Crack is a freely available program designed to find standard Unix DES-encrypted passwords by
standard guessing techniques. It is written to be flexible, configurable and fast, and to be able to
make use of several networked hosts via the Berkeley rsh program (or similar), where possible.
This program checks your users' passwords for "guessable" values. It works by encrypting a list of
likely passwords and seeing if the result matches any of your user's encrypted passwords (which
must be provided to it). It is surprisingly effective and easy to use....