Alice and Bob start exchanging messages using the session key. Mallory, who also has that key, can now decipher the entire conversation. An independent third party that everyone trusts is needed to foil the attack. When Alice receives the signed public key from Bob, she can verify the third party's signature. This way she knows that the public key really belongs to Bob, and not Mallory.
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.