Security hole exposed in 802.11b wireless LANs

A security weakness in the encryption standard used within IEEE-based wireless LANs has been uncovered.

Three cryptographers have described a practical way of attacking the key scheduling algorithm of the RC4 cipher, 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 paper's authors 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" and the number used to scramble the data being transmitted. Once the key is recovered, it can be used to decrypt the messages.

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 attackers to uncover the process.

Industry group, The Wireless Ethernet Compatibility Alliance (WECA) said enterprise users should continue to use WEP because only skilled cryptoanalysts would be able to attack the weakness. Enterprises could also use several existing tools for additional security, such as Virtual Private Networks (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 David Cohen, WECA's chairman. However, these schemes do not interoperable with each other.

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

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 Systems.

Cohen says the paper provides a more practical approach to breaking RC4 than previous publications and 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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John Cox

Computerworld
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