19216811.liveAppendix B Wireless LANs
VMG1312-B Series User’s Guide
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WPA2-PSK (WPA2-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 wireless
client will be granted access to a WLAN.
If the AP or the wireless clients do not support WPA2, just use WPA or WPA-PSK depending on
whether you have an external RADIUS server or not.
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