Amey-Thakur TLS and certificates
A certificate proves a server is who it claims to be, and the chain proves the certificate came from someone the client already trusts. Nearly every TLS incident is one of three things: an expired certificate, an incomplete chain, or a name that does not match.
Method
- Match the certificate to every name it serves. The name the client typed must appear in the subject alternative names, including both apex and www when both are used. A wildcard covers one label only, so it does not cover a nested subdomain.
- Serve the full chain, not just the leaf. Servers must send the leaf plus any intermediates. Browsers often paper over a missing intermediate from cache while API clients and mobile apps fail outright, so test with a client that has no cache.
- Automate renewal and alert well before expiry. Short-lived certificates are the norm, so renewal must be automatic and monitored, with an alert weeks ahead rather than hours. Expiry is the most common TLS outage and the most preventable.
- Keep the private key private and rotate on exposure. Keys never leave the host or key store, never enter version control, and are replaced rather than merely re-issued if they might have leaked (see secrets-management). A certificate for a leaked key is worth nothing.
- Read handshake failures literally. Unknown authority means a chain or trust store problem, name mismatch means the wrong certificate, and a protocol alert means the two ends share no acceptable settings. Fix what the error names instead of lowering security until it connects.
- Prefer modern defaults and disable what is broken. Current protocol versions and cipher suites only, HSTS once you are confident, and stapled revocation to speed validation (see api-security). Old protocol support is a liability, not compatibility.
Boundaries
- TLS secures data in transit between two endpoints; it says nothing about storage, authorization, or what the server does with the data afterwards (see data-encryption, authn-design).
- A valid certificate proves control of a name, not that the operator is honest. Trust in the party is a separate question.
- Terminating TLS at a proxy leaves the hop beyond it in the clear unless you re-encrypt; decide deliberately whether that segment is trusted.