with a new “cohort” identifier which represents not a single user but a group of users with similar interests. Advertisers can then build a list of the sites that all the users in a cohort visit, but not the history of any individual user. If the interests of users in a cohort are truly similar, this cohort identifier can be used for ad targeting. Google has run an experiment with FLoC; from that they’ve stated that FLoC provides 95% of the per-dollar conversion rate when compared to interest-based ad targeting using tracking cookies.