Bedienungsanleitung ZyXEL AG-120 (Seite 102 von 119) (Englisch)

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ZyXEL AG-120 User’s Guide

102 Appendix D Wireless Security

TKIP uses 128-bit keys that are dynamically generated and distributed by the authentication

server. AES (Advanced Encryption Standard) is a block cipher that uses a 256-bit

mathematical algorithm called Rijndael. They both include 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.

WPA and WPA2 regularly change and rotate 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.

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), with TKIP and AES it is more difficult to decrypt data on a Wi-Fi

network than WEP and difficult for an intruder to break into the network.

The encryption mechanisms used for WPA(2) and WPA(2)-PSK are the same. The only

difference between the two is that WPA(2)-PSK uses a simple common password, instead of

user-specific credentials. The common-password approach makes WPA(2)-PSK susceptible to

brute-force password-guessing attacks but it’s still an improvement over WEP as it employs a

consistent, single, alphanumeric password to derive a PMK which is used to generate unique

temporal encryption keys. This prevent all wireless devices sharing the same encryption keys.

(a weakness of WEP)

User Authentication

WPA and WPA2 apply IEEE 802.1x and Extensible Authentication Protocol (EAP) to

authenticate wireless stations using an external RADIUS database. WPA2 reduces the number

of key exchange messages from six to four (CCMP 4-way handshake) and shortens the time

required to connect to a network. Other WPA2 authentication features that are different from

WPA include key caching and pre-authentication. These two features are optional and may not

be supported in all wireless devices.

Key caching allows a wireless client to store the PMK it derived through a sucessful

authentication with an AP. The wireless client uses the PMK when it tries to connect to the

same AP and does not need to go with the authentication process again.

Pre-authentication enables fast roaming by allowing the wireless client (already connecting to

an AP) to perform IEEE 802.1x authentication with another AP before connecting to it.

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