Definition
ACME is the IETF-standard protocol (RFC 8555) for automating issuance and renewal of TLS certificates. Let’s Encrypt operates the largest public ACME deployment.
Key Points
- ARI: ACME Renewal Information lets CAs signal optimal renewal windows—critical as lets-encrypt shortens tls-certificates lifetimes (64 days from Feb 2027; 45 days from Feb 2028) (2026-10-07-lets-encrypt-64-day-certs-official)
- Rate limits: Renewals of existing names are exempt from new-order limits; shorter lifetimes increase renewal frequency without changing per-domain issuance caps (2026-10-07-lets-encrypt-64-day-certs-official)
- Operations: Replace hardcoded cron offsets (83/80/60 days) with lifetime-relative scheduling