In this article by Cameron Buchanan, author of the book Kali Linux Wireless Penetration Testing Beginner’s Guide.

(For more resources related to this topic, see here.)

“640K is more memory than anyone will ever need.”

                                                                            Bill Gates, Founder, Microsoft

Even with the best of intentions, the future is always unpredictable. The WLAN committee designed WEP and then WPA to be foolproof encryption mechanisms but, over time, both these mechanisms had flaws that have been widely publicized and exploited in the real world.

WLAN encryption mechanisms have had a long history of being vulnerable to cryptographic attacks. It started with WEP in early 2000, which eventually was completely broken. In recent times, attacks are slowly targeting WPA. Even though there is no public attack available currently to break WPA in all general conditions, there are attacks that are feasible under special circumstances.

In this section, we will take a look at the following topics:

• Different encryption schemas in WLANs
• Cracking WEP encryption
• Cracking WPA encryption

 

WLAN encryption

WLANs transmit data over the air and thus there is an inherent need to protect data confidentiality. This is best done using encryption. The WLAN committee (IEEE 802.11) formulated the following protocols for data encryption:

• Wired Equivalent Privacy (WEP)
• Wi-Fi Protected Access (WPA)
• Wi-Fi Protection Access v2 (WPAv2)

In this section, we will take a look at each of these encryption protocols and demonstrate various attacks against them.

WEP encryption

The WEP protocol was known to be flawed as early as 2000 but, surprisingly, it is still continuing to be used and access points still ship with WEP enabled capabilities.

There are many cryptographic weaknesses in WEP and they were discovered by Walker, Arbaugh, Fluhrer, Martin, Shamir, KoreK, and many others. Evaluation of WEP from a cryptographic standpoint is beyond the scope, as it involves understanding complex math. In this section, we will take a look at how to break WEP encryption using readily available tools on the BackTrack platform. This includes the entire aircrack-ng suite of tools—airmon-ng, aireplay-ng, airodump-ng, aircrack-ng, and others.

The fundamental weakness in WEP is its use of RC4 and a short IV value that is recycled every 224 frames. While this is a large number in itself, there is a 50 percent chance of four reuses every 5,000 packets. To use this to our advantage, we generate a large amount of traffic so that we can increase the likelihood of IVs that have been reused and thus compare two cipher texts encrypted with the same IV and key.

Let’s now first set up WEP in our test lab and see how we can break it.

Time for action – cracking WEP

Follow the given instructions to get started:

1. Let’s first connect to our access point Wireless Lab and go to the settings area that deals with wireless encryption mechanisms:

   

2. On my access point, this can be done by setting the Security Mode to WEP. We will also need to set the WEP key length. As shown in the following screenshot, I have set WEP to use 128bit keys. I have set the default key to WEP Key 1 and the value in hex to abcdefabcdefabcdefabcdef12 as the 128-bit WEP key. You can set this to whatever you choose:

   

3. Once the settings are applied, the access point should now be offering WEP as the encryption mechanism of choice. Let’s now set up the attacker machine.
4. Let’s bring up Wlan0 by issuing the following command:

   ifconfig wlan0 up

5. Then, we will run the following command:

   airmon-ng start wlan0

6. This is done so as to create mon0, the monitor mode interface, as shown in the following screenshot. Verify that the mon0 interface has been created using the iwconfig command:

   

7. Let’s run airodump-ng to locate our lab access point using the following command:

   airodump-ng mon0

8. As you can see in the following screenshot, we are able to see the Wireless Lab access point running WEP:

   

9. For this exercise, we are only interested in the Wireless Lab, so let’s enter the following command to only see packets for this network:

   airodump-ng –bssid 00:21:91:D2:8E:25 --channel 11 --write WEPCrackingDemo mon0

   The preceding command line is shown in the following screenshot:

   

10. We will request airodump-ng to save the packets into a pcap file using the –write directive:

    

11. Now let’s connect our wireless client to the access point and use the WEP key as abcdefabcdefabcdefabcdef12. Once the client has successfully connected, airodump-ng should report it on the screen.
12. If you do an ls in the same directory, you will be able to see files prefixed with WEPCrackingDemo-*, as shown in the following screenshot. These are traffic
    dump files created by airodump-ng:

