Yesterday I blogged about the Security Advisory – Update For Minimum Certificate Key Length. I would like to take the opportunity to give some more information on it.

The reaction on the advisory is interesting so far. Some customers expect mainly older applications to run into a problem. Others tell us that they mandated 2k keys since a long time and are therefore safe. Well, I tend to agree and disagree (cool statement J). I guess the huge majority of the keys are not less than 1024bits. This is to be expected and I would be surprised of public CAs would have issued certs recently which cause challenges – but sometimes I get surprised in the oddest ways.

My real worry are systems that connect through and authenticated or encrypted channel to older, mostly embedded systems. Are the keys on these systems long enough?

There is a way to figure this out. If you look into the KB article which comes with the advisory (KB2661254), there is a section called Resolutions and in there it is explained how the updated can be put on a "logging only" mode:

Allow key lengths of less than 1024 bits by using registry settings

Microsoft does not recommend customers use certificates less than 1024 bits long. Customers may however need a temporary workaround while a longer term solution is developed to replace RSA certificates with a key length of less than 1024 bits length. In these cases, Microsoft is providing the customers the ability to change the way the update functions. Customers configuring these settings are accepting the risk that an attacker may be able to break their certificates and use them to spoof content, perform phishing attacks, or perform Man-in-the-Middle attacks.

So, my recommendation today definitely is to deploy the update in the logging-only mode and figure out, whether you have a problem and how big it is – October is coming soon.