ZyXEL Communications Zyair B-1000 v.2, User Manual

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ZyAIR Access Point Series User’s Guide
6-10 Wireless Security
6.9 Introduction to WPA
Wi-Fi Protected Access (WPA) is a subset of the IEEE 802.11i security specification draft. Key differences
between WPA and WEP are user authentication and improved data encryption.
6.9.1 User Authentication
WPA applies IEEE 802.1x and Extensible Authentication Protocol (EAP) to authenticate wireless clients
using an external RADIUS database. You can’t use the ZyAIR’s Local User Database for WPA
authentication purposes since the Local User Database uses EAP-MD5 which cannot be used to generate
keys. See later in this chapter and the appendices for more information on IEEE 802.1x, RADIUS and EAP.
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.
6.9.2 Encryption
WPA improves data encryption by using Temporal Key Integrity Protocol (TKIP), 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.