Page 241 - DCAP311_DCAP607_WIRELESS_NETWORKS

Unit 14: Authentication

802.11 Client Authentication Process as shown in figure 14.7 are describe below: Notes
1. Probe Requests and Responses: Once the client becomes active on the medium, it searches
for access points in radio range using the 802.11 management frames known as probe
request frames. The probe request frame is sent on every channel the client supports in
an attempt to find all access points in range that match the SSID and client-requested data
rates. All access points that are in range and match the probe request criteria will respond
with a probe response frame containing synchronization information and access point load.
The client can determine which access point to associate to by weighing the supported data
rates and access point load. Once the client determines the optimal access point to connect
to, it moves to the authentication phase of 802.11 network access.
2. Open Authentication: Open authentication is a null authentication algorithm. The access
point will grant any request for authentication. It might sound pointless to use such
an algorithm, but open authentication has its place in 802.11 network authentication.
Authentication in the 1997 802.11 specification is connectivity-oriented. The requirements
for authentication are designed to allow devices to gain quick access to the network. In
addition, many 802.11-compliant devices are hand-held data-acquisition units like bar
code readers. They do not have the CPU capabilities required for complex authentication
algorithms.
Open authentication consists of two messages:
zz The authentication request
zz The authentication response
Open authentication allows any device network access. If no encryption is enabled on the
network, any device that knows the SSID of the access point can gain access to the network.
With WEP encryption enabled on an access point, the WEP key itself becomes a means of
access control. If a device does not have the correct WEP key, even though authentication
is successful, the device will be unable to transmit data through the access point. Neither
can it decrypt data sent from the access point (Figure 14.8).
Figure 14.8: Open Authentication with Differing WEP Keys

Source: http://www.cisco.com/warp/public/cc/pd/witc/ao1200ap/prodlit/wswpf_wp.pdf
3. Shared Key Authentication: Shared key authentication is the second mode of authentication
specified in the 802.11 standard. Shared key authentication requires that the client configure
a static WEP key. Figure 14.9 describes the shared key authentication process.
(a) The client sends an authentication request to the access point requesting shared key
authentication

(b) The access point responds with an authentication response containing challenge text

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