Intent to Ship: Protected Audience: Auction Nonce Hardening

[Orr Bernstein]

Contact emails

or...@google.com, paulj...@chromium.org, cara...@chromium.org

Explainer

https://github.com/WICG/turtledove/pull/1322

Specification

https://github.com/WICG/turtledove/pull/1313

Summary

Additional bids are a feature of the Protected Audience auction that provide buyers with a way to include server-constructed contextual bids in the auction, which allows negative targeting of those bids. We've identified a potential privacy risk with the current implementation, as well as a potential solution that addresses that risk. Additional bids come from buyers, but are transported to the auction by the auction's seller. To prevent replay of additional bids, additional bids rely on an auction nonce — a unique number created by and used by the browser to uniquely identify that auction. However, this introduces a privacy risk, in that all buyers see the same auction nonce, and could use that auction nonce as a key to join distinct bid requests for an auction. This proposal allows sellers to introduce an additional nonce that gets combined with the browser-provided one so that buyers see different combined nonces across bid requests, preventing the joining of bid requests. The combined nonce is generated through a one-way hash (SHA-256) to prevent the construction of a combined nonce that matches a previous combined nonce, which could otherwise be used to facilitate the replay of an additional bid.

Blink component

Blink>InterestGroups

TAG review

For Protected Audience: https://github.com/w3ctag/design-reviews/issues/723

TAG review status

Completed for Protected Audience, resolved unsatisfied.

Risks

Interoperability and Compatibility

Optional new functionality that does not break existing use.

Gecko & WebKit: For Protected Audiences in general - Negative from Mozilla. No signal from Webkit.

Edge: Edge is running an Origin Trial of the Ad Selection API which shares a Web API and services protocol with Protected Audience.

Web developers: Requested by ad tech in GitHub issue #1198.

Debuggability

Ad-Auction-Additional-Bid response headers are visible in the DevTools Network tab, and each can be trivially decoded into an auction nonce, a seller nonce, and a base-64 encoded signed additional bid. Errors encountered while decoding and parsing the signed additional bid are presented in the DevTools console. Additional bids are debuggable via DevTools debugging of Protected Audience scoring scripts.

Will this feature be supported on all six Blink platforms (Windows, Mac, Linux, ChromeOS, Android, and Android WebView)?

It will be supported on all platforms that support Protected Audience, so all but WebView.

Is this feature fully tested by web-platform-tests?

Yes 

Flag name on chrome://flags

None

Finch feature name

FledgeSellerNonce

Requires code in //chrome?

False

Estimated milestones

Shipping on desktop and Android in M132.

Anticipated spec changes

None

Link to entry on the Chrome Platform Status

https://chromestatus.com/feature/5081571282124800

This intent message was generated by Chrome Platform Status.

[Vladimir Levin]

On Mon, Nov 25, 2024 at 9:15 AM 'Orr Bernstein' via blink-dev <blin...@chromium.org> wrote:

Contact emails

or...@google.com, paulj...@chromium.org, cara...@chromium.org

Explainer

https://github.com/WICG/turtledove/pull/1322

Specification

https://github.com/WICG/turtledove/pull/1313

Summary

Additional bids are a feature of the Protected Audience auction that provide buyers with a way to include server-constructed contextual bids in the auction, which allows negative targeting of those bids. We've identified a potential privacy risk with the current implementation, as well as a potential solution that addresses that risk. Additional bids come from buyers, but are transported to the auction by the auction's seller. To prevent replay of additional bids, additional bids rely on an auction nonce — a unique number created by and used by the browser to uniquely identify that auction. However, this introduces a privacy risk, in that all buyers see the same auction nonce, and could use that auction nonce as a key to join distinct bid requests for an auction. This proposal allows sellers to introduce an additional nonce that gets combined with the browser-provided one so that buyers see different combined nonces across bid requests, preventing the joining of bid requests. The combined nonce is generated through a one-way hash (SHA-256) to prevent the construction of a combined nonce that matches a previous combined nonce, which could otherwise be used to facilitate the replay of an additional bid.

According to the explainer, the auction nonce (generated by the browser, and given to the seller (?)) is combined with a seller generated nonce to generate a bid nonce that buyers see. That's to make sure that buyers can't use the auction nonce to figure out other bids that are happening for the same auction, right? Then the bid nonce is returned back to the seller. I presume this is to identify which auction the bid is for? What I don't understand is that the bid nonce is then returned to the browser, but the browser only knows the auction nonce so wouldn't it have no way to match that with an auction because it doesn't know seller generated nonce for this bid?

Another unrelated question, does this have any separate implications for Trusted Execution Environments? Specifically, does this apply to both or only to "local" auctions?

Thanks,

Vlad

 

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