2.4.1. Public Key Infrastructure (PKI)

💡 First Principle: PKI is the trust framework that makes asymmetric cryptography work at scale. It solves a critical problem: if someone gives you a public key, how do you know it's really theirs? PKI introduces a trusted third party — the Certificate Authority — that vouches for the binding between a public key and an identity.

Public key — shared openly, used to encrypt data sent to the key owner or verify their digital signatures. Think of it as a mailbox slot — anyone can put mail in, but only the owner retrieves it.

Private key — kept secret, used to decrypt data encrypted with the corresponding public key or create digital signatures. The entire system's security depends on keeping private keys private.

Certificate Authority (CA) hierarchy — trust flows from the Root CA (offline, heavily protected) through Intermediate CAs (online, issue day-to-day certificates). If a Root CA is compromised, the entire PKI collapses — that's why root CAs are kept offline in secured facilities. Intermediate CAs limit blast radius: revoking one doesn't invalidate the entire chain.

Certificate types serve different purposes: Domain Validation (DV) proves domain ownership only, Organization Validation (OV) verifies the organization exists, and Extended Validation (EV) requires the most rigorous identity verification. Wildcard certificates (*.example.com) cover all subdomains. Subject Alternative Name (SAN) certificates cover multiple specific domains in one certificate.

Key escrow — a copy of the encryption key held by a trusted third party, allowing authorized recovery if the key holder is unavailable. Controversial because it creates a potential single point of compromise.

⚠️ Exam Trap: Public key encrypts; private key decrypts. For digital signatures, it's reversed: private key signs, public key verifies. Many candidates mix these up. Encryption protects data going TO someone (their public key). Signatures prove data came FROM someone (their private key).