This is a lot to cover in one question and I think I front-loaded too much time on the CA hierarchy and didn't leave enough room for revocation.
Start by clarifying requirements and constraints (e.g., scale, security level, compliance). Then walk through the PKI design layer by layer: CA hierarchy, certificate lifecycle, revocation, key storage, validation, and high availability. Emphasize trade-offs and justify your choices based on the scenario.
Pro tip: Highlight the importance of offline root CA and hardware security modules (HSMs) for key protection, and discuss how to handle revocation at scale with OCSP stapling and CRL distribution points. Also, mention automation for certificate rotation to avoid outages.
Ask about scale (number of certificates, requests per second), security requirements (key sizes, algorithms), compliance (e.g., WebTrust), and existing infrastructure. This shapes the design.
Propose a root CA (offline, air-gapped) and intermediate CAs for different purposes (e.g., TLS, client auth). Consider cross-signing or multiple roots for redundancy.
Describe the enrollment process (e.g., ACME, SCEP), signing with intermediate CAs, and automated rotation. Include key generation and storage (HSM, TPM, secure enclaves).
Explain revocation mechanisms (CRL, OCSP, OCSP stapling) and client-side validation (chain building, revocation checking, trust store management). Discuss scalability of revocation.
Detail how to make CAs highly available (load balancing, clustering, geo-distribution) and scale issuance (caching, sharding, rate limiting). Address disaster recovery.
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