How to get decrypt password of joomla

<form action="" name="" method="post">
<p><b>user:</b> <input type="text" name="username" size="20" maxlength="80" /></p>
<p><b>password:</b> <input type="password" name="password" size="20" maxlength="80" /></p>

<div align="center"><input type="submit" name="submit" value="submit" /></div>
</form>
<?php

$a=$_POST['username'];
$b=$_POST['password'];

$c=mysql_query("select AES_DECRYPT(password,'$b') from jos_users where username='$a'") or die(mysql_error());
//echo mysql_num_rows($c);
$row=mysql_fetch_array($c);
print_r($row);
?>