I still see several wireless networks with weak or even no security enabled. However if somebody uploads illegally content, like copyrighted movies, you will be held responsible as holder of the internet connection.
As you may not know what kind of security your router has as it has been installed by somebody else or you can't change the configuration you can use a software stack on Ubuntu Linux to test security. Do NOT use this to crack a network you do not own!
This blog entry is based on this article. I found that some information is outdated or instructions were missing so I wrote this new article.
I am using Ubuntu Linux 11.10 / 32 Bit on a Lenovo Thinkpad.

First you should install some requirements:

sudo apt-get install git gtk-sharp2 libncurses5-dev gettext mono-dmcs pyrit

Now install aircrack-ng:

sudo apt-get install aircrack-ng

To install crunch-wordlist open a terminal and cd to your home directory

mkdir crunch-wordlist
cd crunch-wordlist
wget http://sourceforge.net/projects/crunch-wordlist/files/crunch-wordlist/crunch-3.1.tgz
tar xvfz crunch-3.1.tgz
cd crunch3.1
gedit Makefile

Navigate to line 44 and modify it so it reads:

crunch: crunch.c
@echo "Building binary..."
$(CC) $(CFLAGS) $(LFS) $? -o $@ -lm
@echo ""

Now continue to make the binaries:

make
sudo make install

To get mdk3 open your terminal in your home directory:

mkdir mdk3
cd mdk3
wget http://homepages.tu-darmstadt.de/~p_larbig/wlan/mdk3-v6.tar.bz2
bunzip2 mdk3-v6.tar.bz2
tar xvf mdk3-v6.tar
cd mdk3-v6
gedit Makefile

Edit the second line so it reads:

LINKFLAGS = -pthread

Now continue on the terminal:

make
sudo make install

To install the nice WEPCRACKGUI open another terminal in your home directory

git clone git://wepcrackgui.git.sourceforge.net/gitroot/wepcrackgui/wepcrackgui
/ cd wepcrackgui
./configure
make
cd GWepCrackGui/bin/Debug/
gksudo ./wepcrack

You should now see a program start like this:

You may need to go to Options - Preferences to edit the location of crunch. By default it installs to /pentest/passwords/crunch.
For better results I had to go to Driver specific options and change from AireplayConservative to WpaSupplicant.

If you like to test if your password is too easy to guess because it is one of the most common passwords, read here and here.

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