    

13. If you notice the airodump-ng screen, the number of data packets listed under the #Data column is very few in number (only 68). In WEP cracking, we need a large number of data packets, encrypted with the same key to exploit weaknesses in the protocol. So, we will have to force the network to produce more data packets. To do this, we will use the aireplay-ng tool:

    

14. We will capture ARP packets on the wireless network using Aireplay-ng and inject them back into the network to simulate ARP responses. We will be starting Aireplay-ng in a separate window, as shown in the next screenshot. Replaying these packets a few thousand times, we will generate a lot of data traffic on the network. Even though Aireplay-ng does not know the WEP key, it is able to identify the ARP packets by looking at the size of the packets. ARP is a fixed header protocol; thus, the size of the ARP packets can be easily determined and can be used to identify them even within encrypted traffic. We will run aireplay-ng with the options that are discussed next. The -3 option is for ARP replay, -b specifies the BSSID of our network, and -h specifies the client MAC address that we are spoofing. We need to do this, as replay attacks will only work for authenticated and associated client MAC addresses:

    

15. Very soon you should see that aireplay-ng was able to sniff ARP packets and started replaying them into the network. If you encounter channel-related errors as I did, append –ignore-negative-one to your command, as shown in the following screenshot:

    

16. At this point, airodump-ng will also start registering a lot of data packets. All these sniffed packets are being stored in the WEPCrackingDemo-* files that
    we saw previously:

    

17. Now let’s start with the actual cracking part! We fire up aircrack-ng with the option WEPCRackingDemo-0*.cap in a new window. This will start the aircrack-ng software and it will begin working on cracking the WEP key using the data packets in the file. Note that it is a good idea to have Airodump-ng collect the WEP packets, aireplay-ng do the replay attack, and aircrack-ng attempt to crack the WEP key based on the captured packets, all at the same time. In this experiment, all of them are open in separate windows.
18. Your screen should look like the following screenshot when aircrack-ng is working on the packets to crack the WEP key:

    

19. The number of data packets required to crack the key is nondeterministic, but generally in the order of a hundred thousand or more. On a fast network (or using aireplay-ng), this should take 5-10 minutes at most. If the number of data packets currently in the file is not sufficient, then aircrack-ng will pause, as shown in the following screenshot, and wait for more packets to be captured; it will then restart the cracking process:

    

20. Once enough data packets have been captured and processed, aircrack-ng should be able to break the key. Once it does, it proudly displays it in the terminal and exits, as shown in the following screenshot:

    

21. It is important to note that WEP is totally flawed and any WEP key (no matter how complex) will be cracked by Aircrack-ng. The only requirement is that a large enough number of data packets, encrypted with this key, are made available to aircrack-ng.

What just happened?

We set up WEP in our lab and successfully cracked the WEP key. In order to do this, we first waited for a legitimate client of the network to connect to the access point. After this, we used the aireplay-ng tool to replay ARP packets into the network. This caused the network to send ARP replay packets, thus greatly increasing the number of data packets sent over the air. We then used the aircrack-ng tool to crack the WEP key by analyzing cryptographic weaknesses in these data packets.

Note that we can also fake an authentication to the access point using the Shared Key Authentication bypass technique. This can come in handy if the legitimate client leaves the network. This will ensure that we can spoof an authentication and association and continue to send our replayed packets into the network.

Have a go hero – fake authentication with WEP cracking

In the previous exercise, if the legitimate client had suddenly logged off the network, we would not have been able to replay the packets as the access point will refuse to accept packets from un-associated clients.

While WEP cracking is going on. Log off the legitimate client from the network and verify that you are still able to inject packets into the network and whether the access point accepts and responds to them.

WPA/WPA2

WPA( or WPA v1 as it is referred to sometimes) primarily uses the TKIP encryption algorithm. TKIP was aimed at improving WEP, without requiring completely new hardware to run it. WPA2 in contrast mandatorily uses the AES-CCMP algorithm for encryption, which is much more powerful and robust than TKIP.

Both WPA and WPA2 allow either EAP-based authentication, using RADIUS servers (Enterprise) or a Pre-Shared key (PSK) (personal)-based authentication schema.

