如今你要申请一个QQ号，需要输入很复杂的验证码：验证码由若干个汉字组成，还加上了花哩唬哨的背景，使得有些汉字实在难以辨认。腾讯这么做，是为了防止有人利用软件批量获取QQ号码----每次提交都要输入随机生成的验证码，这是软件难以做到的。
最初的验证码，只是几个随机生成的数字。但是道高一尺魔高一丈，很快就有能识别数字的软件了，“收藏家”们利用这种软件批量获取帐号，或是探测密码，因为软件可以不知疲倦地不断submit。于是，出现了图片形式的验证码，还要加上无规则的背景，既然人眼都难以分辨，想来软件分辨起来就有一定的难度。但是腾讯开始采用汉字图片做验证码，是不是意味着破解验证码的技术又有了新进展，带背景的数字或字母图片形式的验证码，也可以被软件分辨了？
且不管那么多，我们来谈谈ASP生成验证码的办法吧。
还是从最简单的例子说起，来了解一下验证码的基本思路。
第一个例子，在显示表单的同时，生成一个4位的随机数做为验证码，利用session传递该验证码，在数据处理页面，比对用户输入的验证码与session中的值是否一致。顺便说一句，我曾见过有糊涂的仁兄利用hidden类型的input控件传递验证码，孰不知，通过查看页面源代码，该数据是暴露无遗的。还有password型的input中的数据也一样。
<%
Response.Buffer = true
Dim CheckCode
Response.Write "<html><body>"
'======***表单提交后:***======
if Request.ServerVariables("REQUEST_METHOD")="POST" then
Dim founderr,callform,msg
founderr = false '是否有错误发生
callform = false '是否调用表单
msg = "" '提示信息
'==========================验证码验证开始============================
dim sessionCode
sessionCode = session("chkCode")
session("chkCode") = ""
CheckCode = trim(Request.Form("chkcode"))
if CheckCode = "" then
msg = msg + "<li>请填写验证码"
founderr = true
callform = true
elseif cstr(CheckCode) <> cstr(sessionCode) then
msg = msg + "<li>验证码不正确"
founderr = true
callform = true
end if
'==============================验证码验证结束==========================
if founderr = true then
call message("500")
if callform = true then call myform()
else
msg = "<li>操作成功！"
call message("500")
end if
'======***页面初始化(表单提交前)***======
else
CheckCode = ""
call myform()
end if
Response.Write "</body></html>"
REM 子过程，定义表单
Sub myform()
Response.Write "<table width=500 border=1 cellspacing=0 cellpadding=5 align=center>"&_
"<form name=form1 method=post action=post.asp>"&_
"<tr height=30><td rowspan=2 width=100 align=center>验证码</td>"&_
"<td><input type=text name=chkcode size=10> "&_
getChkCode()&"</td></tr>"&_
"<tr height=30><td>请填写文本框右侧的验证码</td></tr>"&_
"<tr height=30><td colspan=2 align=center>"&_
"<input type=submit name=Submit value=提交></td></tr>"&_
"</form></table>"
end Sub
Rem 子函数，生成验证码(四位随机数)
function getChkCode()
dim ranNum
randomize
ranNum=int(9000*rnd)+1000
session("chkCode") = ranNum
getChkCode = ranNum
end function
Rem 提示信息
sub message(w)
Response.Write "<table width="&w&" border=1 cellspacing=0 cellpadding=5 align=center>"&_
"<tr height=30><td>提示信息</td></tr>"&_
"<tr valign=top><td style=""color:red;"">"&msg&"</td></tr></table>"
end sub
%>
以上代码展示生成验证码的一般思路，生成一个四位随机数作为验证码，这是最简单，同时也是最不安全的一种方法。
或许你想到了，可以以每位数字对应一张图片，生成图片型的验证码，就象图片型计数器那样处理。这并不是一个好想法，它跟数值型的验证码没有本质上的区别，并不能提高安全性。
下面，介绍如何生成图片型的验证码。
首先，可以通过ASP生成xbm型的验证码，这的的确确是一张xbm格式的图片，而且，你可以任意设置图片的大小。为了简单起见，我们仍以数字为例。
可以把生成验证码的代码独立出来，命名为checkcode.asp：
<%
on error resume next
dim i
dim countdata
countdata="1234567890"
dim rou,chkcode,chklen
chkcode=""
chklen = 4
randomize
for i=1 to 4
rou = int(rnd*10)
chkcode = chkcode + cstr(rou)
next
dim strDigits
strDigits = Array(_
