At the center of encryption and authentication is the notion of a cryptographic key. Security endpoints use keys to encrypt and decrypt data. The IPSec protocols create Security Association keys that are directional. As shown in Figure 1, the key that is used to encrypt outbound data on one host is used to decrypt the same data on the remote host, while the key that is used to encrypt data on the remote host is used to decrypt data on the local host.
Figure 1. Symmetric encryption
This type of encryption is known as symmetric, because it requires that both hosts use the same keys on the same data.