Step 4 - Performing the Crack
Once a packet is successfully captured and the ARP replay starts, aireplay-ng will look something like Figure 8. Once again, the key is the "sent N packets", which now indicates the number of ARP packets injected by the spoofed STA.
Figure 8: aireplay with ARP replay running
You can now switch back to your airodump window and you should see that the #/s column should have increased from about zero to somewhere in the hundreds, as shown in Figure 9.
Figure 9: airodump with ARP replay running
You need to leave this running until the number in the #Data column reaches at least 300,000 IVs for a WEP 64 key or around 1,500,000 for a WEP 128 key. The problem is, with a "zero knowledge" attack, you don't know the length of the key, since it is not contained in any packets.
Since we knew we had set a 128 bit key, we waited until we had more than the suggested 1,500,000 IVs, which took about an hour, with the target AP and all notebooks involved in the same room. Under normal conditions with an AP located some distance away, it would take longer. We then opened a third shell window and started aircrack-ng:
aircrack-ng -b [AP BSSID] [capture file(s) name]
Note that the command can take a wildcard so that it uses all capture files. For our example, the command was:
aircrack-ng -b 00:06:25:B2:D4:19 capturefile*.ivs
Aircrack will start to chug through the captured packets trying to find the WEP key. This may take some time, and in some cases aircrack-ng will quit without finding the key, but offer some suggestions for things you might try. But when it succeeds, the aircrack screen will look like Figure 10.
Figure 10: aircrack-ng with key found
The 128 bit WEP key is in hexadecimal form and can be entered directly into a wireless client, omitting the ":".