Generatore di password

<form name = "form_password">
   <input type = "text"   name  = "password"       readonly = "readonly" />
   <input type = "button" value = "NUOVA PASSWORD" onClick  = "genera()" />
</form>
 
<script>
function genera()
{
   var consonanti = String("bcdfglmnprstvz"); 
   var vocali     = String("aeiou");          
   var nc         = consonanti.length;
   var nv         = vocali.length;

   var str = "";
   for(var i=1; i <= 4; i++)
   {
      x1  = Math.floor(nc * Math.random()); 
      x2  = Math.floor(nv * Math.random()); 
      c1  = consonanti.charAt(x1); 
      c2  = vocali.charAt(x2);     
      str = str+c1+c2;
   }
   document.form_password.password.value = str;
}