What will it mean when Windows operating systems reject encryption keys smaller than 1024 bit soon?

Microsoft will soon release an update to its current operating systems (Windows XP and higher; Windows Server 2003 and higher), which will block the use of cryptographic keys that are less than 1024 bits in length. This announcement was made quite a while ago, but most links go to a rather specialized place, the “Windows PKI blog”. And honestly, who besides some geeks are really reading such a blog?

The consequence is that certificates with key lengths of 512 bits will be blocked, leading to error messages. These errors can occur when browsing the web, when trying to enroll certificates, when creating or consuming S/MIME secure eMail, when installing ActiveX controls, or when installing applications. Most things will work smoothly, but some legacy components and applications might fail.

That might cause some trouble in organizations once the update – which clearly makes sense from a security perspective – is deployed. Unfortunately, it is not that easy to handle this issue. Microsoft’s approach described in the blog post mentioned above is not what I’d call straightforward. There is a lot of valuable information on how to deal with that issue, but it requires a lot of administrative work.

However, some vendors like Entrust and Venafi offer solutions to discover certificates used across your network. Both are tools that provide you a sort of “Enterprise Certificate Management” as part of the Enterprise Key Management initiatives you should have running anyway. If you haven’t started with such an initiative, it is long past the time to do so – EKM/ECM makes a lot of sense for discovering, managing, and protecting all your certificates and keys across the enterprise. More at the lower end of the set of available tools you find the Qualys SSL Labs SSL Server Test, which allows you to run an in-depth analysis of SSL keys used by publicly available websites. That at least might provide some information for troubleshooting.

The reason behind this all is simple: Certificates with a key length of 512 bit have been successfully cracked. This is related to the Flame malware, the reason why Microsoft finally decided to block the 512 bit keys. Some information about the relationship between Flame and the Microsoft security update are found in the Microsoft blog post mentioned above.

A question that could be raised is whether 1024 bit key lengths then will be sufficient or whether we will face the next update soon. An important fact is that encryption strength is exponential to the key length, due to the algorithms used. So it is not about just doubling computing power. However, there is some likelihood that we will see larger algorithms being cracked over time. That requires a lot of knowledge and computing power because there is no simple algorithm known yet. There might be one (which would make virtually all of today’s security useless) but most security experts doubt that. So we will have to wait and see. In the meantime, you should try to get a better grip on all the keys and certificates used in your organization – that at least will allow you to react quicker and with less work on the Microsoft update and future changes in that area.


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