Re: [w3ctag/design-reviews] Cookies Having Independent Partitioned State (CHIPS) (#654)

> Would it be correct to frame this as a side channel of the Google Privacy Sandbox being built to support specific usecases?

No, this proposal does not introduce any side channels. The explainer and the S&P questionnaire explicitly discuss potential side channels, and where applicable, solutions for preventing them. We decided to address these head-on, because similar side-channel attacks [were discussed](https://github.com/privacycg/storage-partitioning/issues/11) in the context of other partitioning work, and we wanted to make sure that these were given consideration

> How is this impacted by CNAME mapping and efforts to muddle 1st party context? Will IDs stored in this storage be accessible to Javascript for 3rd party triangle syncs and therefore any IDs stored in this partition be ~100% open to be used as a cross-site ID? 

Could you expand upon the attack you are envisioning? By definition, identifiers stored in partitioned cookies are available to the owning domain/host **only within the same top-level context** that makes up the [partition key](https://github.com/WICG/CHIPS#partitioning-model). This is what prevents their use for tracking across sites.

> Have other browsers considered this? 

Yes. In fact, Firefox and Safari are [currently either shipping or discussing](https://github.com/WICG/CHIPS#alternate-cookie-partitioning-designs) partitioning of third-party cookies. In short, there is cross-browser alignment that this is a useful semantic for the web, there differences in the syntactical/deployment approaches.




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