Cryptographic Improvements in Microsoft Windows

You might remember that in June 2013 we released Security Advisory 2854544 announcing additional options for enterprise customers to manage their digital certificate handling configuration on the Windows platform. The particular functionality announced in Security Advisory 2854544 was first built into Windows 8, Windows Server 2012, and Windows RT and then back-ported to other operating systems. At the time, we also announced our plan to release additional updates to this advisory – all aimed at bolstering Windows cryptography and certificate-handling infrastructure. These efforts are not in response to any specific incident, but rather just the continuing evolution of how he handle digital certificate to ensure the safest possible computing environment for our customers.

What’s New Today?

Today, we have released the next in that planned set of advisories. Security Advisory 2862973 announces the immediate availability of an update to restrict the use of the MD5 hashing algorithm in digital certificates that are a part of the Microsoft Root Program. We plan to release this update broadly through Windows Update on February 11, 2014 after customers have a chance to assess the impact of this update and take necessary actions in their enterprise. It is available today on the Microsoft Download Center.

This MD5 update is enabled by a new framework for management of cryptography, described briefly below and in more detail on Microsoft Technet. These updates are meant to enhance customer privacy and security. Strong cryptography improves the functionality of signing features which allow users to validate the source and trustworthiness of content. It also improves the functionality of the underlying cryptography algorithms, increasing the cost of attacker efforts to perform content spoofing, man-in-the-middle (MiTM), and phishing attacks.

We’ll look at the new cryptographic framework update first. It provides a number of features Administrators can use to monitor and deprecate weak cryptography. The features introduced focus on increasing the strength of asymmetric cryptography as used in the platform and deprecating hashing algorithms such as MD5.

Increasing the strength of asymmetric cryptography

Asymmetric cryptography is used to encrypt data / share secrets between two or more entities. Instead of using a single shared key to unlock a secret (symmetric-key cryptography), each entity has a public and private key to encrypt, decrypt and sign. In practice, the strength of asymmetric cryptography is dependent on the key size, algorithm used, and the trusted third party who validates the keys.

Recent updates address these properties by providing users with options to manage which cryptographic algorithms are used (RSA, DSA or ECDSA), options to set minimum key length, and options to set allowed hashing algorithms for code signing and other functions.

All updates and the current status of cryptographic improvements for cryptography are centrally tracked as a part of Microsoft Security Advisory 2854544 (Updates to Improve Cryptography and Digital Certificate Handling in Windows).

Timeline of Cryptographic improvements 2012-2013

Microsoft Security Advisory 2661254 This update set a mandatory key length for RSA keys of 1024 bits or stronger by restricting the use of certificates with RSA keys less than 1024 bits in length.

KB Article 2813430 This update establishes functionality that enables consumers to update trusted and disallowed Certificate Trust Lists (CTL) in non-internet connected environments. CTL’s are a list of approved hashes or certificates approved by a trusted third party. This update allows customers more control in which parties they trust.

KB Article 2862966 This update established a framework for managing asymmetric cryptography by adding features for monitoring and directly controlling which cryptographic algorithms are used (RSA, DSA or ECDSA), options to set minimum key length as well as to set which hashing algorithms will be permitted for code signing and other functions. All functionality is documented in detail on Microsoft TechNet.

Microsoft Security Advisory 2862973 This update released today as Downloadable Content (DLC) gives customers the option to restrict the use of certificates that utilize the MD5 hashing algorithm as part of their digital signature. We recommend that customers download and test the update in their environment at the earliest opportunity, this will be especially useful for environments that have little or no inventory of their cryptographic and certificate dependencies. This update will only affect MD5 as used in server authentication, code signing and time stamping. There are exceptions for certain certificates and timestamps, please see KB 2862973 for additional details. Microsoft is planning to release this update through Windows Update on February 11, 2014 after customers have a chance to assess the impact of this update and take necessary actions in their enterprise.

- William Peteroy, MSRC