crypt

crypt 
string
, 
salt

Used by the passwd function on Unix systems to produce a unique 13-character string (stored in the system's password file) from the first 8 characters of the given string and the 2-character salt . The Perl function operates the same way, and returns a 13-character string with the first 2 characters being the salt . crypt uses a modified version of the Data Encryption Standard, which produces a one-way encryption; the resulting string cannot be decrypted to determine the original string. crypt can be used to check that a password is correct by comparing the string from the function to the string found in /etc/passwd (if you have permission to do this):

if (crypt ($guess, $pass) eq $pass) {
       # guess is correct
   }

The variable $pass is the password string from the password file. crypt merely uses the first two characters from this string for the salt argument.