Configuring Certificate Authority with OpenSSL
Overview
A Certificate Authority (CA) is the trust anchor in a PKI hierarchy, responsible for issuing, signing, and revoking digital certificates. This skill covers building a two-tier CA hierarchy (Root CA + Intermediate CA) using OpenSSL and the Python cryptography library, including CRL distribution, OCSP responder configuration, and certificate policy management.
When to Use
- When deploying or configuring configuring certificate authority with openssl capabilities in your environment
- When establishing security controls aligned to compliance requirements
- When building or improving security architecture for this domain
- When conducting security assessments that require this implementation
Prerequisites
- Familiarity with cryptography concepts and tools
- Access to a test or lab environment for safe execution
- Python 3.8+ with required dependencies installed
- Appropriate authorization for any testing activities
Objectives
- Create a Root CA with self-signed certificate
- Create an Intermediate CA signed by the Root CA
- Issue server and client certificates from the Intermediate CA
- Configure Certificate Revocation Lists (CRLs)
- Implement certificate policies and constraints
- Build a complete PKI hierarchy programmatically
Key Concepts
CA Hierarchy
Root CA (offline, air-gapped)
|
+-- Intermediate CA (online, operational)
|
+-- Server Certificates
+-- Client Certificates
+-- Code Signing Certificates
Certificate Extensions
| Extension | Purpose | Critical |
|---|---|---|
| basicConstraints | CA:TRUE/FALSE, pathLenConstraint | Yes |
| keyUsage | keyCertSign, cRLSign, digitalSignature | Yes |
| extendedKeyUsage | serverAuth, clientAuth, codeSigning | No |
| subjectKeyIdentifier | Hash of public key | No |
| authorityKeyIdentifier | Issuer's key identifier | No |
| crlDistributionPoints | URL to CRL | No |
| authorityInfoAccess | OCSP responder URL | No |
Security Considerations
- Root CA private key must be stored offline (air-gapped HSM)
- Use minimum 4096-bit RSA or P-384 ECDSA for CA keys
- Set path length constraints on intermediate CAs
- Implement certificate policies (OIDs)
- Enable CRL and OCSP for revocation checking
- Audit all certificate issuance operations
Validation Criteria
- Root CA self-signed certificate is valid
- Intermediate CA certificate chains to Root CA
- Issued certificates chain to Intermediate -> Root
- Path length constraints are enforced
- CRL is generated and accessible
- Revoked certificates appear in CRL
- Certificate policies are correctly embedded