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ZyXEL G-220F User’ Guide
Appendix E 88
WPA
User Authentication
WPA applies IEEE 802.1x and Extensible Authentication Protocol (EAP) to authenticate
wireless stations using an external RADIUS database.
Encryption
WPA improves data encryption by using Temporal Key Integrity Protocol (TKIP) or
Advanced Encryption Standard (AES), Message Integrity Check (MIC) and IEEE 802.1x.
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 PMK to dynamically generate unique data
encryption keys to encrypt every data packet that is wirelessly communicated between the AP
and the wireless stations. This all happens in the background automatically.
AES (Advanced Encryption Standard) also uses a secret key. This implementation of AES
applies a 128-bit key to 128-bit blocks of data.
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 decrypt 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.