In light of the recent spate of certificate authority controversies, the next entry in my series of Hiawatha tutorials will focus on one particular countermeasure: HTTP Public Key Pinning, or HPKP for short. In a nutshell, HPKP helps to avoid the scenario where an antagonist issues an SSL certificate for your domain which is signed by a rogue CA. Normally the client browser will implicitly trust any cert which is signed by a valid certificate authority, even if that CA happens to be, say the Hong Kong Post Office or the China Certification Authority, for example.
To avoid this, HPKP tells a capable client via an HTTP header that, for your particular domain, it should trust only certs signed by a particular set of keys. After the first time they’ve visited your site, even if an adversary issues a certificate for your domain which isn’t signed by one of the keys in your HPKP list, the browser will behave as if the certificate is untrusted and issue the appropriate warning to the user. While imperfect, this method can at least present a fairly effective hurdle to would-be attackers looking to harass your users.