How to modify the RequireAsChecksum registry entry after you install security update 899587

This article contains information about how to modify the RequireAsChecksum registry entry. This registry entry helps provide additional protection after you install Microsoft security update 899587. This security update is documented in Security Bulletin MS05-042.

For more information, click the following article number to view the article in the Microsoft Knowledge Base:

Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall your operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.

Security update 899587 contains some security-related changes to functionality. Security Bulletin MS05-042 addresses the externally reported security vulnerabilities. However, in addition to the changes that are listed in each "Vulnerability Details" section of Security Bulletin MS05-042, security update 899587 includes another change in functionality. An optional, but recommended, registry entry (RequireAsChecksum) has been added to help provide additional protection for potential future PKINIT-related vulnerabilities. The RequireAsChecksum registry entry is located under the following registry subkeys:

• Microsoft Windows XP:
  HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\Kerberos\RequireAsChecksum
• Microsoft Windows 2000 and Microsoft Windows Server 2003:
  HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\Kerberos\Parameters\RequireAsChecksum

Possible values for the RequireAsChecksum registry entry are as follows:

• RequireAsChecksum = 1 or any other non-zero value
  When this setting is turned on, the client accepts only replies that are compliant with the latest PKINIT revision (PKINIT-27) from the domain controller for smart card logon.
• RequireAsChecksum = 0
  When this setting is turned off, the client accepts replies that are compliant with the new revision or with older revisions.

Note When the registry entry is not present, the computer acts as if the setting is disabled.

The smart card logon fails when all the following conditions are true:

• The logon attempt is initiated by the client.
• Security update 899587 is installed on the client.
• The value of the RequireAsChecksum registry entry is set to 1 on the client.
• The domain controller that replies to the authentication request does not have security update 899587 installed.

We recommend that you turn on the registry setting on client computers only after security update 899587 has been deployed to all domain controllers in the domain.

Note You must restart a Windows 2000-based computer after you modify this registry entry. However, a restart is not required for computers that are running Windows XP or Windows Server 2003.