Why not Convergence? (07 Sep 2011)
In light of recent events, I've had several requests to implement Convergence in Chrome. For those who don't know and, frankly, for anyone interested in SSL, I highly recommend watching Moxie's talk on the subject from this year's Black Hat. You can also check out the project website.
Moxie, having actually thought about the issue and coded something up, has already done a thousand times more to address the problem than almost anyone else. But I don't think that Convergence is something we would add in Chrome:
Although the idea of trust agility is great, 99.99% of Chrome users would never change the default settings. (The percentage is not an exaggeration.) Indeed, I don't believe that an option for setting custom notaries would even meet the standards for inclusion in the preferences UI.
Given that essentially the whole population of Chrome users would use the default notary settings, those notaries will get a large amount of traffic. Also, we have a very strong interest for the notaries to function, otherwise Chrome stops working. Combined, that means that Google would end up running the notaries. So the design boils down to Chrome phoning home for certificate validation. That has both unacceptable privacy implications and very high uptime requirements on the notary service.
It also doesn't address the two problems that Moxie highlights: internal servers and captive portals. It's not clear how either would work in this design, at least without giving up on security and asking the user. (These two problems, captive portals esp, are the bane of many an idea in this area.)
None of the above argues against allowing Convergence as an extension for those who wish to run it. We don't currently have an extension API for controlling certificate decisions and I'm not inherently opposed to one. It would be additional complexity and something that we would have to support in the future, so it's not without costs, but mostly it's not there because nobody has written it and I'm afraid that I don't have any plans to do so.