ASP prevent message Irrigation Instance code

ASP Prevent message Irrigation principle: Every time you submit a message, to enter a randomly generated 4-bit authentication code.

The following code is used in ASP
generate a random 4-digit number:

<%
Dim Key
Randomize Timer
Key=int ((8999) *rnd +1000)
%>

Show in form:

Authentication Code:<%=key%>
<input type= "text" name= "rekey" size= "8" maxlength= "4" >
<input value= "<%=key%>" type= "hidden" name= "key" >

Check after form submission:

key=request.form ("key")
rekey=request.form ("rekey")
if rekey<> key then
Response.Write ("<script language=javascript> Alert" Please enter the correct authentication code!) '); History.back () </script> ")
Response. End ()
End If