source: bootcd/isolinux/syslinux-6.03/utils/sha1pass @ e16e8f2

#!/usr/bin/perl

use bytes;
use Digest::SHA1;
use MIME::Base64;

sub random_bytes($) {
    my($n) = @_;
    my($v, $i);

    if ( open(RANDOM, '<', '/dev/random') ||
         open(RANDOM, '<', '/dev/urandom') ) {
        read(RANDOM, $v, $n);
    } else {
        # No real RNG available...
        srand($$ ^ time);
        $v = '';
        for ( $i = 0 ; $i < $n ; $i++ ) {
            $v .= ord(int(rand() * 256));
        }
    }

    return $v;
}


($pass, $salt) = @ARGV;

unless (defined($salt)) {
    $salt = MIME::Base64::encode(random_bytes(6), '');
}
$pass = Digest::SHA1::sha1_base64($salt, $pass);

print '$4$', $salt, '$', $pass, "\$\n";