Last December, a group of security researchers published a practical attack on CAs that issue certificates signed with MD5-based signatures (http://www.win.tue.nl/hashclash/rogue-ca/). As a result, some browser developers are planning to drop support of MD5 certificates at some point. The exact time frame depends in part on how many HTTPS websites are still using MD5 certificates. Johnathan Nightingale of Mozilla crawled the top 1 million HTTPS sites and published his findings (http://blog.johnath.com/2009/01/21/ssl-information-wants-to-be-free/). With Google Chrome's usage statistics collection service, we can help answer this usage question by measuring how often Google Chrome users are encountering MD5 certificates. We'd like to share these measurements with other browser developers.
The following statistics were gathered on our Dev channel releases, which are weekly releases based on the tip of the Chromium source tree. Also note that only Google Chrome users who opted in to usage statistics collection send their numbers to us.
The counts shown below reflect periods of time on the order of a week, but didn't seem to vary significantly across over a month of investigating. For each kind of certificate, we collected two counts. One count reflects the number of SSL connections made to sites using that kind of certificate, while the other count reflects the number of Google Chrome sessions involving that kind of certificate. The latter can provide an estimator of the percentage of users that would be impacted by dropping MD5 certificate support. Note that we ignore the root CA certificates because the signatures in the root CA certificates are irrelevant to an attack of this sort.
1. The number of SSL connections with certain kinds of certificates. Note that an HTTP keep-alive connection is counted once even though multiple HTTP requests may be sent over it.
2. The number of Google Chrome sessions in which the user has seen certain kinds of certificates.
The percentage of MD5 certificates of the secure Web, weighted by site popularity and reported by users of Google Chrome's Dev channel releases who opted in to usage statistics collection, is 365152/16408141 = 2.23%. Looking at the per session statistics, we see that the number of user sessions that would be impacted by no longer supporting MD5 certificates would be 33134/542846 = 6.1%. Either of these percentages, though significantly smaller than reported by Nightingale, suggests that there would be a large user impact if MD5 certificate support were suddenly removed.
A second alternative being considered is to remove support for MD5 intermediate CA certificates. Although this would not completely preclude an attack, it would significantly raise the cost of attacks. The percentage of MD5 intermediate CA certificates is notably lower, at 480/16408141 < 0.003%. Looking at a per session statistics, we can see that only 58/542846 = .011% would be impacted by such a change. As a result, it should be possible for browsers to drop support of MD5 intermediate CA certificates sooner, which will make the MD5 certificate attack much more expensive.
In the future we hope to use the statistics collection service to gather other information of interest to crypto practitioners, such as which SSL cipher suites are being used in practice and how many HTTPS sites use certificates with certification path validation errors.