"0","0x3c","0x66","0x66","0x66","0x66","0x66","0x66","0x66","0x66","0x3c",_
"1","0x30","0x38","0x30","0x30","0x30","0x30","0x30","0x30","0x30","0x30",_
"https://files.jxasp.com/image/2","0x3c","0x66","0x60","0x60","0x30","0x18","0x0c","0x06","0x06","0x7e",_
"3","0x3c","0x66","0x60","0x60","0x38","0x60","0x60","0x60","0x66","0x3c",_
"4","0x30","0x30","0x38","0x38","0x34","0x34","0x32","0x7e","0x30","0x78",_
"5","0x7e","0x06","0x06","0x06","0x3e","0x60","0x60","0x60","0x66","0x3c",_
"6","0x38","0x0c","0x06","0x06","0x3e","0x66","0x66","0x66","0x66","0x3c",_
"7","0x7e","0x66","0x60","0x60","0x30","0x30","0x18","0x18","0x0c","0x0c",_
"8","0x3c","0x66","0x66","0x66","0x3c","0x66","0x66","0x66","0x66","0x3c",_
"9","0x3c","0x66","0x66","0x66","0x66","0x7c","0x60","0x60","0x30","0x1c")
dim iCharWidth,iCharHeight,theBit,theNum,iRow,k,theOffset
dim imageStr
imageStr = ""
iCharWidth = 8
iCharHeight= 10*1
Response.ContentType ="image/x-xbitmap"
Response.Expires =0
Response.Write "#define counter_width "&iCharWidth*chklen&chr(13) & chr(10)
Response.Write "#define counter_height "&iCharHeight&chr(13) & chr(10)
Response.Write "static unsigned char counter_bits[]={"
for iRow=0 to iCharHeight-1
for i=1 to chklen
theBit=mid(chkcode,i,1)
k=0
do while k<ubound(strDigits)
if strDigits(k) = theBit then exit do
k=k+iCharHeight+1
loop
if k>=ubound(strDigits) then k=0
theOffset = k + 1
imageStr = imageStr + (strDigits(theOffset+iRow))&","
next
next
imageStr = left(imageStr,(len(imageStr)-1))
Response.Write imageStr
Response.Write "};"
session("chkCode") = chkcode
%>
在post.asp中，定义表单时，相应的代码改为：
REM 子过程，定义表单
Sub myform()
Response.Write "<table width=500 border=1 cellspacing=0 cellpadding=5 align=center>"&_
"<form name=form1 method=post action=default.asp>"&_
"<tr height=30><td rowspan=2 width=100 align=center>验证码</td>"&_
"<td><input type=text name=chkcode size=10> "&_
"<img src=checkcode.asp width=32 height=10 border=0></td></tr>"&_
"<tr height=30><td>请填写文本框右侧的验证码</td></tr>"&_
"<tr height=30><td colspan=2 align=center>"&_
"<input type=submit name=Submit value=提交></td></tr>"&_
"</form></table>"
end Sub
以上就是最简单的xbm型的验证码。但是这种格式的验证码，windows xp sp2的用户无法看到，原因是xp sp2取消了对xbm格式文件的支持。不过，可以通过修改注册表解决这个问题：打开注册表，找到HKEY_LOCAL_MACHINE/SOFTWARE/Microsoft/Internet Explorer/Security，用右键增加一个 dword 值，改名为 BlockXBM，其值为 00000000 。改好后，重新打开游览器就可以了。也可以用文件编辑器编辑一文件，内容如下：
REGEDIT4
[HKEY_LOCAL_MACHINE/SOFTWARE/Microsoft/Internet Explorer/Security]
"BlockXBM"=dword:00000000
将该文件保存为扩展名为reg的注册表文件，然后双击该文件即可。
除了xbm格式的验证码，也可以利用Adodb.Stream对象，开发出更复杂的BMP型验证码，可以从http://www.netop.cc/downloads/bmpchkcode.rar得到相关程序。利用该程序，可以生成数字、字母混合，彩色，且有彩色背景的验证码。
对于租用空间的朋友来说，使用第三方组件受到很大限制。如果拥有自己的服务器，可以试一下利用ASP绘图组件shotgraph生成验证码，该验证码是GIF格式的，具有很广泛的通用性。该组件可从http://www.netop.cc/downloads/shotgraph.zip得到。压缩包里有详细的示例程序和说明文档。
总之，生成验证码的办法很多，现在有很多第三方组件可以帮助你生成各种样式的验证码，上网找找看吧。
