Aircrack-ng is an 802.11a/b/g WEP/WPA cracking program that can recover a 40-bit, 104-bit, 256-bit or 512-bit WEP key once enough encrypted packets havebeen gathered. Also it can attack WPA1/2 networks with some advanced methods or simply by brute force.
Howto Install Aircrack-ng in ubuntu
open a terminal and type
$sudo apt-get install aircrack-ng
or use Synaptic package manager