Saturday, August 24, 2013

EffectiveDate (thisupdate), NextUpdate and NextCRLPublish


The validity time of a certificate revocation list (CRL) is critical for every public key infrastructure. By default, most applications verify the validity of certificates against a CRL.

Two CRL types exist: base CRLs and delta CRLs. In case where no delta CRL is used, certificates are treated as invalid if the base CRL is not available or expired. If a delta CRL is in use, the delta and base CRL must be available and valid to succeed with certificate verification.

The information provided in this article applies for both, the base CRL and the delta CRL generation.

When you look at a CRL, there is information about the Next update, the Next CRL Publish and the Effective date of the CRL. The term Effective date is used in the Windows certificate dialog while certutil.exe and the RFC name this fieldthisupdate.
  • Effective Date (aka thisupdate) - The date that a CRL became effective. The effective time, by default, is set to 10 minutes prior to the current date and time to allow for clock synchronization issues.
  • Next CLR Publish - This non-critical CRL extension indicates the date and time when a Windows CA will publish a new CRL. When a Windows computer uses a CRL for certificate verification it also examines the Next CRL Publish extension. If the Next CRL Publish date is already in the past, it connects to the CRL distribution points (referenced in the certificate) and attempts a download of a newer CRL. Note: If CRL is locally cached, and under certain conditions, download of new CRL might be skipped, even if Next CRL Publish date is already in the past. For more information, please see time after the Next CRL Publish and before the Next Update is a buffer time to allow Windows computers retrieval of a CRL before the CRL has actually expired.
  • Next Update - The date and time that a Windows client considers as the expiration date of the CRL. From an operational viewpoint, this is the most critical information. If this date passes, Windows computers will invalidate certificates that are checked against this CRL.
For more information about relation between the above three fields, see

Now we know the CRL attributes that control the CRL validity. The question is how these dates are calculated by a Windows CA. Read on to find out!

Under the Certification Services configuration hive in the registry two values control the overlap period for the base CRL and two registry values define the overlap period for delta CRL creation:

You can verify the settings for the above registry keys on your CA computer with the following commands:
certutil -getreg CA\CRLOv*
certutil -getreg CA\CRLDeltaOv*
If the registry values are set and valid, the overlap period for a base or delta CRL is initially calculated by the CA as:
OverlapPeriod = CRLOverlapUnits * CRLPeriod

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