WPA/WPA2 PSK is vulnerable to a dictionary attack. The inputs required for this attack are the four-way WPA handshake between client and access point, and a wordlist that contains common passphrases. Then, using tools such as Aircrack-ng, we can try to crack the WPA/WPA2 PSK passphrase.

An illustration of the four-way handshake is shown in the following screenshot:

The way WPA/WPA2 PSK works is that it derives the per-session key, called the Pairwise Transient Key (PTK), using the Pre-Shared Key and five other parameters—SSID of Network, Authenticator Nounce (ANounce), Supplicant Nounce (SNounce), Authenticator MAC address (Access Point MAC), and Suppliant MAC address (Wi-Fi Client MAC). This key is then used to encrypt all data between the access point and client.

An attacker who is eavesdropping on this entire conversation by sniffing the air can get all five parameters mentioned in the previous paragraph. The only thing he does not have is the Pre-Shared Key. So, how is the Pre-Shared Key created? It is derived by using the WPA-PSK passphrase supplied by the user, along with the SSID. The combination of both of these is sent through the Password-Based Key Derivation Function (PBKDF2), which outputs the 256-bit shared key.

In a typical WPA/WPA2 PSK dictionary attack, the attacker would use a large dictionary of possible passphrases with the attack tool. The tool would derive the 256-bit Pre-Shared key from each of the passphrases and use it with the other parameters, described earlier, to create the PTK. The PTK will be used to verify the Message Integrity Check (MIC) in one of the handshake packets. If it matches, then the guessed passphrase from the dictionary was correct; if not, it was incorrect.

Eventually, if the authorized network passphrase exists in the dictionary, it will be identified. This is exactly how WPA/WPA2 PSK cracking works! The following figure illustrates the
steps involved:

In the next exercise, we will take a look at how to crack a WPA PSK wireless network. The exact same steps will be involved in cracking a WPA2-PSK network using CCMP(AES) as well.

Time for action – cracking WPA-PSK weak passphrases

Follow the given instructions to get started:

1. Let’s first connect to our access point Wireless Lab and set the access point to use WPA-PSK. We will set the WPA-PSK passphrase to abcdefgh so that it is vulnerable to a dictionary attack:

   

2. We start airodump-ng with the following command so that it starts capturing and storing all packets for our network:

   airodump-ng –bssid 00:21:91:D2:8E:25 –channel 11 –write WPACrackingDemo mon0"

   The following screenshot shows the output:

   

3. Now we can wait for a new client to connect to the access point so that we can capture the four-way WPA handshake, or we can send a broadcast deauthentication packet to force clients to reconnect. We do the latter to speed things up. The same thing can happen again with the unknown channel error. Again, use –-ignore-negative-one. This can also require more than one attempt:

   

4. As soon as we capture a WPA handshake, the airodump-ng tool will indicate it in the top-right corner of the screen with a WPA handshake followed by the access point’s BSSID. If you are using –ignore-negative-one, the tool may replace the WPA handshake with a fixed channel message. Just keep an eye out for a quick flash of a WPA handshake.
5. We can stop the airodump-ng utility now. Let’s open up the cap file in Wireshark and view the four-way handshake. Your Wireshark terminal should look like the following screenshot. I have selected the first packet of the four-way handshake in the trace file in the screenshot. The handshake packets are the one whose protocol is EAPOL:

   

6. Now we will start the actual key cracking exercise! For this, we need a dictionary of common words. Kali ships with many dictionary files in the metasploit folder located as shown in the following screenshot. It is important to note that, in WPA cracking, you are just as good as your dictionary. BackTrack ships with some dictionaries, but these may be insufficient. Passwords that people choose depend on a lot of things. This includes things such as which country users live in, common names and phrases in that region the, security awareness of the users, and a host of other things. It may be a good idea to aggregate country- and region-specific word lists, when undertaking a penetration test:

   

7. We will now invoke the aircrack-ng utility with the pcap file as the input and a link to the dictionary file, as shown in the following screenshot. I have used nmap.lst , as shown in the terminal:

   

8. aircrack-ng uses the dictionary file to try various combinations of passphrases and tries to crack the key. If the passphrase is present in the dictionary file, it will eventually crack it and your screen will look similar to the one in the screenshot:

   

9. Please note that, as this is a dictionary attack, the prerequisite is that the passphrase must be present in the dictionary file you are supplying to aircrack-ng. If the passphrase is not present in the dictionary, the attack will fail!

