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A practical knowledge base for software architects and developers working with X.509 certificates, TLS, and public key infrastructure.

Fundamentals

The standard behind every TLS certificate — its history, structure, and where it is used.

Every field and extension in an X.509v3 certificate, from Serial Number to Authority Key Identifier.

PEM, DER, PKCS#7, PKCS#12 — how to identify, use, and convert between certificate formats.

Root CAs, intermediates, leaf certificates, and how browsers build and verify trust.

When to use each, trust implications, browser warnings, and the role of Let's Encrypt.

Practical fixes for ERR_CERT_AUTHORITY_INVALID, hostname mismatch, expired certs, and more.

Mutual TLS authentication — how it works, where it is used, and how to set it up.

Certificate Authorities, lifecycle management, OCSP, CRL, Certificate Transparency, and ACME.