WPA PSK Crackers: Loose Lips Sink Ships - Page 2

By Lisa Phifer

March 23, 2007

Now, coWPAtty starts trudging through the supplied dictionary file, trying out words as possible passphrases, trying to come up with the right PSK. The right PSK will be one that, when used as a PMK with all of these observed values, ends up generating a matching MIC. This is shown in the following extra-verbose coWPAtty output:

$cowpatty -vv -r capturefile -s soho-psk -f veryshortlist
cowpatty 2.0 - WPA-PSK dictionary attack. 
Collected all necessary data to mount crack against passphrase.
AA is:
         0020 a64f 31e4                           . .O1.
SPA is:
         000c 41da f2e7                           ..A...
snonce is:
         ed12 afbd a8c5 8305 0032 e5b5 2953 82d2  .........2..)S..
         7956 fd58 4a63 43ba fe49 135f 2695 2a0f  yV.XJcC..I._&.*.
anonce is:
         477b a8dc 6d7e 80d0 1a30 9d35 891d 868e  G{..m~...0.5....
         b82b cc3b 5d52 b5a9 a42c 4cb7 fd34 3a64  .+.;]R...,L..4:d
keymic is:
         f3a0 f691 4e28 a2df 1030 61a4 1ee8 3878  ....N(...0a...8x
eapolframe is:
         0103 005f fe01 0900 0000 0000 0000 0000  ..._............
         0200 0000 0000 0000 0000 0000 0000 0000  ................
         0000 0000 0000 0000 0000 0000 0000 0000  ................
         0000 0000 0000 0000 0000 0000 0000 0000  ................
         0000 0000 0000 0000 0000 0000 0000 0000  ................
         00f3 a0f6 914e 28a2 df10 3061 a41e e838  .....N(...0a...8
         7800                                     x. 00
Starting dictionary attack.  Please be patient.
Testing passphrase: secretsecret
Calculating PMK for "secretsecret".
Calculating PTK with collected data and PMK.
Calculating hmac-MD5 Key MIC for this frame.
The PSK is "secretsecret".

Making a Bad Situation Worse

 A bare minimum PSK (8 lowercase letters) has 26^8 possible combinations, so even just trying all 208,827,064,576 of those possible passphrases would take far too long. Working from a dictionary file cuts that effort by just trying all 8-characters words. Huge password dictionaries are readily available for use with conventional Windows/UNIX password crackers like John the Ripper, and they can be fed into PSK crackers. But multi-pass hashing for every word in those files still takes time -- depending on SSID and PSK length, a lot of time. In the above example, coWPAtty tested just 36 passphrases per second.

Unfortunately, conventional password crackers know how to get around this problem using rainbow tables -- long lists of pre-computed password hashes. The latest version of coWPAtty can now use a variation on rainbow tables to speed PSK cracking by three orders of magnitude. For example, a 2006 Shmoocon demo showed coWPAtty testing 18,000 passphrases per second using a pre-hashed WPA PSK lookup table.

Remember that WPA PSKs combine the passphrase with the WLAN's SSID. A pre-hashed PSK lookup table therefore depends on the target WLAN's name. Given enough time, storage and CPU, you can generate a table for any SSID using the "genpmk" program now included with coWPAtty. Although genpmk itself doesn't really take less time, the generated tables can be fed into coWPAtty to crack individual PSKs much, much faster. Or you could feed coWPAtty one of the pre-hashed WPA PSK lookup tables posted online, representing 170,000 words hashed against the top 1000 most common SSIDs.

How to Protect Yourself

At this point, you have probably noticed several vulnerabilities that WPA PSK crackers exploit. There is nothing you can do about some of those vulnerabilities (like the values exchanged during the four-way handshake). But there are several steps you can take to mitigate other vulnerabilities and protect your WLAN against WPA PSK crackers.

  1. WPA PSKs are particularly easy to guess if you choose a passphrase that is composed of word(s) easily found in a password dictionary, or words that anyone might easily associate with you (e.g., your surname, your pet's name). The strongest passphrases are not words but randomly-generated strings that mix case, letters and numbers.
  2. WPA PSKs that are too short are also much easier to guess. When configuring a passphrase, the IEEE 802.11i standard strongly recommends using at least 20 characters. If you can't come up with your own long random strings, try a random password generator (e.g., keygen, wlankeygen).
  3. Some products let you configure actual 256-bit PSK values, rather than entering the ASCII passphrase used to generate the WPA PSK. In a few products, you can enter the PSK as a hex string. In others, pushing a button causes the AP and station to derive their own random PSK (e.g., Broadcom SecureEasySetup). In January, the Wi-Fi Alliance announced Wi-Fi Protected Setup (WPS), a certification program intended to simplify configuration -- including strong PSK generation. Just 17 products have been certified to date, but more will emerge.
  4. Given the existence of published WPA PSK lookup tables for common SSIDs, it can also be helpful to give your WLAN a unique SSID. Using a common or default SSID increases the odds of a successful WPA PSK dictionary attack against your WLAN.
  5. WPA PSK crackers can be avoided altogether by stepping up to WPA/WPA2-Enterprise. Small businesses should give this very serious consideration. Aside from crackers, passphrases have all the usual password drawbacks, like when workers share your passphrase or lose a configured laptop. If 802.1X sounds too hard, check out entry-level solutions (e.g., Witopia SecureMyWiFi, ZyXEL ZyAir B-1000).
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