为什么需要验证码

有效防止某个黑客对某一个特定注册用户，用特定程序暴力破解方式进行不断的登陆尝试，即防止暴力注册/穷取密码/服务器拒绝等可能影响网站正常运行的行为。

验证码的运行机制是什么

在随机产生验证码的同时生成了一Session，然后通过表单递交数据与Session中保存的验证码进行比较，正确即验证码通过，否则出错提示。

1，XBM文件

这是一个很简单的显示XBM图片的例子，数字掩码是主要。

<%@ Language=javascript %>
<%
//一个最简单的例子
//可广泛应用于计数器、时钟等任何需要动态生成图形的地方
//通过改变ContentType的值，比如改为 "image/gif"可以作出更漂亮的图形

Response.ContentType ="image/x-xbitmap";
Response.Write ("#define counter_width 8\r\n"); //图形宽
Response.Write ("#define counter_height 10\r\n");//图形高
Response.Write ("static unsigned char counter_bits[]={\r\n");
Response.Write ("0x3c,0x66,0x60,0x60,0x30,0x18,0x0c,0x06,0x06,0x7e");//2的掩码
Response.Write ("};\r\n");

%>

2，黑白XBM随机验证码（原作者：张三疯）

xbm.asp：主要作用，随机显示四位验证码，同时产生Session("validateCode")

<!--#include file="numcode.asp"-->
<%
Response.Buffer = True
With Response
.Expires = -1
.AddHeader "Pragma","no-cache"
.AddHeader "cache-ctrol","no-cache"
End With

Dim num
Randomize
num = Int(7999 * Rnd + 2000)
Session("validateCode") = num

Dim Image
Dim Width, Height
Dim digtal
Dim Length
Dim sort
Dim hc
Length = 4
hc = chr(13) & chr(10)

Redim sort(Length)

digital = ""
For I = 1 To Length - Len(num)
digital = digital & "0"
Next
For I = 1 To Len(num)
digital = digital & Mid(num, I, 1)
Next
For I = 1 To Len(digital)
sort(I) = Mid(digital, I, 1)
Next
Width = 8 * Len(digital)
Height = 10

Response.ContentType = "image/x-xbitmap"

Image = "#define counter_width " & Width & hc
Image = Image & "#define counter_height " & Height & hc
Image = Image & "static unsigned char counter_bits[] = {" & hc

For I = 1 To Height
For J = 1 To Length
Image = Image & a(sort(J),I) & ","
Next
Next

Image = Left(Image, Len(Image) - 1)
Image = Image & "};" & hc

Response.Write Image
%>

其中包含文件<!--#include file="numcode.asp"-->的主要内容就是数字的掩码

numcode.asp

<%
Dim a(10,10)

a(0,1) = "0x3c" '数字0
a(0,2) = "0x66"
a(0,3) = "0xc3"
a(0,4) = "0xc3"
a(0,5) = "0xc3"
a(0,6) = "0xc3"
a(0,7) = "0xc3"
a(0,8) = "0xc3"
a(0,9) = "0x66"
a(0,10)= "0x3c"

a(1,1) = "0x18" '数字1
a(1,2) = "0x1c"
a(1,3) = "0x18"
a(1,4) = "0x18"
a(1,5) = "0x18"
a(1,6) = "0x18"
a(1,7) = "0x18"
a(1,8) = "0x18"
a(1,9) = "0x18"
a(0,10)= "0x7e"

a(2,1) = "0x3c" '数字2
a(2,2) = "0x66"
a(2,3) = "0x60"
a(2,4) = "0x60"
a(2,5) = "0x30"
a(2,6) = "0x18"
a(2,7) = "0x0c"
a(2,8) = "0x06"
a(2,9) = "0x06"
a(2,10)= "0x7e"

a(3,1) = "0x3c" '数字3
a(3,2) = "0x66"
a(3,3) = "0xc0"
a(3,4) = "0x60"
a(3,5) = "0x1c"
a(3,6) = "0x60"
a(3,7) = "0xc0"
a(3,8) = "0xc0"
a(3,9) = "0x66"
a(3,10)= "0x38"

a(4,1) = "0x38" '数字4
a(4,2) = "0x3c"
a(4,3) = "0x36"
a(4,4) = "0x33"
a(4,5) = "0x33"
a(4,6) = "0x33"
a(4,7) = "0xff"
a(4,8) = "0x30"
a(4,9) = "0x30"
a(4,10)= "0xfe"

a(5,1) = "0xfe" '数字5
a(5,2) = "0xfe"
a(5,3) = "0x06"
a(5,4) = "0x06"
a(5,5) = "0x3e"
a(5,6) = "0x60"
a(5,7) = "0xc0"
a(5,8) = "0xc3"
a(5,9) = "0x66"
a(5,10)= "0x3c"

a(6,1) = "0x60" '数字6
a(6,2) = "0x30"
a(6,3) = "0x18"
a(6,4) = "0x0c"
a(6,5) = "0x3e"
a(6,6) = "0x63"
a(6,7) = "0xc3"
a(6,8) = "0xc3"
a(6,9) = "0x66"
a(6,10) ="0x3c"

a(7,1) = "0xff" '数字7
a(7,2) = "0xc0"
a(7,3) = "0x60"
a(7,4) = "0x30"
a(7,5) = "0x18"
a(7,6) = "0x18"
a(7,7) = "0x18"
a(7,8) = "0x18"
a(7,9) = "0x18"
a(7,10)= "0x18"

a(8,1) = "0x3c" '数字8
a(8,2) = "0x66"
a(8,3) = "0xc3"
a(8,4) = "0x66"
a(8