802.11 wireless technologies have transformed computer networking, allowing portable computer users to roam freely and remain connected and making it possible to place desktop computers in locations where it's difficult to run network cabling. However, wireless networking presents security issues. Because the data goes over the airwaves, it's easy to intercept; any "war driver" with the right equipment can detect and plug into an unprotected wireless network.
That means encryption is essential, but the most common encryption scheme for wireless, the Wired Equivalent Privacy (WEP) protocol suffers some serious shortcomings. Now there's a better option: Wi-Fi Protected Access (WPA). In this article, we'll discuss WPA's advantages over WEP, how WPA works, and how to implement it on your Windows network.
How WPA came about
An unencrypted wireless network is wide open to the world. Don't let the specifications for the typical range of wireless transmissions fool you into thinking you're safe. War drivers (hackers who cruise around with their laptops, looking for open wireless networks) can use high gain antennas to extend that range. Assuming you don't want any and everyone connecting to your wireless network, you need to encrypt the communications.
Most Wireless Access Points (WAPs) and wireless Network Interface Cards (NICs) support WEP encryption. WEP offers some protection, but it is notorious for its weaknesses. One problem is that the encryption key length is only 40 bits. The shorter the key, the easier it is to crack. The same key is used by everyone on the network.
The RC4 stream cipher algorithm that WEP uses is vulnerable to attackers who may be able to decipher the transmission and recover the plain text if they are able to intercept two ciphertexts that are encrypted with the same key. To protect against these vulnerabilities, WEP uses an Initialization Vector (IV) to create a different RC4 key for each packet. The problem is that the IV is only 24 bits, which means the same key stream will almost certainly be reused if there is a lot of traffic going through the WAP. All the attacker has to do is be patient and keep collecting ciphertexts; eventually he'll have two that were encrypted with the same key (these are called IV Collisions).
Because of WEP's vulnerabilities, many companies and government agencies that need high security have banned wireless technology. Other organizations wanted the convenience of wireless communications but worried about security breaches. It was essential, if wireless networking was to go to the next level, that better encryption methods be developed.
WPA was developed by the WiFi Alliance in conjunction with the IEEE as an interim wireless security solution that works with existing hardware, in anticipation of the 802.11i wireless security standards that were recently ratified, but are not compatible with all legacy hardware. For those who aren't ready to upgrade all of their wireless hardware and who need more security than WEP can provide, WPA is the answer.
WPA can be thought of as a "bridge" protocol, since it is backward-compatible with WEP and designed to be forward-compatible with 802.11i, also called WPA v.2.
How WPA works
WPA has two big advantages over WEP:
- WPA uses stronger encryption than WEP
- WEP provides for user authentication in the enterprise environment
Encryption is improved through the use of the Temporal Key Integrity Protocol (TKIP). TKIP uses a "temporal key" that has 128 bits. This 128 bit key is mixed with the MAC address of the wireless sender. Then the mixed key is mixed again with an IV value (which, at 49 bits, is double the length of WEP's) to create a unique key that is used to encrypt one packet. WPA creates dynamic session keys, with different keys per user, per session and per packet. This overcomes the WEP vulnerability we described above.
Note that WPA, like WEP, uses RC4 instead of the Advanced Encryption Standard (AES). This is because although AES is a stronger algorithm, it can't be implemented with most legacy NICs, and the goal of WPA's developers was a more secure protocol that would work with existing hardware.
The word temporal means "brief, only lasting a short time." TKIP keys are called "temporal keys" because they're changed often, to prevent the same keys from being used again. TKIP includes a rekeying mechanism to prevent this problem. Each packet has a different key that contains a 48 bit serial number as the IV. This number gets incremented for each new packet in sequence. There are no IV collisions with TKIP.
Having the numbers in sequence also helps to protect against replay attacks, because if a hacker tried to replay packets from a previous wireless connection, the numbers would be out of sequence and the attack would be detectable. To ensure the integrity of the data, TKIP uses an 8 byte Message Integrity Code (MIC) at the end of each message. The method used to generate this MIC is called Michael.
In addition to stronger encryption, WPA provides a way for enterprises to authenticate wireless users with a RADIUS server. The authentication protocol that's used is the Extensible Authentication Protocol (EAP).
The RADIUS server also allows you to set user access policies to control wireless access to your network. For example, you can set time limits on wireless sessions or place restrictions on days and times that users can connect.
WPA provides for mutual authentication—both client ("supplicant" in wireless parlance) and server are authenticated.
What about small business or home networks that want to use WPA but don't have RADIUS servers? In that case, WPA can use a pre-shared key.
Implementing WPA on your Windows network
So you're convinced that you need to switch from WEP to WPA to protect your wireless network? What steps do you need to take to accomplish this? Basically, you must:
- Ensure that your WAPs support WPA.
- Ensure that the wireless NICs on your client computers support WPA.
- Ensure that the operating system on your client computers supports WPA.
