Revocation checking and Chrome's CRL (05 Feb 2012)
When a browser connects to an HTTPS site it receives signed certificates which allow it to verify that it's really connecting to the domain that it should be connecting to. In those certificates are pointers to services, run by the Certificate Authorities (CAs) that issued the certificate, that allow the browser to get up-to-date information.
All the major desktop browsers will contact those services to inquire whether the certificate has been revoked. There are two protocols/formats involved: OCSP and CRL, although the differences aren't relevant here. I mention them only so that readers can recognise the terms in other discussions.
The problem with these checks, that we call online revocation checks, is that the browser can't be sure that it can reach the CA's servers. There are lots of cases where it's not possible: captive portals are one. A captive portal frequently requires you to sign in on an HTTPS site, but blocks traffic to all other sites, including the CA's OCSP servers.
If browsers were to insist on talking to the CA before accepting a certificate, all these cases would stop working. There's also the concern that the CA may experience downtime and it's bad engineering practice to build in single points of failure.
Therefore online revocation checks which result in a network error are effectively ignored (this is called “soft-fail”). I've previously documented the resulting behaviour of several browsers.
But an attacker who can intercept HTTPS connections can also make online revocation checks appear to fail and so bypass the revocation checks! In cases where the attacker can only intercept a subset of a victim's traffic (i.e. the SSL traffic but not the revocation checks), the attacker is likely to be a backbone provider capable of DNS or BGP poisoning to block the revocation checks too.
If the attacker is close to the server then online revocation checks can be effective, but an attacker close to the server can get certificates issued from many CAs and deploy different certificates as needed. In short, even revocation checks don't stop this from being a real mess.
So soft-fail revocation checks are like a seat-belt that snaps when you crash. Even though it works 99% of the time, it's worthless because it only works when you don't need it.
While the benefits of online revocation checking are hard to find, the costs are clear: online revocation checks are slow and compromise privacy. The median time for a successful OCSP check is ~300ms and the mean is nearly a second. This delays page loading and discourages sites from using HTTPS. They are also a privacy concern because the CA learns the IP address of users and which sites they're visiting.
On this basis, we're currently planning on disabling online revocation checks in a future version of Chrome. (There is a class of higher-security certificate, called an EV certificate, where we haven't made a decision about what to do yet.)
Pushing a revocation list
Our current method of revoking certificates in response to major incidents is to push a software update. Microsoft, Opera and Firefox also push software updates for serious incidents rather than rely on online revocation checks. But our software updates require that users restart their browser before they take effect, so we would like a lighter weight method of revoking certificates.
So Chrome will start to reuse its existing update mechanism to maintain a list of revoked certificates, as first proposed to the CA/Browser Forum by Chris Bailey and Kirk Hall of AffirmTrust last April. This list can take effect without having to restart the browser.
An attacker can still block updates, but they have to be able to maintain the block constantly, from the time of revocation, to prevent the update. This is much harder than blocking an online revocation check, where the attacker only has to block the checks during the attack.
Since we're pushing a list of revoked certificates anyway, we would like to invite CAs to contribute their revoked certificates (CRLs) to the list. We have to be mindful of size, but the vast majority of revocations happen for purely administrative reasons and can be excluded. So, if we can get the details of the more important revocations, we can improve user security. Our criteria for including revocations are:
- The CRL must be crawlable: we must be able to fetch it over HTTP and robots.txt must not exclude GoogleBot.
- The CRL must be valid by RFC 5280 and none of the serial numbers may be negative.
- CRLs that cover EV certificates are taken in preference, while still considering point (4).
- CRLs that include revocation reasons can be filtered to take less space and are preferred.
For the curious, there is a tool for fetching and parsing Chrome's list of revoked certificates at https://github.com/agl/crlset-tools.