Digital certificates are the topic of the next section. Digital Certificates A certificate is a data package that completely identifies an entity and is issued by a CA only after that authority has verified the entity's identity. The data package includes the public key that belongs to the entity. The version field identifies the certificate format. The signature algorithm identifies the algorithm used to sign the certificate.
Subject is the name of the user to whom the certificate is issued.
The subject's public key field includes the algorithm name and the public key itself. The last field is the CA's signature. This attachment contains the hash of the original message signed with the sender's private key, as well as the signer certificate. The message is actually encrypted using a random symmetric key, and it is that symmetric key that is encrypted using the recipient's public key and sent along with the message. If a message is being sent to multiple recipients, the symmetric key is encrypted separately by every recipient's public key.