To protect the data you transmit from the WAN boot server to the client, you can generate a Hashed Message Authentication Code (HMAC) key. You install this hashing key on both the WAN boot server and the client. The WAN boot server uses this key to sign the data to be transmitted to the client. The client then uses this key to verify the integrity of the data that is transmitted by the WAN boot server. After you install a hashing key on a client, the client uses this key for future WAN boot installations.
For instructions about how to use a hashing key, see (Optional) To Create a Hashing Key and an Encryption Key.