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2.5 Wireless Security

A world of danger can lurk at the link layer. We've already discussed ARP-spoofing. But wireless networks add a new dimension. It's not that they extend the attackers' powers; rather, they expand the reach and number of potential attackers.

The most common form of wireless networking is IEEE 802.11b, known to marketeers as WiFi. 802.11 is available in most research labs, at universities, at conferences, in coffeehouses, at airports, and even in peoples' homes. To prevent random, casual access to these networks, the protocol designers added a symmetric key encryption algorithm called Wired Equivalent Privacy (WEP).

The idea is that every machine on the wireless network is configured with a secret key, and thus nobody without the key can eavesdrop on traffic or use the network. Although the standard supports encryption, early versions supported either no encryption at all or a weak 40-bit algorithm. As a result, you can cruise through cities or high-tech residential neighborhoods and obtain free Internet (or intranet!) access, complete with DHCP support! Mark Seiden coined the term war driving for this activity.

Unfortunately, the designers of 802.11 did not get the protocol exactly right. The security flaws resulted from either ignorance of or lack of attention to known techniques. A team of researchers consisting of Nikita Borisov, Ian Goldberg, and David Wagner [2001] discovered a number of flaws that result in attackers being able to do the following: decrypt traffic based on statistical analysis; inject new traffic from unauthorized mobile stations; decrypt traffic based on tricking the access points; and decrypt all traffic after passively analyzing a day's worth of traffic.

This is devastating. In most places, the 802.11 key does not change after deployment, if it is used at all. Considering the huge deployed base of 802.11 cards and access points, it will be a monumental task to fix this problem.

A number of mistakes were made in the design. Most seriously, it uses a stream cipher, which is poorly matched to the task. (See Appendix A for an explanation of these terms.) All users on a network share a common, static key. (Imagine the security of sharing that single key in a community of college students!) The alleged initialization vector (IV) used is 24 bits long, guaranteeing frequent collisions for busy access points. The integrity check used by WEP is a CRC-32 checksum, which is linear. In all cases, it would have been trivial to avoid trouble. They should have used a block cipher; failing that, they should have used much longer IVs and a cryptographic checksum. Borisov et al. [2001] implemented the passive attack.

WEP also comes with an authentication mechanism. This, too, was easily broken [Arbaugh et al., 2001]. The most devastating blow to WEP, however, came from a theoretical paper that exposed weaknesses in RC4, the underlying cipher in WEP [Fluhrer et al., 2001]. The attack (often referred to as the FMS attack) requires one byte of known plaintext and several million packets, and results in a passive adversary directly recovering the key. Because 802.11 packets are encapsulated in 802.2 headers with a constant first byte, all that is needed is the collection of the packets.

Within a week of the release of this paper, researchers had implemented the attack [Stubble-field et al., 2002], and shortly thereafter, two public tools Airsnort and WEPCrack appeared on the Web.

Given the availability of these programs, WEP can be considered dead in the water. It provides a sense of security, without useful security. This is worse than providing no security at all because some people will trust it. Our recommendation is to put your wireless network outside your firewall, turn on WEP as another, almost useless security layer, and use remote access technology such as an IPsec VPN or ssh to get inside from the wireless network.

Remember that just because you cannot access your wireless network with a PCMCIA card from the parking lot, it does not mean that someone with an inexpensive high gain antenna cannot reach it from a mile (or twenty miles!) away. In fact, we have demonstrated that a standard access point inside a building is easily reachable from that distance.

On the other hand, you cannot easily say "no" to insiders who want wireless convenience. Access points cost under $150; beware of users who buy their own and plug them into the wall jacks of your internal networks. Periodic scanning for rogue access points is a must. (Nor can you simply look for the MAC address of authorized hosts; many of the commercial access points come with a MAC address cloning feature.)

2.5.1 Fixing WEP

Given the need to improve WEP before all of the hardware is redesigned and redeployed in new wireless cards, the IEEE came up with a replacement called Temporal Key Integrity Protocol (TKIP). TKIP uses the existing API on the card—namely, RC4 with publicly visible IVs—and plays around with the keys so that packets are dynamically keyed. In TKIP, keys are changed often (on the order of hours), and IVs are forced to change with no opportunity to wrap around. Also, the checksum on packets is a cryptographic MAC, rather than the CRC used by WEP. Thus, TKIP is not vulnerable to the Berkeley attacks, nor to the FMS one. It is a reasonable workaround, given the legacy issues involved. The next generation of hardware is designed to support the Advanced Encryption Standard (AES), and is being scrutinized by the security community.

It is not clear that the link layer is the right one for security. In a coffeeshop, the security association is terminated by the store: is there any reason you should trust the shopkeeper? Perhaps link-layer security makes some sense in a home, where you control both the access point and the wireless machines. However, we prefer end-to-end security at the network layer or in the applications.

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