X.509

What is X.509?

X.509 is the international standard that defines the format and structure of digital certificates used in SSL/TLS communications. Established by the International Telecommunication Union (ITU), X.509 serves as the universal template that ensures all SSL certificates contain the necessary information in a standardized format that browsers, servers, and other applications can understand and process.

X.509 Certificate Structure

X.509 certificates contain several mandatory and optional fields organized in a hierarchical structure:

• Version: Indicates which version of the X.509 standard is used
• Serial Number: Unique identifier assigned by the issuing Certificate Authority
• Signature Algorithm: Specifies the cryptographic algorithm used to sign the certificate
• Issuer: Distinguished name of the Certificate Authority that issued the certificate
• Validity Period: Not Before and Not After dates defining the certificate’s lifespan
• Subject: Distinguished name identifying the certificate holder
• Public Key: The public key and algorithm information for encryption
• Extensions: Optional fields for additional functionality like Subject Alternative Names

Standards Evolution and Compliance

X.509 has evolved through several versions, with X.509v3 being the current standard supporting extensions that enable modern features like Subject Alternative Names, Key Usage restrictions, and Certificate Policies. The standard ensures interoperability across different platforms and vendors, making it possible for certificates issued by any compliant CA to work with any compliant application. Understanding X.509 structure is essential for certificate management, debugging SSL issues, and ensuring proper certificate deployment in complex environments.