Rainbow tables are sometimes used in preparation for this kind of operation.  
Rainbow tables are sometimes used in preparation for this kind of operation.  Popular tools to perform this operation include LCP, Cain and Abel and John the Ripper.  This operation is made more difficult on Unix systems through shadow files.  Stretching is a technique to slow-down brute-force approaches to this operation.  One method for performing this operation requires a lot of pre-computed data and is called a dictionary attack.  This operation is easier when the salt is either known or not used, and is complicated by the fact that a one-way trapdoor function was used to store the target of this operation.  This operation is more difficult when the target is longer and made up of non-alpha numeric symbols.  For 10 points, name this process by which one attempts to find a hidden key associated with a login.
Rainbow tables are sometimes used in preparation for this kind of operation.  
Password Cracking [accept equivalents for cracking like password guessing or password retrieving; accept "key" for "password" until mentioned]