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.

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

configuring-certificate-authority-with-openssl