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The use of Counter Mode with Cipher Block Chaining Message Authentication Code Protocol (CCMP) for WPA/WPA2 PSK is being attacked. This is a trivial attack (offline brute force) against the initial key exchange. It should be noted that the IEEE does not recognize this attack.

The concern is that there is a tool called Pyrit which claims to make 7.9 million password guesses per second or about 682.5 Billion per day. This is made possible by using the new Intel i7 chips which have the AES-NI instruction set. One of these chips costs less than $300.

What changes to WPA2-PSK are needed in order to mitigate this attack?

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2 Answers

up vote 9 down vote accepted

To be precise, the "new attack" is an optimization of brute-force, by using a slightly faster way to check whether a key is correct or not, mainly through the knowledge of the first few bytes of plaintext. This offers a speed increase of 50% -- in other words, attacks which took 6 days can now be done in 4 days. To put things in perspective, using a PC from next years will offer the same kind of speedup (but it is cumulative, of course).

The main issue with WPA2-PSK has not changed: the conversion from the password to the cryptographic keys is too fast. It should use the same tricks than for password storage, namely iterating hundreds or thousands of hash function invocations. It would not induce any noticeable slowdown in normal usage, but it would makes things much harder for the attacker.

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I was hoping for something a bit more creative than just key strengthening. +1 anyway. –  Rook May 2 '11 at 22:45
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For something more creative, lookup Password Authenticated Key Exchange protocols -- they combine an asymmetric key exchange (like Diffie-Hellman) with mutual password-based authentication which resists offline dictionary attacks. One exemple is SRP (see RFC 2945). But this implies structural changes quite beyond key strengthening. –  Thomas Pornin May 2 '11 at 23:07
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Strengthening existing (especially if known to be weak) protocols isn't about being creative, it's about avoiding its weaknesses and playing to its strengths. In case of WPA2-PSK merely having a non-default SSID helps a lot, as you cant download a pre-generated table, you have to generate it yourself. –  Marcin May 3 '11 at 2:44
    
Speed increase of 50%... Therefore what took six days now takes four? It would seem that I am missing something... Did you mean three? –  KnightOfNi Jan 30 at 22:26
    
No, I mean 4. "Speed increase of 50%" means "speed multiplied by 1.5", hence time is divided by 1.5. 6/1.5 = 4. –  Thomas Pornin Jan 31 at 0:11
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The best solution is to use strong pass phrase with WPA-PSK. See http://www.diceware.com for suggestions on how to generate a strong pass phrase that is relatively easy to remember.

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