WPA Security Enhancements
March 20, 2003
We're all tired of not having adequate standards-based security for wireless LANs. Learn how the new Wi-Fi Protected Access (WPA) can enhance the security of a multi-vendor Wi-Fi network.
As we've discussed in a previous tutorial, 802.11 Wired Equivalent Privacy (WEP) doesn't provide enough security for most enterprise wireless LAN applications. Because of static key usage, it's fairly easy to crack WEP with off-the-shelf tools. This motivates IT managers to use stronger dynamic forms of WEP.
The problem to date, however, is that these enhanced security mechanisms are proprietary, making it difficult to support multi-vendor client devices. The 802.11i standard will eventually solve the issues, but it's not clear when the 802.11 Working Group will ratify the 802.11i standard.
As a result, the Wi-Fi Alliance has taken a bold step forward to expedite the availability of effective standardized wireless LAN security by defining Wi-Fi Protected Access (WPA) while promoting interoperability. With WPA, an environment having many different types of 802.11 radio NICs, such as public hotspots, can benefit from enhanced forms of encryption.
WPA is actually a snapshot of the current version of 802.11i, which includes Temporal Key Integrity Protocol (TKIP) and 802.1x mechanisms. The combination of these two mechanisms provides dynamic key encryption and mutual authentication, something much needed in WLANs.
As with WEP, TKIP uses the RC4 stream cipher provided by RSA Security to encrypt the frame body and CRC of each 802.11 frame before transmission. The issues with WEP don't really have much to do with the RC4 encryption algorithm. Instead, the problems primarily relate to key generation and how encryption is implemented.TKIP adds the following strengths to WEP:
- 48-bit initialization vectors. WEP produces what's referred to as a "keyschedule" by concatenating a shared secret key with a randomly-generated 24-bit initialization vector (IV). WEP inputs the resulting keyschedule into a pseudo-random number generator that produces a keystream equal to the length of the 802.11 frame's payload. With a 24 bit IV, though, WEP eventually uses the same IV for different data packets. In fact, the reoccurrence of IVs with WEP can happen within an hour or so in busy networks. This results in the transmission of frames having encrypted frames that are similar enough for a hacker to collect frames based on the same IV and determine their shared values, leading to the decryption of the 802.11 frames. WPA with TKIP, however, uses 48-bit IVs that significantly reduce IV reuse and the possibility that a hacker will collect a sufficient number of 802.11 frames to crack the encryption.
- Per-packet key construction and distribution. WPA automatically generates a new unique encryption key periodically for each client. In fact, WPA uses a unique key for each 802.11 frame. This avoids the same key staying in use for weeks or months as they do with WEP. This is similar to changing the locks on a house each time you leave, making it impossible for someone who happened to make a copy of your key to get in.
- Message integrity code. WPA implements the message integrity code (MIC), often referred to as "Michael," to guard against forgery attacks. WEP appends a 4-byte integrity check value (ICV) to the 802.11 payload. The receiver will calculate the ICV upon reception of the frame to determine whether it matches the one in the frame. If they match, then there is some assurance that there was no tampering. Although WEP encrypts the ICV, a hacker can change bits in the encrypted payload and update the encrypted ICV without being detected by the receiver. WPA solves this problem by calculating an 8-byte MIC that resides just before the ICV.
For authentication, WPA uses a combination of open system and 802.1x authentication. Initially, the wireless client authenticates with the access points, which authorizes the client to send frames to the access point. Next, WPA performs user-level authentication with 802.1x. WPA Interfaces to an authentication server, such as RADIUS or LDAP, in an enterprise environment. WPA is also capable of operating in what's known as "pre-shared key mode" if no external authentication server is available, such as in homes and small offices.
An issue that WPA does not fix yet is potential denial of service (DoS) attacks. If someone, such as a hacker or disgruntled employee, sends at least two packets each second using an incorrect encryption key, then the access point will kill all user connections for one minute. This is a defense mechanism meant to thwart unauthorized access to the protected side of the network.
You will be able to upgrade existing Wi-Fi-compliant components to use WPA through relatively simple firmware upgrades. As a result, WPA is a good solution for providing enhanced security for the existing installed base of WLAN hardware.
The eventual 802.11i standard will be backward compatible with WPA; however, 802.11i will also include an optional Advanced Encryption Standard (AES) encryption. AES requires coprocessors not found in most access points today, which makes AES more suitable for new WLAN installations.
Jim Geier provides independent consulting services to companies developing and deploying wireless network solutions. He is the author of the book, Wireless LANs and offers workshops on deploying wireless LANs.
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