Wifi Hacking - WEP - Kali Linux Aircrack-ng suite

• Disclaimer - TLDR; some stuff here can be used to carry out illegal activity, our intention is, however, to educate

Alright, this post is written assuming you have Kali Linux up and running on your computer. If not, here is a post on hacking with kali linux. It will tell you about what Kali Linux is, and how to use it. It will guide you through installation process.
So if you are still following, then just follow these simple steps-

Firstly, create a wireless network to crack. Don't use this method on others. It is illegal. Then proceed with the steps below.

1. Find out the name of your wireless adapter.

Alright, now, your computer has many network adapters, so to scan one, you need to know its name. So there are basically the following things that you need to know-

• lo - loopback. Not important currently.
• eth - ethernet
• wlan - This is what we want. Note the suffix associated.

Now, to see all the adapters, type ifconfig on a terminal. See the result. Note down the wlan(0/1/2) adapter.

2. Enable Monitor mode

Now, we use a tool called airmon-ng to  create a virtual interface called mon. Just type 

airmon-ng start wlan0

 Your monitoring interface will be created - mon0 in case of Kali 1.x, wlan0mon in all other cases. 

3. Start capturing packets

Now, we'll use airodump-ng to capture the packets in the air. This tool gathers data from the wireless packets in the air. You'll see the name of the wifi you want to hack. For kali 2.0 or rolling, replace mon0 with wlan0mon

airodump-ng mon0

4. Store the captured packets in a file 

This can be achieved by giving some more parameters with the airodump command. For Kali 2.0 or rolling, replace mon0 with wlan0mon.

airodump-ng mon0 --write name_of_file

Now the captured packets will be stored in name_of_file.cap
You will have to wait till you have enough data (10000 minimum)
PS: Don't wait too long for this step though. Just understand how the procedure works (including the next sections), and once you are convinced you know what you are doing, proceed to the next tutorial where we use ARP replay to speed up the rate at which we gets packets. Using ARP request replay, we can get 10k packets in a few minutes.

5. Crack the wifi

If all goes well ,then you'll be sitting in front of your pc, grinning, finally you've got 10000 packets (don't stop the packet capture yet). Now, you can use aircrack-ng to crack the password. (in a new terminal)

aircrack-ng name_of_file-01.cap 

The program will ask which wifi to crack, if there are multiple available. Choose the wifi. It'll do its job. If the password is weak enough, then you'll get it in front of you. If not, the program will tell you to get more packets. The program will retry again when there are 15000 packets, and so on.

You'll get the key, probably in this format-
xx:xx:xx:xx:xx
Remove the colons
xxxxxxxxxx is the password of the wireless network

Issues?

Try this-

ifconfig wlan0 up
ifconfig wlan0 down

airmon-ng check kill

rfkill unblock all

Or this- (from Sebastian's comment below)

With the new version, the monitor is wlan0mon, and the monitor isn't working.
A solution is the commands:

ifconfig wlan0mon down
iwconfig wlan0mon mode monitor
ifconfig wlan0mon up

Source: http://stackoverflow.com/questions/32025472/airodump-ng-wlan0mon-doesnt-work

Disconnected from internet (wifi)?

Again, replace mon0 with wlan0mon for Kali 2.0 or rolling.

airmon-ng stop mon0

This is usually sufficient. If wlan0 is not up (check ifconfig or iwconfig), then do this (if you don't know what to do, then do this anyway)-

ifconfig wlan0 up

 If wifi still doesn't start, try this too

service network-manager restart