What just happened?

We set up WPA-PSK on our access point with a common passphrase: abcdefgh. We then use a deauthentication attack to have legitimate clients reconnect to the access point. When we reconnect, we capture the four-way WPA handshake between the access point and the client.

As WPA-PSK is vulnerable to a dictionary attack, we feed the capture file that contains the WPA four-way handshake and a list of common passphrases (in the form of a wordlist) to Aircrack-ng. As the passphrase abcdefgh is present in the wordlist, Aircrack-ng is able to crack the WPA-PSK shared passphrase. It is very important to note again that, in WPA dictionary-based cracking, you are just as good as the dictionary you have. Thus, it is important to compile a large and elaborate dictionary before you begin. Though BackTrack ships with its own dictionary, it may be insufficient at times and might need more words, especially taking into account the localization factor.

Have a go hero – trying WPA-PSK cracking with Cowpatty

Cowpatty is a tool that can also crack a WPA-PSK passphrase using a dictionary attack. This tool is included with BackTrack. I leave it as an exercise for you to use Cowpatty to crack the WPA-PSK passphrase.

Also, set an uncommon passphrase that is not present in the dictionary and try the attack again. You will now be unsuccessful in cracking the passphrase with both Aircrack-ng and Cowpatty.

It is important to note that the same attack applies even to a WPA2 PSK network. I encourage you to verify this independently.

Speeding up WPA/WPA2 PSK cracking

As we have already seen in the previous section, if we have the correct passphrase in our dictionary, cracking WPA-Personal will work every time like a charm. So, why don’t we just create a large elaborate dictionary of millions of common passwords and phrases people use? This would help us a lot and most of the time, we would end up cracking the passphrase. It all sounds great but we are missing one key component here— the time taken. One of the more CPU and time-consuming calculations is that of the Pre-Shared key using the PSK passphrase and the SSID through the PBKDF2. This function hashes the combination of both over 4,096 times before outputting the 256-bit Pre-Shared key. The next step in cracking involves using this key along with parameters in the four-way handshake and verifying against the MIC in the handshake. This step is computationally inexpensive. Also, the parameters will vary in the handshake every time and hence, this step cannot be precomputed. Thus, to speed up the cracking process, we need to make the calculation of the Pre-Shared key from the passphrase as fast as possible.

We can speed this up by precalculating the Pre-Shared Key, also called the Pairwise Master Key (PMK) in 802.11 standard parlance. It is important to note that, as the SSID is also used to calculate the PMK, with the same passphrase and with a different SSID, we will end up with a different PMK. Thus, the PMK depends on both the passphrase and the SSID.

In the next exercise, we will take a look at how to precalculate the PMK and use it for WPA/WPA2 PSK cracking.

Time for action – speeding up the cracking process

We can proceed with the following steps:

1. We can precalculate the PMK for a given SSID and wordlist using the genpmk tool with the following command:

   genpmk –f <chosen wordlist>–d PMK-Wireless-Lab –s "Wireless Lab

   This creates the PMK-Wireless-Lab file containing the pregenerated PMK:

   

2. We now create a WPA-PSK network with the passphrase abcdefgh (present in the dictionary we used) and capture a WPA-handshake for that network. We now use Cowpatty to crack the WPA passphrase, as shown in the following screenshot:

   

   It takes approximately 7.18 seconds for Cowpatty to crack the key, using the precalculated PMKs.

3. We now use aircrack-ng with the same dictionary file, and the cracking process takes over 22 minutes. This shows how much we are gaining because of the precalculation.
4. In order to use these PMKs with aircrack-ng, we need to use a tool called airolib-ng. We will give it the options airolib-ng, PMK-Aircrack –import,and cowpatty PMK-Wireless-Lab, where PMK-Aircrack is the aircrack-ng compatible database to be created and PMK-Wireless-Lab is the genpmk compliant PMK database that we created previously.
5. We now feed this database to aircrack-ng and the cracking process speeds up remarkably. We use the following command:

   aircrack-ng –r PMK-Aircrack WPACrackingDemo2-01.cap

6. There are additional tools available on BackTrack such as Pyrit that can leverage multi CPU systems to speed up cracking. We give the pcap filename with the -r option and the genpmk compliant PMK file with the -i option. Even on the same system used with the previous tools, Pyrit takes around 3 seconds to crack the key, using the same PMK file created using genpmk.

