certificates

Checking the signature is one step of seven. The bugs that matter live in the other six, and in code that switches validation off.

Issuing a certificate is easy. Knowing every certificate you issued, renewing them before they expire, and surviving the day a CA key leaks is the design.

A certificate is a signed statement with a short list of claims: this key, these names, these uses, this period, issued by this CA. Every validation failure you have seen is one of those claims not holding.

There is no single list of trusted roots. Browsers, operating systems, language runtimes and container images each carry their own, updated on their own schedules, and production outages follow from the differences.

Domain-validated certificates prove one thing: whoever asked could answer a challenge on that domain at that moment. Every attack on issuance is a way to answer that challenge without owning the domain.

OpenSSL is two libraries and a command-line tool. Most TLS failures blamed on it are applications that never asked it to check the hostname, or told it to stop checking at all.

A certificate is a CA's signature over a structured statement about your key. Each step from key to padlock exists to stop one specific lie.

The root key is the one thing you can never easily replace, so it signs almost nothing. Everything about intermediates follows from that.

Signed software outlives the certificate that signed it. The scored answer explains how a timestamp proves the signature was made while the certificate was valid, and how revocation dates interact with it.