var fieldStock = new String();
Field.onChanged = getMaskedWithDots;

function getMaskedWithDots()
{
     var caret = Selection.getCaretIndex();
     //文字列数の変化
     var differ = this.text.length-fieldStock.length;
     //追加/変更された文字列
     var changed = new String();
     for(var n=0; n<this.text.length; n++) if(this.text.charAt(n)!="●") changed += this.text.charAt(n);
     //適用
     fieldStock = fieldStock.substr(0,caret-changed.length) + changed + fieldStock.substr(caret-differ);
     
     //マスク
     this.text = "";
     while(this.text.length<fieldStock.length)
     {
          this.text += "●";
     }
     Selection.setSelection(caret, caret);
}