Is it possible to cohabit with an old PKI hierarchy and a new PKI in a same Forest?

“Yes you can have multiple root CAs and even multiple PKIs in a single Active Directory forest. Because of the way the objects are representing those CAs are named and stored, you couldn’t possibly experience a conflict unless you tried to give more than one CA the same CA name.”

http://blogs.technet.com/b/askds/archive/2010/08/23/moving-your-organization-from-a-single-microsoft-ca-to-a-microsoft-recommended-pki.aspx

Tasks to do before to remove the old CA:

“The first thing you’ll want to do is prevent the old CA from issuing any new certificates. You just uninstall it, of course, but that could cause considerable problems. What do you think would happen if that CA’s published CRL expired and it wasn’t around to publish a new one? Depending on the application using those certificates, they’d all fail to validate and become useless. Wireless clients would fail to connect, smart card users would fail to authenticate, and all sorts of other bad things would occur. The goal is to prevent any career limiting outages so you shouldn’t just uninstall that CA.”

“No, you should instead remove all the templates from the Certificate Templates folder using the Certification Authority MMC snap-in on the old CA. If an Enterprise CA isn’t configured with any templates it can’t issue any new certificates. On the other hand, it is still quite capable of refreshing its CRL, and this is exactly the behavior you want. Conversely, you’ll want to add those same templates you removed from the Old And Busted CA into the Certificate Templates folder on the New Hotness Issuing CA.

If you modify the contents of the Certificate Templates folder for a particular CA, that CA’s pKIEnrollmentService object must be updated in Active Directory. That means that you will have some latency as the changes replicate amongst your domain controllers. It is possible that some user in an outlying site will attempt to enroll for a certificate against the Old And Busted CA and that request will fail because the Old And Busted CA knows immediately that it should not issue any certificates. Given time, though, that error condition will fade as all domain controllers get the new changes. If you’re extremely sensitive to that kind of failure, however, then just add your templates to the New Hotness Issuing CA first, wait a day (or whatever your end-to-end replication latency is) and then remove those templates from the Old And Busted CA. In the long run, it won’t matter if the Old And Busted CA issues a few last minute certificates.

At this point all certificate requests within your organization will be processed by the New Hotness Issuing CA, but what about all those certificates issued by the Old And Busted CA that are still in use? Do you have to manually go to each user and computer and request new certificates? Well…it depends on how the certificates were originally requested”.

Manually Requested

If a certificate has been manually requested then, yes, in all likelihood you’ll need to manually update those certificates. I’m referring here to those certificates requested using the Certificates MMC snap-in, or through the Web Enrollment Pages. Unfortunately, there’s no automatic management for certificates requested manually. In reality, though, refreshing these certificates probably means changing some application or service so it knows to use the new certificate. I refer here specifically to Server Authentication certificates in IIS, OCS, SCCM, etc. Not only do you need to change the certificate, but you also need to reconfigure the application so it will use the new certificate. Given this situation, it makes sense to make your necessary changes gradually. Presumably, there is already a procedure in place for updating the certificates used by these applications I mentioned, among others I didn’t, as the current certificates expire. As time passes and each of these older, expiring certificates are replaced by new certificates issued by the new CA, you will gradually wean your organization off of the Old And Busted CA and onto the New Hotness Issuing CA. Once that is complete you can safely decommission the old CA.

And it isn’t as though you don’t have a deadline. As soon as the Old And Busted CA certificate itself has expired you’ll know that any certificate ever issued by that CA has also expired. The Microsoft CA enforces such validity period nesting of certificates. Hopefully, though, that means that all those certificates have already been replaced, and you can finally decommission the old CA.

Automatically Enrolled

Certificate Autoenrollment was introduced in Windows XP, and it allows the administrator to assign certificates based on a particular template to any number of forest users or computers. Triggered by the application of Group Policy, this component can enroll for certificates and renew them when they get old. Using Autoenrollment, once can easily deploy thousands of certificates very, very quickly. Surely, then, there must be an automated way to replace all those certificates issued by the previous CA?

