How do I crack Unix passwords?
Contrary to popular belief, Unix passwords cannot be decrypted. Unix passwords are encrypted with a one way function. The login program encrypts the text you enter at the “password:” prompt and compares that encrypted string against the encrypted form of your password.
Password cracking software uses wordlists. Each word in the wordlist is encrypted with each of the 2600 possible salt values and the results are compared to the encrypted form of the target password.
The best cracking program for Unix passwords is currently Crack by Alec Muffett. For PC-DOS, the best package to use is currently CrackerJack.