Zyxel NBG-418N v2 - Инструкция по эксплуатации - Страница 214

Appendix E Wireless LANs

NBG-418N v2 User’s Guide

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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)

Use r A uthe ntic a tio n 

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.

Wire le ss C lie nt WPA  Sup p lic a nts

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 RA DIUS A p p lic a tio n Exa m p le

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.