Change CA Name Constraints

TL;DR

Yes, you can adjust X509v3 Name Constraints on your internal Certificate Authority (CA) after issuing certificates. However, it’s not a simple ‘edit and save’. You need to revoke existing certificates, update the CA configuration, and then re-issue those certificates with the new constraints.

Understanding Name Constraints

Name Constraints restrict which names a certificate can be issued for. They’re a security feature to prevent rogue certificate issuance within your organisation. Changing them affects all future certificates but doesn’t automatically change existing ones.

Step-by-step Guide

1. Back Up Your CA: Before making any changes, create a full backup of your CA database and configuration files. This is critical for recovery if something goes wrong.
2. Identify Affected Certificates: Determine which certificates were issued *before* the intended change to Name Constraints. You’ll need to revoke these.
   • Use your CA’s management tools (e.g., OpenSSL, Microsoft Certificate Services) to query for certificates issued within a specific date range or matching certain criteria.
3. Revoke Existing Certificates: Revoke the identified pre-change certificates.
   • OpenSSL Example (CRL): If using OpenSSL, you’ll likely be updating a Certificate Revocation List (CRL).

     openssl ca -revoke cert.pem

   • Microsoft CA: Use the Certification Authority console to revoke certificates. Ensure the CRL publication is working correctly.
4. Update CA Configuration: Modify your CA configuration file (e.g., OpenSSL’s openssl.cnf) to reflect the new Name Constraints.
   • Locate the section defining the X509v3 extensions for certificate issuance.
   • Edit the subjectAltName or nameConstraints extension as needed. Here’s an example of a modified nameConstraints section:

     [ req ]
     ... 
     [ v3_ca ]
     subjectAltName = @alt_names
     nameConstraints = critical, dir:./crl_distrib, policy:1.2.3.4.5.6.7.8
     ...

5. Restart CA Service: Restart your CA service to apply the new configuration.
   • The method for restarting varies depending on your operating system and CA software (e.g., systemctl restart apache2, Windows Services Manager).
6. Re-issue Certificates: Re-issue the revoked certificates with the updated Name Constraints.
   • This typically involves creating new Certificate Signing Requests (CSRs) from the original requesters and signing them with the updated CA.
   • Ensure the CSRs are valid before submitting them to the CA.
7. Verify New Certificates: After re-issuance, verify that the new certificates have the correct Name Constraints.
   • Use OpenSSL or a similar tool:

     openssl x509 -in cert.pem -text -noout

   • Check the X509v3 Subject Alternative Name and X509v3 Authority Information Access sections.
8. Monitor CRL: Continuously monitor your CRL to ensure revoked certificates are being distributed correctly.

Important Considerations

• Downtime: Revoking and re-issuing certificates can cause downtime for affected services. Plan accordingly.
• Automation: For large environments, automate the revocation and re-issuance process using scripting or CA management tools.
• Testing: Thoroughly test the changes in a non-production environment before applying them to production.