How does the Newsham 21-bit attack work?

A lot of wireless access point offer a key generation process on basis of an easy to remember passphrase. Unfortunatel some companies implemented a very dangerous algorithm for the generation of 40-bit WEP keys. This algorithm generates only keys with an effective strength of 21-bits. KisMAC is able to bruteforce these keys in very short time.

Access Points from the following companies are confirmed vulnerable: Linksys, D-Link, Belkin and Netgear. There are undoubtably others.
The following companies use other algorithms and are safe: 3Com, Apple.

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