ZyXEL Communications P-660HN-51 User Manual page 279

Select WEP only when the AP and/or wireless clients do not support WPA or WPA2. WEP is less secure
than WPA or WPA2.
Encryption
WPA improves data encryption by using Temporal Key Integrity Protocol (TKIP), Message Integrity Check
(MIC) and IEEE 802.1x. WPA2 also uses TKIP when required for compatibility reasons, but offers stronger
encryption than TKIP with Advanced Encryption Standard (AES) in the Counter mode with Cipher block
chaining Message authentication code Protocol (CCMP).
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 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), 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
clients 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 successful 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.
Appendix C Wireless LANs
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