Protected Audience already supports a mechanism to ensure the authenticity and integrity of information passed into the auction from the seller called directFromSellerSignals. Currently this is implemented by the seller providing subresources in a WebBundle to the browser, which after a year of testing has proved to not be as efficient as originally planned. It either requires an entirely new additional fetch of a WebBundle, or for the seller to rewrite and rework an existing fetch to respond instead with only a WebBundle. This feature is a rewrite of directFromSellerSignals to use an HTTP response header, transferred via HTTPS same-origin with the seller, instead of a WebBundle to optimize performance.
Explainers: https://github.com/WICG/turtledove/pull/695