Certificates can need to be revoked. There are few good solutions, but with Heartbleed a lot of certs need to be revoked.
Certificates are issued by a “Certificate Authority” (CA), which is simply a certificate configured by clients to be trusted to sign other certificates. You probably haven’t made this choice for yourself, someone else did it for you: a browser vendor, an operating system vendor, or perhaps a local administrator. There are over 200 CAs which most clients trust, including government agencies of many and varied nations. That’s how bad the house of cards is, to start with.
But with Heartbleed, it looks as though (non-nation-state) criminals have been gathering data, so those cards are on fire. Revocation is how we put out the fire, for now, and hope that there are few smouldering embers waiting to burst back into flame.
The situation is not pretty.
The oldest mechanism is a “Certificate Revocation List”, or CRL, which is a file signed by the CA’s key listing the serial numbers of all revoked certs. It can get bulky, but it’s something which can be cached and retrieved in bulk periodically. It’s probably about to get too bulky, but we’ll find out over the coming weeks what happens.
Next is the “Online Certificate Status Protocol”, OCSP, which is a way to ask a CA to give a signed statement that a particular existing certificate is still valid. It has a lifetime, perhaps measured in hours or days, instead of the years of the original certificate.
If browsers use OCSP to talk to the CAs, then CAs (or people who sniff their traffic) can track all of the people visiting sites which use certificates issued by that CA. This is a major privacy violation and creates a new tempting target. Also, if the CA appears to be down then the only safe action is to not connect to the site, so the CAs become single points of failure. Who here thinks that a typical Certificate Authority is as good at running a reliable site as is Google?
There’s an alternative, called “OCSP Stapling”, in which the web Server requests the OCSP proof and can give it to clients in the TLS handshake. The server operator already has a business relationship with the CA, has exchanged money (or whatever) and the CA already knows the servers must exist, because they issued the certs. There’s no privacy violation. The server can start trying to renew before the old OCSP proof expired, so that if there’s a server glitch, we can ride it out – as long as the CA OCSP server can respond at some point after the web-server starts trying to renew. (If you know DHCP, think of the T1 timer.) If you have lots of servers, you can arrange to have just one fetch the proof and distribute it to the others.
There is very little client support for OCSP Stapling. Firefox appears to support it now. If you know of other clients, please do let us know!
There is server support of OCSP Stapling in current versions of nginx, Exim (experimental feature, can be enabled at build time), Apache and a few others.
The sensible choices are for clients to prioritize OCSP Stapling support, and to support CRLs. Supporting clients making OCSP queries to CAs is a major violation of trust.
So in summary: the tools people are using today are mostly not adequate to safely recover from Heartbleed and many people will be making large Leaps of Faith to recover.