It may become impractical to continue the exchange of keys when recipients are located worldwide. Another issue facing symmetric key encryption is that of key compromise. If a key is compromised, an attacker will decrypt all messages and traffic using that key. Worse yet, the attacker could modify and falsify the encrypted information prior to sending it on to its original destination. In other cases, the attacker may simply passively read the information without your knowledge.
Many cryptographic systems use a type symmetrical key called a session key.
Session key encryption is a hybrid system utilizing both private and public key infrastructures. In these systems slower and more complex public key cryptographic algorithms are used to encrypt the session key, which is then used in a symmetrical algorithm. In this system the session key is only used in one encryption session prior to being discarded, never to be used again. Asymmetric key encryption is better known as public key encryption. Public key encryption helps to overcome the burdens of key management found with systems such as symmetric key encryption.