Open Authentication and Shared Key Authentication are the two methods that
the 802.11 standard defines for clients to connect to an access point
. The association
process can be broken down into three elements known as probe, authentication,
and association. This section will explain both authentication methods and the
steps the client undergoes during the process. Network EAP will be discussed in
the enterprise WLAN security section.
Open Authentication
The
Open Authentication method performs the entire authentication process in clear
text. This is shown in Figure
. Open
Authentication is basically a null authentication, which means there is no
verification of the user or machine. Open Authentication is usually tied to a
WEP key. A client can associate to the access point with an incorrect WEP key
or even no WEP key. A client with the wrong WEP key will be unable to send or
receive data, since the packet payload will be encrypted. Keep in mind that the
header is not encrypted by WEP. Only the payload or data is encrypted.
Shared Key Authentication
Shared Key Authentication works
similarly to Open Authentication, except that it uses WEP encryption for one
step. Shared key requires the client and the access point to have the same WEP
key. An access point using Shared Key Authentication sends a challenge text
packet to the client, as shown in Figure
. If the client
has the wrong key or no key, it will fail this portion of the authentication
process. The client will not be allowed to associate to the AP. Shared key is
vulnerable to a man-in-the-middle attack, so it is not recommended.
Interoperability
On most access points, including Cisco, it is
possible to use Open Authentication with or without a WEP key. For basic
interoperability requiring WEP, a Cisco Access point will be set up using Open
Authentication. Data Encryption is set to Required, and TKIP, MIC, and BKR are
all disabled.