The WLAN's Weakest Link

Talk About an Oxymoron

Throughout the process, the 802.11i Task Group remained painfully aware that the transition from old station hardware would be slow because of the installed base of wireless networking cards that conform to the Wi-Fi standard. RSN does provide for both RC4 and AES encryption, though it will take new Wi-Fi adapters or fast stations to support the AES encryption. But the principal concession to the RSN migration effort is the inclusion of Transition Security Network (TSN), which is defined only to facilitate migration to an RSN, according to the standard. "A TSN is insecure, since the pre-RSN equipment can compromise the larger network," the standard says. The contradiction of an insecure security network comes from the ways broadcast and multicast traffic are (and aren't) protected and from the inclusion of preshared (manually configured) keys in RSN.

An access point sends broadcast and multicast frames encrypted with the weakest configured security method: WEP (Wired Equivalent Privacy), TKIP (RSN with RC4) or CCMP (RSN with AES). If the AP is configured for TSN, the WEP-encrypted broadcast frames will easily yield the WEP key, exposing all broadcast traffic even if no associated station is using WEP. And if RSN is being deployed with preshared keys because setting up RADIUS and choosing a trustworthy authentication method are too difficult, chances are the same key is being used for WEP and preshared-key RSN! So much for robust security.

Sure, every security system has its weakest link. And RSN does address all three aspects of a security system--authentication, key distribution and data confidentiality. But it provides only legacy approaches to them all, and the legacy 802.11 Wi-Fi standard has shared keys for authentication; it has no key distribution and only weak data confidentiality. Using any of these legacy features in an RSN leaves the network compromised.

He Who Hesitates ...The task group will be working hard on RSN at its next meeting, which starts March 9 in Dallas. But don't wait until RSN is a done deal to start your transition.

Remove the risk of preshared keys in RSN by deploying RADIUS immediately. Many WEP-keyed products already support 802.1x and RADIUS, and several good authentication options are available. EAP-TLS (draft-ietf-pppext-eap-ttls-02.txt) is getting most of the attention lately, but EAP-AKA (draft-arkko-pppext-eap-aka-08.txt) is a viable alternative based on per-station shared secrets, and Wi-Fi Protected Access is an interim version of RSN based on a mid-2002 draft of 802.11i. Use any one of them to get your transition going. As long as legacy wireless stations are in your network, robust wireless security will remain out of reach.

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