ZyXEL Communications ZyAIR G-3000, Руководство пользователя

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ZyAIR G-3000 User’s Guide
83 Chapter 6 Wireless Security
6.10 Introduction to WPA
Wi-Fi Protected Access (WPA) is a subset of the IEEE 802.11i security specification draft.
Key differences between WPA and WEP are user authentication and improved data
encryption.
6.10.1 User Authentication
WPA applies IEEE 802.1x and Extensible Authentication Protocol (EAP) to authenticate
wireless clients using an external RADIUS database. You can’t use the ZyAIR’s Local User
Database for 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.
6.10.2 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.