WPA is an encryption algorithm that takes care of a lot of the vunerablities inherent in WEP. WEP is, by design, flawed. No matter how good or crappy, long or short, your WEP key is, it can be cracked. WPA is different. A WPA key can be made good enough to make cracking it unfeasible. WPA is also a little more cracker friendly. By capturing the right type of packets, you can do your cracking offline. This means you only have to be near the AP for a matter of seconds to get what you need. Advantages and disadvantages.
WPA basically comes in two flavours RADIUS or PSK. PSK is crackable, RADIUS is not so much.
PSK uses a user defined password to initialize the TKIP, temporal key integrity protocol. There is a password and the user is involved, for the most part that means it is flawed. The TKIP is not really crackable as it is a per-packet key but upon the initialization of the TKIP, like during an authentication, we get the password (well the PMK anyways). A robust dictionary attack will take care of a lot of consumer passwords.
Radius involves physical transferring of the key and encrypted channels blah blah blah, look it up to learn more about it but 90% of commerical APs do not support it, it is more of an enterprise solution then a consumer one.
The WPA handshake was designed to occur over insecure channels and in plaintext so the password is not actually sent across. There are some fancy dancy algorithms in the background that turn it into a primary master key, PMK, and the like but none of that really matters cause the PMK is enough to connect to the network.
The only step we need to do is capture a full authenication handshake from a real client and the AP. This can prove tricky without some packet injection, but if you are lucky to capture a full handshake, then you can leave and do the rest of the cracking at home.
We can force an authenication handshake by launching a Deauthentication Attack, but only if there is a real client already connected (you can tell in airodump). If there are no connected clients, you’re outta luck.
Like for WEP, we want to know the channel the WPA is sitting on, but the airodump command is slightly different. We don’t want just IVs so we don’t specify an IV flag. This will produce “lucid.cap” instead of “lucid.ivs”. Assume WPA is on channel 6 and wireless interface is ath0.
./airodump ath0 lucid 6
Dictionary Brute Force
The most important part of brute forcing a WPA password is a good dictionary. Check out http://www.openwall.com/wordlists/ for a ‘really’ good one. It costs money, but it’s the biggest and best I’ve ever seen (40 Million words, no duplicates, one .txt file). There is also a free reduced version from the same site but i’m sure resourceful people can figure out where to get a good dictionary from.
When you have a good dictionary the crack is a simple brute force attack:
./aircrack -a 2 -b 00:23:1F:55:04:BC -w /path/to/wordlist
Either you’ll get it or you won’t… depends on the strength of the password and if a dictionary attack can crack it.
Source : http://www.coderetard.com