Certificates are （）documents attesting to the （） of a public key to an individual or other entity. They allow verification of the claim that a given public key does in fact belong to a given individual. Certificates help prevent someone from using a phony key to （）someone else. In their simplest form， Certificates contain a public key and a name. As commonly used， a certificate also contains an （）date， the name of the CA that issued the certificate， a serial number， and perhaps other information. Most importantly， it contains the digital（）of the certificate issuer. The most widely accepted format for certificates is X.509 ， thus， Certificates can be read or written by any application complying with X.509.