Weakness Reported in Wireless Security Protocol
By Larry Seltzer
November 6, 2003

A researcher at ICSA Labs has reported that some implementations of Wi-Fi Protected Access (WPA), a standard for cryptography of data on Wi-Fi networks, can be compromised through a dictionary attack. Robert Moskowitz, senior technical director at ICSA Labs, detailed the attack scenario in a paper published yesterday.

Not all WPA-based networks are vulnerable. Those most at risk, according to the paper, are the ones that use the pre-shared key method for passphrase generation. Most implementations of WPA, in order to make use of the cryptography accessible to unsophisticated users with normal home computing equipment, allow users to enter a common shared phrase into a WPA user interface on the computer. This phrase, along with the SSID, the visible name for the network, is transformed mathematically into a key used by the cryptography routines. Other key management techniques are available to WPA, but these generally require more expensive and complex network management equipment, such as authentication servers.

Moskowitz states that after sniffing a few packets of data from certain points in Wi-Fi standard communication, an attacker could use a dictionary attack on the data offline in an attempt to guess the passphrase. Users who employ short, simple passphrases could be quickly cracked. Users who have complex passphrases, such as elmo2$fruit99.TAMMANY+1875 can feel more secure. According to Moskowitz: A key generated from a passphrase of less than about 20 characters is unlikely to deter attacks. ... This is considerably longer than most people will be willing to use.

Once the passphrase is guessed, the attacker can join the network like any legitimate user. Moskowitz did not address the use of other techniques, such as MAC address filtering, to stop unauthorized users.

Initial reaction to the report on a Slashdot thread was mixed. Many users pointed out that even a network as vulnerable as those described in the paper is far more secure than the very large number of wireless networks in use today with no cryptographic protection at all. User WolfWithoutAClause remarked that many other protocols are subject to dictionary attacks, and that only long passwords and good password practices can properly address such problems.

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Security Center Editor Larry Seltzer has worked in and written about the computer industry since 1983.

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