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Certificates

Public keys are often contained in certificates. A user can digitally sign messages using his private key, and another user can verify the signature using the public key contained in signer's certificate which was issued by a Certificate Authority (CA) within the PKI.

A public key certificate (sometimes called an "identity certificate") is a file in a specified format (Mac OS X Server uses the x.509 format) which contains:

• The public key half of a public-private key pair.
• The key user's identity information, such as a person's name and contact information.
• A validity period (how long the certificate can be trusted to be accurate).
• The URL of someone with the power to revoke the certificate (its "revocation center").
• The digital signature of either a CA, or the key user himself.