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ZyXEL AG-200 User’s Guide
2-6 WLAN Networking
User Authentication
WPA applies IEEE 802.1x and Extensible Authentication Protocol (EAP) to authenticate wireless clients
using an external RADIUS database.
Therefore, if you don’t have an external RADIUS server, you should use WPA-PSK (WPA -Pre-Shared
Key) that only requires a single (identical) password entered into each access point, wireless gateway and
wireless client. As long as the passwords match, a client will be granted access to a WLAN.
Encryption
WPA improves data encryption by using either Temporal Key Integrity Protocol (TKIP) or Advanced
Encryption Standard (AES), Message Integrity Check (MIC) and IEEE 802.1x.
Temporal Key Integrity Protocol (TKIP) uses 128-bit keys that are dynamically generated and distributed
by the authentication server. It includes a per-packet key mixing function, a Message Integrity Check
(MIC) named Michael, an extended initialization vector (IV) with sequencing rules, and a re-keying
mechanism.
TKIP regularly changes and rotates the encryption keys so that the same encryption key is never used
twice. 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 pair-wise key 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), TKIP makes it much more difficult to decode data on a Wi-Fi network than WEP,
making it difficult for an intruder to break into the network.
The encryption mechanisms used for WPA and WPA-PSK are the same. The only difference between the
two is that WPA-PSK uses a simple common password, instead of user-specific credentials. The common-
password approach makes WPA-PSK susceptible to brute-force password-guessing attacks but it’s still an
improvement over WEP as it employs an easier-to-use, consistent, single, alphanumeric password.
2.2.4 WPA-PSK Application Example
A WPA-PSK application looks as follows.
Step 1. First enter identical passwords into the AP and all wireless clients. The Pre-Shared Key (PSK)
must consist of between 8 and 63 ASCII characters (including spaces and symbols).
Step 2. The AP checks each client’s password and (only) allows it to join the network if it matches its
password.
Step 3. The AP derives and distributes keys to the wireless clients.