Re:Ask HN: Why aren't websites adopting client certs in lieu...
by geofft on Wednesday November 23, 2016 @11:42AM
(#13023778)
I don't understand the security argument you're making. Are you claiming that, if I use client certs, I am protected against a rogue CA issuing a fake certificate for web.whatsapp.com? How?
If you're thinking of a protocol like Kerberos, then yes, you can derive a shared secret because there's a single-point-of-trust authentication entity (the KDC) which has knowledge of both your password and the server's password/key, and yes, your password certifies that you're talking to the right server (as long as the KDC is trustworthy). But that's not how TLS mutual auth works.
A login through a QR code (basically a token) is just normal TLS with the same MiTM risk. Its just an application layer login.