4.3 WPA: a Subset of 802.11i

Work on 802.11i began in 2001 after the weaknesses in WEP were made public by several teams of researchers. However, as with any standards body, the IEEE does not always work as fast as some people would like.

In mid-2002, the Wi-Fi Alliance, an industry consortium, proposed a subset of 802.11i, based on draft 3 from the IEEE working group , and called it Wireless Protected Access (WPA). The upcoming full IEEE implementation is also being referred to as WPA v2.

WPA, as a subset of the 802.11i proposed standard, incorporates two major features:

• Use of 802.1x for authentication

• Use of the Temporal Key Integrity Protocol (TKIP)

Chipsets supporting WPA began to become available in 2003. As of this writing, many access points either support WPA out of the box or have firmware updates available that include WPA.

WPA is not only an encryption mechanism but also includes 802.1x authentication, so support is required on the client for the authentication mechanism. As of this writing, your options are very limited regarding WPA support in Linux.

A few vendors have released updated firmware for older radio cards with WPA support; Apple AirPort cards, the Linksys WPC-11, and the Dell TrueMobile 1150 all have updates available.

WPA Support in Access Points

WPA and 802.1x are starting to become available in new access points, and earlier models are getting firmware updates that support WPA. The Linksys WRT54G and D-Link 900AP+ can both support WPA after a firmware upgrade. Newer Linksys and D-Link models are packaged with this support already enabled. Enterprise-level access points from Cisco, Proxim, and others also support WPA and are starting to advertise themselves as "802.11i-ready."

The Dell 1150 card is a rebranded Orinoco card; Agere has drivers on its web site listed "for evaluation only" that include this same update. However, Proxim, the new owner of the Orinoco brand, has nothing on its web site about WPA for older cards.

All of this is interesting but not immediately useful, however, because you can't use any of these cards under Linux and take advantage of the WPA code in the cards. Why? Because their associated Linux drivers do not support WPA. As of early 2004, you have two options if you want to use WPA under Linux, which we discuss below. In order to take advantage of these methods , you should understand how 802.1x works.

4.3.1 802.1x Authentication

802.1x was originally designed for wired Ethernet networks. It is a port-based authentication mechanism; when a client is authenticated, traffic is allowed to flow from the Ethernet port of the client through the authenticating device and out into the secured network.

In a wireless network, the principle is the same. Your notebook client is required to authenticate to the access point. If authentication does not occur, wireless frames are not allowed to be sent through the access point to the wired network.

802.1x authenticates users via a four-part process:

1. The Supplicant (the client that wants to access a network resource) connects to the Authenticator (whose resource is needed).

2. The Authenticator asks for credentials from the Supplicant and passes the credentials to the Authenticating Server.

3. The Authenticating Server authenticates the Supplicant on behalf of the Authenticator.

4. If the Supplicant is authenticated, access is then granted.

Note that before the authentication is performed, all the communications go through an uncontrolled port. After authentication, the controlled port is used.

For the Authenticating Server to authenticate the Supplicant, the Extensible Authentication Protocol (EAP) is used. EAP supports multiple authentication mechanisms and was originally developed for PPP.

There are many variants of EAP. Here are some that you may come across in wireless security literature:

EAP-MD5

EAP-MD5 uses the challenge/response method to allow a server to authenticate a user by requesting a username and password. EAP-MD5 does not provide mutual authentication and is vulnerable to an offline dictionary attack.

EAP-Transport Layer Security (EAP-TLS)

EAP-TLS is based on X.509 (an ITU standard specifying the contents of a digital certificate) certificates. It is currently the most commonly used EAP type for securing wireless networks. However, EAP-TLS requires the use of Public Key Infrastructure (PKI), which is not feasible to be implemented on small networks.

Protected EAP (PEAP)

To counter the complexity of using EAP-TLS, PEAP was proposed as an alternative. PEAP uses a server-side certificate to allow the authentication of the server. It creates an EAP-TLS tunnel and then uses other authentication methods over the tunnel. EAP methods such as MD5, MS-CHAP, and MS-CHAP v2 are supported. PEAP was proposed as an IETF standard by Microsoft, Cisco, and RSA.

EAP Tunneled TLS (EAP-TTLS)

EAP-TTLS is similar to PEAP. It creates a tunnel between the user and the RADIUS server. It supports EAP methods such as MD5, MS-CHAP, and MS-CHAP v2.

Lightweight EAP (LEAP)

LEAP is Cisco's proprietary version of EAP, which works mostly with Cisco's wireless cards, RADIUS servers, and access points.

Microsoft Challenge-Handshake Authentication Protocol Version 2 (MS-CHAP v2)

Originally designed by Microsoft as a PPP authentication protocol, MSCHAP v2 is a password-based, challenge-response, mutual authentication protocol that uses the Message Digest 4 (MD4) and Data Encryption Standard (DES) algorithms to encrypt responses. MS-CHAP v2 is now an EAP type in Windows XP.

In the wireless world, suppose a notebook PC needs to connect to an access point. The notebook PC is the Supplicant, and the access point is the Authenticator. The access point, as the Authenticator, maintains a list of users and passwords and acts as the Authenticating Server. For small networks, this is not an issue; for large networks, however, this is an additional overhead in maintenance and a potential security risk, because it means that users must have another account and password.

In this case, the access point is told to refer to an external RADIUS server. RADIUS was developed by Livingston (now part of Lucent) for use in large dial-up modem pools, and is widely used by ISPs as the authentication mechanism for PPP and PPPoE users. The protocol is now defined by RFCs 2058, 2138, and 2139.

A RADIUS server maintains the user and password list, and performs authentication on behalf of the access point. The RADIUS server in this scenario is the Authenticating Server. Frequently, a RADIUS server is merely a method to transform authentication from some other source ”for example, NIS, LDAP, or Kerberos authentication from a corporate network, which is then used by the RADIUS server to authenticate clients .