*
 *  * **A validity interval.** A certificate should not be used before its validity interval starts
 *    or after it ends.
 *
 *  * **A public key.** This cryptographic key is used for asymmetric encryption digital signatures.
 *    Note that the private key is not a part of the certificate!
 *
 *  * **A signature issued by another certificate's private key.** This mechanism allows a trusted