Overview#Session Key is a relatively short-lived and often single-use Symmetric Key used for encryption of messages in one communication session.
A Session Key should be strong enough to remain secure for the lifespan of the session.
Session Key can be derived from a Hash Function.
Asymmetric Key Cryptography is too slow for many purposes, and all secret key algorithms require that the key is securely distributed. By using an Asymmetric Keys to encrypt the Session Key for another, faster, Symmetric Key overall performance can be improved considerably.