ZyXEL Communications Prestige 662HW Series, User Manual

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Prestige 662HW Series User’s Guide
Wireless LAN Setup 6-9
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.
6.7.2 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.
6.8 WPA-PSK Application Example
A WPA-PSK application looks as follows.
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).
2.
The AP checks each client’s password and (only) allows it to join the network if it matches its
password.
3.
The AP derives and distributes keys to the wireless clients.
4.
The AP and wireless clients use the TKIP encryption process to encrypt data exchanged between
them.