Serious security weakness in 802.11b wireless LANs exposed

A new and potentially more serious security weakness in the encryption standard used within IEEE-based wireless LANs has been uncovered by three leading cryptographers.

The cryptographers have described a practical way of attacking the key scheduling algorithm of the RC4 cipher that uses fewer resources than previous attacks, thus making it potentially easier to compromise the networks. The findings have been published in a paper entitled "Weaknesses in the key scheduling algorithm of RC4". The RC4 cipher forms the basis of the Wired Equivalent Privacy (WEP) encryption that is used in IEEE 802.11b (also known as WiFi) wireless networks.

The authors have discovered several ways to uncover patterns in packets of information passing over wireless LANs. These patterns can be used to figure out the WEP encryption "key," which is the number used to scramble the data being transmitted. Once the key is recovered, it can be used to decrypt the messages.

There have been other problems uncovered in the WEP structure, as published in previous reports, but the latest discovery is more significant because an attack could be carried out faster and with fewer resources.

David Cohen, chairman of the Wireless Ethernet Compatibility Alliance (WECA), an industry group that tests compatibility between wireless LAN products, said: "Earlier papers were mainly theoretical. This paper provides a much more practical approach to breaking R4."

The authors say using a longer key, one of 128 bits compared to the current WEP standard of 40 bits, does not make it significantly harder for the new attack to discover the key.

WECA said enterprise users should continue to use WEP because only skilled cryptoanalysts would be able to attack the weakness. Enterprises also could use several existing tools for additional security, such as VPNs, IPSec, and RADIUS authentication servers.

In addition, many wireless LAN vendors have introduced proprietary encryption schemes because of the known weaknesses in WEP, according to Cohen. However, these schemes are not interoperable with each other.

The authors of the document are Adi Shamir (who also co-invented the widely-used RSA public key encryption system), Itsik Mantin a computer science student at the Weizmann Institute, and Scott Fluhrer, a cryptographer with Cisco.

Cohen says the discovery also lends fresh urgency to the work of two IEEE groups grappling with the 802.11 vulnerabilities.

One, the 802.11.1x group, is focused on overall network security and authentication, and the other, the 802.11I group, is making use of some of the 1x work to overhaul the identified WEP vulnterabilities. That work, according to Cohen, is scheduled to be finalized by the end of the year and vendors are likely to have products out soon.

A PostScript file of the document can be found at http://www.crypto.com/papers/others/rc4_ksaproc.ps. The full version of the paper will be presented at the Selected Areas in Cryptography (SAC) 2001 conference later this month in Toronto.

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