Offline Root Certificate Authority - Windows Server

Every organization that has an Active Directory structure, or any other service that uses SSL/TLS services (ie. HTTPS, RDP, etc.), should have a certificate authority. This certificate authority is used to verify that you are indeed talking directly with the server you think you are and that the connection is secure. Having a certificate authority is also required to enable secure LDAP (LDAPS), though this can also be done with an external certificate authority.

Choosing to host your offline root CA on a Windows-based system, rather than Linux, will feel more comfortable to those who are not familiar to Linux. However, since this server will be shutdown 99% of the time and should never be on the network once configured, it may seem like a waste of a server license.

Why have an offline Root CA?

If a root CA is in some way compromised (broken into, hacked, stolen, or accessed by an unauthorized or malicious person), then all of the certificates that were issued by that CA are also compromised. Since certificates are used for data protection, identification, and authorization, the compromise of a CA could compromise the security of an entire organizational network. For that reason, many organizations that run internal PKIs install their root CA offline. That is, the CA is never connected to the company network, which makes the root CA an offline root CA. Make sure that you keep all CAs in secure areas with limited access.¹

– Microsoft TechNet

Requirements

• Latest Windows Server available installed on a single-purpose physical server
  • You will need to be able to transfer files to and from this device via USB drive.
  • We’ll be using Windows Server 2022 for this example.
  • This system will be used as an offline root CA, therefore it will be powered off after we’re done.
• USB drive formatted with ExFAT or FAT32 file system

Ensure the Server is Updated

1. Update the server with Windows Update

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At this point, there is no need for the server to be connected to the network. You should remove the network cable from the ethernet port. If this is a device with a wireless card, ensure it is not connected to the wireless or even remove the WLAN card.

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Create CAPolicy.inf File

1. Open Powershell as Administrator
2. Create a new file with notepad.exe called CAPolicy.inf in C:\Windows

   cd C:\Windows
   notepad.exe CAPolicy.inf
   

3. Answer Yes to create the new file
4. Copy this text into the file

   [Version]
   Signature="$Windows NT$"
   [Certsrv_Server]
   RenewalKeyLength=4096
   RenewalValidityPeriod=Years
   RenewalValidityPeriodUnits=20
   AlternateSignatureAlgorithm=1
   

5. Save and close the CAPolicy.inf file

Install the Active Directory Certificate Services Role

1. Open Server Manager, if not already open
2. Click Manage in the top right corner
3. Click Add Roles and Features
4. On Before You Begin, click Next
5. On Installation Type, ensure Role-based or feature-based installation is selected and click Next
6. On Server Selection, ensure this server is selected and click Next
7. On Server Roles, check Active Directory Certificate Services, on the pop-up click Add Features, then click Next
8. On Features, click Next
9. On AD CS, click Next
10. On Role Services, ensure Cerificate Authority is checked and click Next
11. On Confirmation, click Install

Configure the Certificate Authority Service

1. If the Installation Wizard is still open, click Configure Active Directory Certificate Services on the destination server. Otherwise, in Server Manager click the flag icon in the upper right which should have a warning symbol (⚠️), then Post-deployment Configuration
2. On Credentials, click Next
3. On Role Services, check Certification Authority, then click Next
4. On Setup Type, click Next (Standalone CA is the only available option)
5. On CA Type, select Root CA and click Next
6. On Private Key, ensure Create a new private key is selected, then click Next
7. On Cryptography, set the Key length to 4096
8. On CA Name, set the Common name to a descriptive value, such as “Contoso Root CA”, then click Next
9. On Validity Period, set the validity period to 20 Years.
10. On Certificate Database, click Next
11. On Confirmation, click Configure

Perform Post Installation Configuration

1. Open Powershell ad Administrator
2. Define the Active Directory Configuration Partition Distinguished Name (Substitute your DC values)

   certutil -setreg CA\DSConfigDN "CN=Configuration,DC=ad,DC=example,DC=edu"
   

3. Define CRL Period Units and CRL Period

   certutil -setreg CA\CRLPeriodUnits 52
   certutil -setreg CA\CRLPeriod "Weeks"
   certutil -setreg CA\CRLDeltaPeriodUnits 0
   

4. Define CRL Overlap Period Units and CRL Overlap Period

   certutil -setreg CA\CRLOverlapPeriodUnits 12
   certutil -setreg CA\CRLOverlapPeriod "Hours"
   

5. Define Validity Period Units for all issued certificates by this CA

   certutil -setreg CA\ValidityPeriodUnits 10
   certutil -setreg CA\ValidityPeriod "Years"
   

Restart the Certificate Authority Service and Publish the CRL

1. Restart the CA service

   Restart-Service certsvc
   

2. Publish the Certificate Revocation List (CRL)

   certutil -crl
   

Copy the Root CA Certificate and CRL to USB Drive

You should now copy *.crl and *.crt to a USB drive. The two files can be found at C:\Windows\system32\CertSrv\CertEnroll.

You will need the root CA certificate and CRL file when you configure the Windows Enterprise Subordinate CA

Shutdown and Store the Machine

You may now shutdown the server and store it. The only times the server will be needed are when you stand up a new subordinate (intermediate) certificate authority, or when a sub-CA needs its certificate renewed. Sub-CA’s should renew their certificates every ten years or so.

Footnotes

1. https://social.technet.microsoft.com/wiki/contents/articles/2900.offline-root-certification-authority-ca.aspx ↩︎