The server and client have a shared key. When one wants to send a message to another, the source creates a digest value by using the shared key and the message contents as inputs to a keyed hashing function (MD5 by default). The source then includes the digest value in the authentication option. When the message arrives at the destination, the destination searches for the shared key and generates the digest value itself. If the digest value in the authentication option of the message matches the digest value generated by the destination, then the message is secure and comes from a trusted source. This table is used to define containers for keys to be used for a single client.