3.2.22 crypt

crypt PLAINTEXT, SALT

This function encrypts a string exactly in the manner of crypt(3). This is useful for checking the password file for lousy passwords.[2] Only the guys wearing white hats are allowed to do this.

[2] What you really want to do is prevent people from adding the bad passwords in the first place.

To see whether a typed-in password $guess matches the password $pass obtained from a file (such as /etc/passwd), try something like the following:

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

Note that there is no easy way to decrypt an encrypted password apart from guessing. Also, truncating the salt to two characters is a waste of CPU time, although the manpage for crypt(3) would have you believe otherwise.

Here's an example that makes sure that whoever runs this program knows their own password:

$pwd = (getpwuid ($<))[1];
$salt = substr $pwd, 0, 2;

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

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

Of course, typing in your own password to whoever asks for it is unwise.

The crypt function is unsuitable for encrypting large quantities of data. Find a library module for PGP (or something like that) for something like that.