Get the Whitepaper called Wi-Foo Ninjitsu Exploitation. It touches on methods of cracking keys, tools involved, and how to break basic simple defenses.
You can download it from the following link: https://packetstormsecurity.com/files/download/75158/wifoo-ninjitsu.txt
Source: https://packetstormsecurity.com/files/75158/Whitepaper-Called-Wi-Foo-Ninjitsu-Exploitation.html