Shared Key Authentication

There are two types of authentication methods in the 802.11 protocol: Open System and Shared Key. Open System (sometimes called Null Authentication) does not utilize a password. Under Open System, a client simply requests to be authenticated and the AP grants access. This is useful in environments like public hotspots, where free and open access is intentional.

Shared key, however, requires knowledge of the WEP key. That's right: The WEP key used to encrypt data (see Chapter 3 for more details) is also used to authenticate clients and access points. It works like this:

• Step 1: The client sends an authentication request to the AP.
  
  
• Step 2: The AP sends the client 128 bytes of challenge text.
  
  
• Step 3: The client encrypts the challenge text with its WEP key and sends the challenge response back to the AP.
  
  
• Step 4: The AP uses its knowledge of the WEP key to validate the challenge response and determine if the client does know the shared secret key.
  
  
• Step 5: The AP responds to the client with a success or failure message.