As a matter of fact, there is.

As described above, the new PKI is up and ready to start issuing digital certificates. The old CA is still up and running, but all the templates have been removed from the Certificate Templates folder so it is no longer issuing any certificates. But you still have literally thousands of automatically enrolled certificates outstanding that need to be replaced. What do you do?

In the Certificates Templates MMC snap-in, you’ll see a list of all the templates available in your enterprise. To force all holders of a particular certificate to automatically enroll for a replacement, all you need to do is right-click on the template and select Reenroll All Certificate Holders from the context menu.

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What this actually does is increment the major version number of the certificate template in question. This change is detected by the Autoenrollment component on each Windows workstation and server prompting them to enroll for the updated template, replacing any certificate they may already have. Automatically enrolled user certificates are updated in the exact same fashion.

Now, how long it takes for each certificate holder to actually finish enrolling will depend how many there are and how they connect to the network. For workstations that are connected directly to the network, user and computer certificates will be updated at the next Autoenrollment pulse.

Note: For computers, the autoenrollment pulse fires at computer startup and every eight hours thereafter. For users, the autoenrollment pulse fires at user logon and every eight hours thereafter. You can manually trigger an autoenrollment pulse by running certutil -pulse from the command line. Certutil.exe is installed with the Windows Server 2003 Administrative Tools Pack on Windows XP, but it is installed by default on the other currently supported versions of Windows.

For computers that only connect by VPN it may take longer for certificates to be updated. Unfortunately, there is no blinking light that says all the certificate holders have been reenrolled, so monitoring progress can be difficult. There are ways it could be done — monitoring the certificates issued by the CA, using a script to check workstations and servers and verify that the certificates are issued from the new CA, etc. — but they require some brain and brow work from the Administrator.

There is one requirement for this reenrollment strategy to work. In the group policy setting where you enable Autoenrollment, you must have the following option selected: Update certificates that use certificate templates.

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If this policy option is not enabled then your autoenrolled certificates will not be automatically refreshed.

Remember, there are two autoenrollment policies — one for the User Configuration and one for the Computer Configuration. This option must be selected in both locations in order to allow the Administrator to force both computers and users to reenroll for an updated template.

But I Have to Get Rid of the Old CA!

As I’ve said earlier, once you’ve configured the Old And Busted CA so that it will no longer issue certificates you shouldn’t need to touch it again until all the certificates issued by that CA have expired. As long as the CA continues to publish a revocation list, all the certificates issued by that CA will remain valid until they can be replaced. But what if you want to decommission the Old And Busted CA immediately? How could make sure that your outstanding certificates would remain viable until you can replace them with new certificates? Well, there is a way.

All X.509 digital certificates have a validity period, a defined interval time with fixed start and end dates between which the certificate is considered valid unless it has been revoked. Once the certificate is expired there is no need to check with a certificate revocation list (CRL) — the certificate is invalid regardless of its revocation status. Revocation lists also have a validity period during which time it is considered an authoritative list of revoked certificates. Once the CRL has expired it can no longer be used to check for revocation status; a client must retrieve a new CRL.

You can use this to your advantage by extending the validity period of the Old And Busted CA’s CRL in the CA configuration to match (or exceed) the remaining lifetime of the CA certificate. For example, if the Old And Busted CA’s certificate will be valid for the next 4 years, 3 months, and 10 days, then you can set the publication interval for the CA’s CRL to 5 years and immediately publish it. The newly published CRL will remain valid for the next five years, and as long as you leave that CRL published in the defined CRL distribution points — Active Directory and/or HTTP — clients will continue to use it for checking revocation status. You no longer need the actual CA itself so you can uninstall it.

One drawback to this, however, is that you won’t be able to easily add any certificates to the revocation list. If you need to revoke a certificate after you’ve decommissioned the CA, then you’ll need to use the command line utility certutil.exe.

Certutil.exe -resign “Old And Busted CA.crl” +<serialNumber>

Of course, this requires that you keep the private keys associated with the CA, so you’d better back up the CA’s keys before you uninstall the role.”