By Juan Aguirre | April 24, 2017
Wi-Fi security has not always been the best. The first attempt at securing Wi-Fi access points was termed Wired Equivalent Privacy (WEP). WEP is a security algorithm that was implemented on IEEE 802.11 wireless networks. The original 802.11 wireless standard was ratified in 1997 to include this enhancement. Due to it’s many known vulnerabilities, this encryption method has mostly been replaced by WPA (Wi-Fi Protected Access) and WPA2 (Wi-Fi Protected Access II).
"A WEP key is a kind of security passcode for Wi-Fi devices. WEP keys enable a group of devices on a local network to exchange encrypted messages with each other while hiding the contents of the messages from easy viewing by outsiders" (Mitchell, 2017). The WEP key is the combination of a shared secret and the IV (Initialization Vector). The IV is a short 24-bit value and it is sent in the clear text portion of the message. The short length of the IV forces it to repeat itself. There exist a 50% possibility that the IV will repeat itself after only 5000 packets. This is dangerous because the reuse of the same IV produces identical key streams. This would allow an attacker to perform a successful analytic attack. Despite the many discovered and exploited vulnerabilities, we still see a number of access points using WEP encryption.
I heard about some folks at yashira.org who went out on a drive on a Wi-Fi hunt and found an access point using WEP. They quickly sniffed all the traffic they could and have now come to us with a challenge.
In this challenge, the folks at yashira.org have come to us with the capture file of all the traffic from the access point using WEP encryption. Our task is to find out the WEP key.
To solve this challenge we are going to be using aircrack. Aircrack is a complete suite of tools that are used to assess Wi-Fi network security. Aircrack is a very complete tool that has many usage options, all the options are important, they are there for a reason. With the correct options and data, aircrack can be used to crack any WEP key within minutes. A complete manual on how to install and use this tool can be found at: Aircrack Guide
WEP has many vulnerabilities and before we can exploit them we should understand how the basic authentication works. WEP’s authentication consist of a four-step handshake. 1. The client sends an authentication request to the access point 2. The access point responds with a clear text challenge 3. The client encrypts the challenge text using the configured WEP key and sends it back in another authentication request. 4. The access point decrypts the response. If it matches the challenge text, it sends back a positive response.
With some basic understanding of how WEP works we can start trying to crack the key. The first thing we need to do is look at the capture file and understand it. So let’s open a terminal on kali and run the basic aircrack command on it $aircrack-ng redwifi.cap. This command will attempt to decrypt the WEP key with the PTW method by default unless specified otherwise.
From this we see that we do not have enough IV packets to easily crack the key, we need an other way to do this. A dictionary attack is good but in order for it to work, we need more information.
If we had access to the network all we would need to do is listen for a long enough period of time and then we would have enough IVs and we would not need to perform a dictionary attack. If you are as impatient as I am and you do not want to sit around for hours waiting to get enough IVs, you can inject the traffic yourself. Wireless device packet injection allows you inject ARP traffic to the access point and then, with a sniffing tool, gather enough IV to crack the key. Since we no longer have access to the network and we count only with the capture file, we must try something else.
Although the previous command did not work it revealed some important information. We now have both the ESSID and the BSSID. The ESSID "WLAN_1F" tells us a lot. After a couple of questions to Mr.Google we can find that all the ESSID of type "WLAN_XX" are automatically generated and belong to a series of routers from Telefonica, a service provider. Further readings on the topic and on how the ESSID is generated reveal that the creation of the ID is based on the BSSID. There exist a number of tools that generate WEP dictionaries for Telefonica routers based on the BSSID and the ESSID. Wlandecrypter is one of them. We can generate a dictionary with the following command: $wlandecrypter [BSSID] [ESSID] wordlistdoc.lst
Now that we have the capture file and the dictionary file we can move on to the actual attack. We will be performing a brute force attack using a dictionary to reduce the number of forceful key attempts. To do this we can use the following command: $aircraack -w wordlistdoc.lst redwifi.cap
And there it is, we managed to successfully crack and decrypt the WEP key.
Given some basic knowledge, WEP encryption is relatively easy to decrypt and if the attacker has enough time to sniff a lot of traffic and obtain a large amount of IVs then the attacker can always successfully decrypt the key. If you are still using WEP encryption this should give you a couple of reasons to upgrade and if for some reason there is a technical restriction that forces you to use WEP, then here are two pieces of advice that can help you implemented in a more secure manner. 1. You must perform a periodic change of your keys. This will make it a lot more difficult for an attacker to perform a successful brute force attack on your network. 2. Use encrypted tunneling protocols which can provide secure data transmission over an insecure network. Such protocol include but are not limited to: a. IPSec b. Secure Shell
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