I don't understand this. Can someone weight in with an explanation? Convergence works just fine without TACK: I can set up two or more notaries on some VPS somewhere, and my browser would check if the notaries see the same certificate on that server I am trying to connect to as my browser. Seems secure to me: no external CA involved, the certificate on the web server can be self signed, and a MITM attack would need to hack two or more external servers to be successful. How does TACK fit in all of this?