1. To decrypt a password, or to bypass a copy protection scheme. See crackz for more about copy protection. History: When the UNIX operating system was first developed, passwords were stored in the file /etc/passwd. This file was made readable by everyone, but the passwords were encrypted so that a user could not figure out who a person's password was. The passwords were encrypted in such a manner that you could test a password to see if it was valid, but you really couldn't decrypt the entry. (Note: not even administrators are able to figure out user's passwords; they can change them, but not decrypt them). However, a program called "crack" was developed that would simply test all the words in the dictionary against the passwords in /etc/passwd. This would find all user accounts whose passwords where chosen from the dictionary. Typical dictionaries also included people's names since a common practice is to choose a spouse's or child's name. Contrast: A "crack" program is one that takes existing encrypted passwords and attempts to find some that are "weak" and easily discovered. However, it is not a "password guessing" program that tries to login with many passwords, that is known as a grind Key point: The sources of encrypted passwords typically include the following: /etc/passwd from a UNIX system SAM or SAM._ from a Windows NT system <username>.pwl from a Windows 95/98 system sniffed challenge hashes from the network Key point: The "crack" program is a useful tool for system administrators. By running the program on their own systems, they can quickly find users who have chosen weak passwords. In other words, it is a policy enforcement tool. Tools: on UNIX, the most commonly used program is called simply "crack". On Windows, a popular program is called "l0phtCrack" from http://www.l0pht.com/. From Hacking-Lexicon