Private prefetch proxyAt a high level, Chrome gives the name of a website to the CONNECT proxy which then creates a secure communication channel between Chrome and that website. By design, the proxy was operated for and by Google, and was restricted to navigations originating from Google owned surfaces, thereby ensuring that the proxy could only see information that’s already accessible such as the names of the websites prefetched from surfaces that we authored. The end-to-end encrypted communication channel provided by the CONNECT proxy means that the proxy is only able to relay further requests without being able to see the content of the communication. Furthermore, given that the content of the communication is encrypted end-to-end between Chrome and the destination site, it also means that intermediaries can’t see the prefetched domains, nor the content of the prefetched resources. Likewise, since the proxy is relaying the prefetching requests, the destination website only sees the IP address of the proxy, not the user’s IP address. Finally, prefetching was restricted to websites that had no cookies or other local state, thereby preventing the destination site from identifying a user via information previously stored on their device.
“Prefetching websites via a CONNECT proxy prevents leaking user information”
Rebuilding pre-rendering on a solid foundationAs mentioned in the introduction, our goal is to bring instant experiences to the whole web through prefetching and pre-rendering techniques that are privacy and developer friendly. This proof of concept experiment and the follow-up discussions with the community represent small, but nevertheless important, steps toward that goal. Indeed, any pre-rendering feature is inherently built on top of a prefetching capability, which must abide by the same privacy principles. There are a few more challenges that we’ll need to solve to bring back an improved, privacy and developer friendly, pre-rendering feature. To be clear, we see this successful proof of concept experiment as a positive signal for iterating on a viable web platform solution, so that more websites can both benefit from, and take advantage of the approach outlined in this blog post. We are hopeful that what we’ve learned from past efforts, as well as new learnings we’ve gained thanks to this proof-of-concept experiment, will help foster a fruitful collaborative effort with the rest of the community.