Another early stage idea to share: cross-browser anonymous conversion reporting

Hello again everyone,

On the call this coming Thursday, I'd love to get feedback on an early stage idea that I've written up and put into the repo.

The idea is to try to find a privacy-preserving way to support "cross-browser conversions". Example would be I see an ad on a mobile website and click on it, but I decide to complete the purchase from my Desktop computer.

As with previous documents I've shared, this is definitely not a perfect proposal. It's super early stage and I'm just hoping to start a discussion and get feedback.

Here's the high level overview:
https://github.com/w3c/web-advertising/blob/master/cross-browser-anonymous-conversion-reporting.md

This links to 2 other documents. One details in great depth how two browsers belonging to the same person might discover one another:
https://github.com/w3c/web-advertising/blob/master/enabling-browsers-that-belong-to-the-same-person-to-discover-one-another.md

Another contemplates how a malicious attacker might attempt to attack this protocol in an effort to compromise someone's privacy, and how we might make that hard to do:
https://github.com/w3c/web-advertising/blob/master/threat-modeling-for-cross-browser-anonymous-conversion-reporting.md

Happy to answer any questions people have on the Thursday call.

Looking forward to your feedback!

--Ben