CA Certificate Key Usage Extension

TL;DR

Yes, an Intermediate CA can extend its certificate key usage by issuing a new certificate for itself. However, it’s crucial to understand the implications and follow best practices to avoid breaking trust chains or introducing vulnerabilities.

Solution Guide

1. Understand Certificate Key Usage (Key Usage)
   • Key Usage defines what a certificate can be used for – digital signature, key encipherment, data encipherment, etc.
   • It’s specified in the certificate itself and is part of the X.509 standard.
   • Changing Key Usage requires re-issuing the certificate.
2. Why Extend Key Usage?

   You might need to extend key usage if you initially issued a certificate with limited purposes and now require additional functionality (e.g., adding server authentication).

3. The Process: Re-issuing the Intermediate CA Certificate
   • Generate a new Certificate Signing Request (CSR): Use your existing private key for the Intermediate CA. Do not generate a new key pair! This is critical to maintain trust.

   openssl req -new -key intermediate_ca.key -out intermediate_ca.csr

   • Modify the CSR (if needed): Ensure the Common Name (CN) and Subject Alternative Names (SANs) are correct.
   • Configure Key Usage in your CA Configuration: Update your OpenSSL or other CA tool’s configuration file to include the new desired key usages. For example, if you want to add server authentication:

     [ ca ]
     default_ca = CA_default
     
     [ CA_default]
     ...other settings...
     san = email:[email protected], DNS:intermediate.example.com
     keyUsage = keyCertSign, cRLSign, digitalSignature, keyEncipherment, serverAuth 

   • Issue the New Certificate: Use your Root CA to sign the new CSR for the Intermediate CA.

     openssl ca -config openssl.cnf -extensions req_ext -days 3650 -in intermediate_ca.csr -out intermediate_ca.crt

4. Important Considerations
   • Trust Chain: The new Intermediate CA certificate must be distributed to all relying parties (servers, clients, etc.) that trust the original Intermediate CA. This is often done via a Certificate Revocation List (CRL) or Online Certificate Status Protocol (OCSP).
   • Revoke the Old Certificate: Once the new certificate is widely deployed, revoke the old Intermediate CA certificate to prevent its misuse. This is essential for security.

     openssl ca -config openssl.cnf -revoke intermediate_ca.crt

   • CRL Distribution: Ensure your CRL is updated and accessible after revocation.
   • Testing: Thoroughly test the new certificate before deploying it widely. Verify that all required functionalities work as expected.
   • Private Key Security: Protect the Intermediate CA’s private key at all costs. Compromise of this key would allow an attacker to issue fraudulent certificates.
5. Using a Certificate Management Tool: Consider using a dedicated certificate management tool (e.g., Cert-Manager, xCA) to automate the process and reduce errors. These tools often handle trust chain updates and revocation automatically.