John the Ripper is a free and Open Source password cracker. Its primary purpose is to detect weak Unix passwords and It is one of the most popular password testing and breaking programs. It combines a number of password crackers into one package, autodetects password hash types, and includes a customizable cracker. JTR is available in official Ubuntu repositories. A custom JTR build includes a nice gpg2john binary that converts your key file into a format that JTR understands.