March 8, 2019

What is X.509 Certificate?

Part of what makes the web and web development a bit confusing is the way acronyms are thrown about. Despite being basic to internet security, X.509’s name doesn’t elucidate the technology well.

Reading: X.509 on Wikipedia.

X.509 is a standard. It defines how public/private key pairs should be formatted. For example, the TLS/SSL protocol uses X.509 to describe certificates for HTTPS encryption. Unfortunately, the Wikipedia article does not do much to elucidate the standard, so I’ll just lay out the basic structure of an X.509 certificate and list a buch of follow up questions

  • Certificate
    • Version Number
    • Serial Number
    • Signature Algorithm ID
    • Issuer Name
    • Validity Period: Not Before; Not After
    • Subject Name
    • Subject Public Key Info: Public Key Algorithm; Subject Public Key
  • Certificate Signature Algorithm
  • Certificate Signature

Here’s how my browser describes my SSL certificate for this webpage: SSL cert as described by Firefox

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