PKI (Public Key Infrastructure) is the system of CAs, certificates, policies, and procedures enabling trust in public keys. Provides: authentication, encryption key distribution, digital signatures.
PKI underpins HTTPS, S/MIME, code signing, smart card auth, and VPN auth. Enterprise PKI: internal root CA (offline) → intermediate CA (online) → end-entity certificates. Certificate management is operationally complex — automate with ACME/SCEP where possible.