Cohort IDs can be collected over time to create cross-site tracking IDs

In https://github.com/WICG/floc/issues/99, it is stated that "FLoC is not useful for tracking." I don't think that's accurate. As far as I know, the user's cohort will not be partitioned per first party site so multiple sites can observe the cohort ID in sync as it changes week after week. A hash of the cohorts seen so far will likely get more and more unique as the weeks go by. Websites or tracker scripts on websites can expose arrays of the cohorts they've seen to help all trackers identify the user, like this: ``` let cohortCollectionForWebsiteA = [ "week01_2022" : "0666", "week03_2022" : "A566", "week04_2022" : "2111", "week05_2022" : "1171", "week07_2022" : "749B", ] let cohortCollectionForWebsiteB = [ "week01_2022" : "0666", "week02_2022" : "0030", "week05_2022" : "1171", "week06_2022" : "7311", "week07_2022" : "749B", ] ``` Trackers send these to a server for matching across websites, in the example above, resulting in the intersection `[ "week01_2022", "week05_2022", "week07_2022" ]`. The cohort collections can be tied to PII on sites that have access to such information about the user. This would allow a tracker with just a collection on one site to call a server and get back PII for that user. If cohorts were partitioned (maybe they are?), the tracking effort would take longer but observed partitioned cohort IDs can be sorted to potentially create a unique ID across websites. You get something like snippets of a DNA that eventually become

Posted by johnwilander on Apr 14, 2021, 5:21 AM

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