- ITU Recommendation X.509
- Official web pages containing the standards published by the International Telecommunication Union. - Digital ID Introduction - Introductory level overview of digital certificates.
- Handbook of Applied Cryptography - Introduction to the practical aspects of conventional and public-key cryptography. (PDF and Postscript sample chapters)
- RFC 4158: Internet X.509 Public Key Infrastructure: Certification Path Building - Provides guidance and recommendations to developers building X.509 public-key certification paths within their applications.
- Wikipedia: X.509 - Encyclopedia article about the ITU-T standard for a public key infrastructure. Includes history, certificate details, and references.
- X.509 Style Guide - Document by Peter Gutmann intended as a set of implementation notes and guidelines for developers.
