## Appendix B - The X.509 Certificate

The X.509 protocols include a structure for public-key certificates. A CA assigns a unique name to each user and issues a signed certificate containing this name and the user's public key. The following diagram shows an X.509 certificate.

X.509 Certificate

These are the meanings for each field.

Field | Meaning
Version | Identifies the certificate format.
Serial Number | Is unique to the CA.
Algorithm Identifier | Identifies the algorithm used to sign the certificate, together with any necessary parameters.
Issuer | The name of the CA.
Period of Validity | A pair of dates. The certificate is valid during the time period between the two.
Subject | The name of the user.
Subject's Public Key | Contains the public key algorithm name, any necessary parameters, and the public key.
Signature | The CA's signature.

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