Re:A Potential Privacy Model for the Web
by jefftk on Wednesday August 28, 2019 @01:10PM
(#20821762)
The first bullet point below that title is "First parties have a way to delegate access to a user identity to specific 3p's, as long as that delegated identity remains sharded by 1p." The model is proposing that if you allow Google to have access to user identities on your site, and I allow Google to have access to user identities on my site, the browser should prevent Google from joining those identities and detecting that the same user visited both of our sites.
(Disclosure: I work for Google, like the author of the article. Speaking only for myself.)
The problem with this, is that third parties can also be first parties and have their own data. The obvious examples are Facebook and Google today, when you use any service where they act as a third party, they may mix your identity with their first party id.