- Ensure that the wireless client software on the client computers support WPA.
Verifying hardware support
First, make sure that the WAP supports WPA. You might need to update the WAP's firmware. Contact the WAP vendor or check the vendor's Web site. If you're buying a new WAP, check the label on the box. If it was certified by the Wi-Fi Alliance after August 2003, it is required to support WPA.
Next, ensure that the client computers' wireless NICs support Wireless Zero Configuration. You might need to upgrade the driver. Contact the NIC vendor or check the vendor's Web site.
Verifying software support
Once you know the hardware supports WPA, consider the operating system. If your wireless client computers are running Windows XP with Service Pack 2, you'll have the easiest time of it. The Windows WPA client software is installed as part of SP2. If the clients run XP with SP1, the easiest route is to update them to SP2. However, you might not be able to do that if they are running applications that have conflicts with SP2. In that case, you can download the Support Patch for Wi-Fi Protected Access from the Microsoft Download Center.
Microsoft also offers a wireless update rollup package for Windows XP that corrects some problems with WPA. You can read about it in Microsoft KB Article 826942.
You can use WPA with either XP Professional or XP Home Edition. If you're using an operating system prior to XP, you'll need to get a third party client. For example, you can download the Odyssey WLAN client from Funk Software.
Configuring the WAP
Configuration of your WAP will depend on the hardware you have. You'll need to access the WAP's configuration utility (usually done through a Web page) and set up WPA options. You may have the following options to choose from:
- Whether to use a pre-shared key or an authentication server. If your network doesn't have a RADIUS server, you'll have to use a pre-shared key. If you select to use a pre-shared key, you'll need to enter it. This should be a strong key (password). The same key will need to be entered in the client configuration. If you select to use an authentication server, you'll need to identify the RADIUS server to be used for authentication and accounting.
- Whether to use the TKIP algorithm or AES. AES is more secure, but may not work with legacy hardware because most older hardware doesn't have the processing power to use it.
Configuring the client software
After you install the WPA patch for XP, you'll need to reboot. The patch installs as Hotfix (SP2) Q815485. If you need to uninstall it for some reason, you can use the Add/Remove Programs applet to remove the hotfix.
To configure WPA in Windows XP, perform the following steps:
- Click Start | Control Panel and click the Network applet.
- Right-click the wireless connection and select Properties.
- Click the Wireless Networks tab.
- Select the wireless network name (SSID) that corresponds to the WAP that you configured for WPA.
- Click the Configure button.
- In the Network Authentication drop-down box, select WPA if you are using a RADIUS server for authentication, or WPA-PSK if you are using a pre-shared key.
- In the Data Encryption drop-down box, select TKIP or AES (the selection must correspond to the algorithm you configured on the WAP).
- If you are using a pre-shared key, enter it in the Network Key text box and again in the Confirm Network Key text box. Note that the key will not be displayed as you type but will be replaced by asterisks.
- Click OK to close the dialog box.
You should now be able to connect to your WAP securely using WPA.
Configuring the RADIUS server
If you want to use EAP authentication with WPA, you'll need to configure a RADIUS authentication server. You'll also need a certification server to issue digital certificates. You can do this with a Windows 2000 Server or Server 2003 computer on an Active Directory network, which comes with IAS (Internet Authentication Service) and the CA (Certification Authority) service. You'll need to install IAS and Certificate services, as they are not installed by default.
Install IAS through the Add/Remove Programs applet in Control Panel by starting the Windows Components Wizard, clicking Network Services and clicking Details, then selecting Internet Authentication Service. You may need to insert the Windows Server installation CD. Certificate Services is also installed as a Windows Component.
You can install the CA on the same server on which you install IAS.
You'll need to add your WAP as a RADIUS client on the IAS server and then use the New Remote Access Policy Wizard to create a policy for wireless access.
To add a remote access policy, open the IAS console and expand the Internet Authentication Service node, then right click Remote Access Policies. Click New Remote Access Policy. This starts the wizard. Select "wireless access" as the access method. If you're using certificate authentication, you need to install a user certificate on your wireless clients.
For fault tolerance, you may want to create both a primary and a backup IAS server. You'll need to copy the configuration of the primary IAS server to the backup IAS server. To save your IAS configuration, use the command line tool netsh. Type netsh aaaa show config > (path\file.txt).
Wireless communications have a "bad rep" when it comes to security, but you can make your wireless network more secure by replacing WEP, the standard encryption protocol, with WPA. Once you know the history of WPA and its advantages over WEP, you can configure your Windows network to use WPA both with and without an authentication server.
Debra Littlejohn Shinder, MCSE, MVP is a technology consultant, trainer, and writer who has authored a number of books on computer operating systems, networking, and security. Deb is a tech editor, developmental editor, and contributor to over 20 additional books on subjects such as the Windows 2000 and Windows 2003 MCSE exams, CompTIA Security+ exam, and TruSecure's ICSA certification.