js text encryption converts English into numbers

<SCRIPT LANGUAGE="JavaScript">
<!-- Begin
var str_in;
var str_out = "";
var num_in;
var num_out = "";
var e = "Enter Text!";

function str_to_num(form) {
num_out = "";
if(form.input.value == "") alert(e);
else {
str_in = escape(form.input.value);
for(i = 0; i < str_in.length; i++) {
num_out += str_in.charCodeAt(i) - 23;
}
form.output.value = num_out;
form.input.value = "";
   }
}

function num_to_str(form) {
str_out = "";
if(form.output.value == "") alert(e)
else {
num_out = form.output.value; 
for(i = 0; i < num_out.length; i += 2) {
num_in = parseInt(num_out.substr(i,[2])) + 23;
num_in = unescape('%' + num_in.toString(16));
str_out += num_in;
}
form.input.value = unescape(str_out);
form.output.value = "";
   }
}
//  End -->
</script>
<center>
<form name=encryptform>
    <table>
      <tr>
        <td align=center>
          <input name=input type=text size=40 value="JavaScript Fairyland">
        </td>
        <td align=center>
          <input type=button value="<--" onClick="javascript:num_to_str(this.form)">
          <br>
          <input type=button value="-->" onClick="javascript:str_to_num(this.form)">
        </td>
        <td align=center>
          <input name=output type=text size=40>
        </td>
      </tr>
    </table>
</form>
</center>