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Appendix C Wireless LANs

EMG1302-R10A User’s Guide

194

The RADIUS server distributes a Pairwise Master Key (PMK) key to the AP that then sets up a key

hierarchy and management system, using the PMK to dynamically generate unique data encryption

keys to encrypt every data packet that is wirelessly communicated between the AP and the wireless

clients. This all happens in the background automatically.

The Message Integrity Check (MIC) is designed to prevent an attacker from capturing data packets,

altering them and resending them. The MIC provides a strong mathematical function in which the

receiver and the transmitter each compute and then compare the MIC. If they do not match, it is

assumed that the data has been tampered with and the packet is dropped.

By generating unique data encryption keys for every data packet and by creating an integrity

checking mechanism (MIC), with TKIP and AES it is more difficult to decrypt data on a Wi-Fi

network than WEP and difficult for an intruder to break into the network.

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 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 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.

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