USS Clueless              Voyages of a restless mind

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Stardate 20010803.2235 (On Screen): So I guess we're seeing it proved yet again that secure crypto isn't guaranteed even in this age of uncrackable ciphers, if you make mistakes. 802.11 is a standard for wireless networking, used in a number of existing products (such as this and this). It was already known that it was vulnerable, and there had been previous cracks. However, the earlier cracks were slow and they required interaction with the system, and therefore the assault could be detected. The new one, however, is devastating.

First, this crack is totally passive. It's possible to crack the crypto simply by listening, which means that there's no way for the network to know it is being attacked. Second, the 40-bit key used by 802.11 can be cracked in fifteen minutes. Third and worst is that the crack scales linearly with the number of bits of the key. In other words, going to a 128 bit key would only increase the crack time to about an hour.

What should have happened is that each base station should have shipped from the factory with a unique public and private key burned into its flash memory. Then what would happen upon opening a session is that the remote would query for the public key, which would be sent in clear. The remote would calculate a session key, encrypt it and send it to the base station which would decrypt it, and then you'd use DES or AES thereafter. Then the only issue is how randomly the session keys were chosen by the remote, which is a much less serious problem. But even if a given session key were cracked, that would do no good for any other sessions since each would use its own. And if the public key for any given base station was cracked it wouldn't endanger any other base station.

Of course, that wasn't possible because 802.11 was designed back in the dark days when the US Government was imposing controls on strong encryption, which is why it is using a 40-bit key in the first place. But there's a difference between the size of the key and the actual strength of the encryption. The DVD CSS uses a 40 bit key but because of screwups it's actually only 26 bits strong (and can be cracked by brute force in a couple of seconds). Now it turns out that the 802.11 crypto is even weaker. If there's a lesson to be learned here it's that private citizens really do need strong crypto, and government controls on same do more harm than good. (But we knew that already.) (discuss)