What just happened?

We looked at various different tools and techniques to speed up WPA/WPA2-PSK cracking. The whole idea is to pre-calculate the PMK for a given SSID and a list of passphrases in
our dictionary.

Decrypting WEP and WPA packets

In all the exercises we have done till now, we cracked the WEP and WPA keys using various techniques. What do we do with this information? The first step is to decrypt data packets we have captured using these keys.

In the next exercise, we will decrypt the WEP and WPA packets in the same trace file that we captured over the air, using the keys we cracked.

Time for action – decrypting WEP and WPA packets

We can proceed with the following steps:

1. We will decrypt packets from the WEP capture file we created earlier: WEPCrackingDemo-01.cap. For this, we will use another tool in the Aircrack-ng suite called airdecap-ng. We will run the following command, as shown in the following screenshot, using the WEP key we cracked previously:

   airdecap-ng -w abcdefabcdefabcdefabcdef12 WEPCrackingDemo-02.cap

   

2. The decrypted files are stored in a file named WEPCrackingDemo-02-dec.cap. We use the tshark utility to view the first ten packets in the file. Please note that you may see something different based on what you captured:

   

3. WPA/WPA2 PSK will work in exactly the same way as with WEP, using the airdecap-ng utility, as shown in the following screenshot, with the following command:

   airdecap-ng –p abdefg WPACrackingDemo-02.cap –e "Wireless Lab"

   

What just happened?

We just saw how we can decrypt WEP and WPA/WPA2-PSK encrypted packets using Airdecap-ng. It is interesting to note that we can do the same using Wireshark. We would encourage you to explore how this can be done by consulting the Wireshark documentation.

Connecting to WEP and WPA networks

We can also connect to the authorized network after we have cracked the network key. This can come in handy during penetration testing. Logging onto the authorized network with the cracked key is the ultimate proof you can provide to your client that his network is insecure.

Time for action – connecting to a WEP network

We can proceed with the following steps:

1. Use the iwconfig utility to connect to a WEP network, once you have the key. In a past exercise, we broke the WEP key—abcdefabcdefabcdefabcdef12:

   

What just happened?

We saw how to connect to a WEP network.

Time for action – connecting to a WPA network

We can proceed with the following steps:

1. In the case of WPA, the matter is a bit more complicated. The iwconfig utility cannot be used with WPA/WPA2 Personal and Enterprise, as it does not support it. We will use a new tool called WPA_supplicant for this lab. To use WPA_supplicant for a network, we will need to create a configuration file, as shown in the following screenshot. We will name this file wpa-supp.conf:

   

2. We will then invoke the WPA_supplicant utility with the following options: –D wext -i wlan0 –c wpa-supp.conf to connect to the WPA network we just cracked. Once the connection is successful, WPA_supplicant will give you the message: Connection to XXXX completed.
3. For both the WEP and WPA networks, once you are connected, you can use dhcpclient to grab a DHCP address from the network by typing dhclient3 wlan0.

What just happened?

The default Wi-Fi utility iwconfig cannot be used to connect to WPA/WPA2 networks. The de-facto tool for this is WPA_Supplicant. In this lab, we saw how we can use it to connect to a WPA network.

Summary

In this section, we learnt about WLAN encryption. WEP is flawed and no matter what the WEP key is, with enough data packet samples: it is always possible to crack WEP. WPA/WPA2 is cryptographically un-crackable currently; however, under special circumstances, such as when a weak passphrase is chosen in WPA/WPA2-PSK, it is possible to retrieve the passphrase using dictionary attacks.

Resources for Article:

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Further resources on this subject:

• Veil-Evasion [article]
• Wireless and Mobile Hacks [article]
• Social Engineering Attacks [article]

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