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HomeSafe User’s Guide

Wireless Security 7-7

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.

7.5.3 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.

Figure 7-5 WPA - PSK Authentication

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Zyxel-communications Parental Control Gateway HS100/HS100W Instrukcja Użytkownika Strona 100