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Appendix C Wireless LANs
NWD-270N User’s Guide
81
The encryption mechanisms used for WPA(2) and WPA(2)-PSK are the same. The only
difference between the two is that WPA(2)-PSK uses a simple common password, instead of
user-specific credentials. The common-password approach makes WPA(2)-PSK susceptible to
brute-force password-guessing attacks but it’s still an improvement over WEP as it employs a
consistent, single, alphanumeric password to derive a PMK which is used to generate unique
temporal encryption keys. This prevent all wireless devices sharing the same encryption keys.
(a weakness of WEP)
User Authentication
WPA and WPA2 apply IEEE 802.1x and Extensible Authentication Protocol (EAP) to
authenticate wireless clients using an external RADIUS database. WPA2 reduces the number
of key exchange messages from six to four (CCMP 4-way handshake) and shortens the time
required to connect to a network. Other WPA2 authentication features that are different from
WPA include key caching and pre-authentication. These two features are optional and may not
be supported in all wireless devices.
Key caching allows a wireless client to store the PMK it derived through a successful
authentication with an AP. The wireless client uses the PMK when it tries to connect to the
same AP and does not need to go with the authentication process again.
Pre-authentication enables fast roaming by allowing the wireless client (already connecting to
an AP) to perform IEEE 802.1x authentication with another AP before connecting to it.
Wireless Client WPA Supplicants
A wireless client supplicant is the software that runs on an operating system instructing the
wireless client how to use WPA. At the time of writing, the most widely available supplicant is
the WPA patch for Windows XP, Funk Software's Odyssey client.
The Windows XP patch is a free download that adds WPA capability to Windows XP's built-
in "Zero Configuration" wireless client. However, you must run Windows XP to use it.
WPA(2) with RADIUS Application Example
To set up WPA(2), you need the IP address of the RADIUS server, its port number (default is
1812), and the RADIUS shared secret. A WPA(2) application example with an external
RADIUS server looks as follows. "A" is the RADIUS server. "DS" is the distribution system.
1 The AP passes the wireless client's authentication request to the RADIUS server.
2 The RADIUS server then checks the user's identification against its database and grants
or denies network access accordingly.
3 A 256-bit Pairwise Master Key (PMK) is derived from the authentication process by the
RADIUS server and the client.
4 The RADIUS server distributes the PMK to the AP. The AP then sets up a key hierarchy
and management system, using the PMK to dynamically generate unique data encryption
keys. The keys are used to encrypt every data packet that is wirelessly communicated
between the AP and the wireless clients.