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