The Perl crypt function is a one way function, you can use it to encrypt a string, but there is no equivalent decryption function. This function is not useful for cryptography because you can’t decrypt the information. You can rather use it to check a password:

• first you encrypt a password with a specific salt and store it somewhere
• when a user types in a password, that is encrypted using the same salt as the stored password
• the two pieces of encrypted data are compared and if they match, the password is correct

$encryptedString = crypt $string, $salt

• $string which is the string to be encrypted
• $salt that is used to select an encrypted version from many variations

Using the DES encryption method, the salt variable could be any two characters from the set:

{ '.', '/', 0 .. 9, 'A' .. 'Z', 'a' .. 'z }.

This set of characters is just a recommendation, you can include more characters here. The salt is stored in the encrypted string as the first two characters for later comparisons. Small changes in the $string or $salt will result in large changes in the resulting encrypted string. Please note that you need to use the same salt for the stored encrypted string and for the string you want to check.

To make things more clearly, let’s see an example:

#!/usr/local/bin/perl

use strict;
use warnings;

# initialze the string to be encrypted
my $psw = '1qazpoiu^^';
my $salt = 'ab';
# crypt and store the encrypted string in a variable
my $encryptedPsw = crypt $psw, $salt;
# print the encrypted password
print "$encryptedPsw\n"; 
# it displays: abzmCVGjNHc3M
# please note the presence of the salt at the 
# begining of the encrypted string

# read a password from STDIN
print "Your password:";
chomp (my $readPsw = <STDIN>);
if($encryptedPsw eq crypt ($readPsw, $encryptedPsw)) {
  print "Password OK\n";
} else {
  print "Password NOK\n";
}

abzmCVGjNHc3M
Your password:1qazpoiu^^
Password OK

It’s a good practice to use random characters for the salt as shown in the following example:

#!/usr/local/bin/perl

use strict;
use warnings;

sub encryptingPsw {
  my $psw = shift;
  my $count = shift; 
  my @salt = ('.', '/', 'a'..'z', 'A'..'Z', '0'..'9');  
  my $salt = "";
  $salt.= $salt[rand(63)] foreach(1..$count);
  print "\$salt = $salt\n";
  crypt($psw, $salt);
}

my $encryptedPsw = &encryptingPsw('ab#$cd', 4);
print "$encryptedPsw\n";

$salt = dBli
dBubxBHYJE73E

• the password to be encrypted
• the number of characters of the salt string

The characters of the $salt string are selected using the rand function and are concatenating using the . operator. At the end of the subroutine the $salt is printed and the Perl crypt function is called to encrypt the input password with the random salt. You don’t need to use the return function explicitly because the subroutine will return by default the result of the last executed statement – in our case the encrypted password returned by the Perl crypt function.

If you look at the output you can notice that only the first two characters from the salt are present at the beginning of the encrypted string, because only the first two characters from salt are used in this implementation of the Perl crypt function. But if for other implementation of the Perl crypt function more than two characters will be used, the previous example subroutine will work for you.

The next example is for a Unix/Linux platform and assures that whoever runs this program knows his password:

#!/usr/local/bin/perl

use strict;
use warnings;

# get the user password
$pwd = (getpwuid($<))[1];

system "stty -echo";
print "Password: ";
chomp($word = <STDIN>);
print "\n";
system "stty echo";

if (crypt($word, $pwd) ne $pwd) {
   die "Sorry wrong password\n";
} else {
   print "ok, correct password\n";
}

($name,$passwd,$uid,$gid,$quota,$comment,$gcos,$dir,$shell,$expire)

where the password element has the index 1. So (getpwuid($<))[1] will return the password of the current user.

The system function is used to put first the terminal into 'no echo' mode, then just read the password normally. After reading the password, the terminal will be put back in the 'echo' mode.

Finally, the Perl crypt function will be used to check up if the password typed by the user is correct. (Unix / Linux servers use DES in order to encrypt the passwords).