<%
 Sub ImageValidCode(sName)
 Response.Expires = -1 
 Response.AddHeader "Pragma","no-cache"
 Response.AddHeader "cache-ctrol","no-cache"
 Response.ContentType = "Image/BMP"
 Randomize
 Dim i, ii, iii
 Const cOdds = 4 '杂点出现的机率
 Const cAmount = 36 ' 文字数量
 Const cCode = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ" '颜色的数据(字符，背景)
 Dim vColorData(1)
 vColorData(0) = ChrB(255) & ChrB(000) & ChrB(000) '字符颜色
 vColorData(1) = ChrB(000) & ChrB(255) & ChrB(255) '背景颜色
 Dim vCode(4), vCodes
 For i=0 To 3
 vCode(i) = Int(Rnd * cAmount)
 vCodes = vCodes & Mid(cCode, vCode(i)+1, 1)
 Next
 Session.Value(sName) = vCodes'记录入Session
 Dim vNumberData(35)
 vNumberData(0) = "1110000111110111101111011110111101001011110100101111010010111101001011110111101111011110111110000111"
 vNumberData(1) = "1111011111110001111111110111111111011111111101111111110111111111011111111101111111110111111100000111"
 vNumberData(2) = "1110000111110111101111011110111111111011111111011111111011111111011111111011111111011110111100000011"
 vNumberData(3) = "1110000111110111101111011110111111110111111100111111111101111111111011110111101111011110111110000111"
 vNumberData(4) = "1111101111111110111111110011111110101111110110111111011011111100000011111110111111111011111111000011"
 vNumberData(5) = "1100000011110111111111011111111101000111110011101111111110111111111011110111101111011110111110000111"
 vNumberData(6) = "1111000111111011101111011111111101111111110100011111001110111101111011110111101111011110111110000111"
 vNumberData(7) = "1100000011110111011111011101111111101111111110111111110111111111011111111101111111110111111111011111"
 vNumberData(8) = "1110000111110111101111011110111101111011111000011111101101111101111011110111101111011110111110000111"
 vNumberData(9) = "1110001111110111011111011110111101111011110111001111100010111111111011111111101111011101111110001111"
 vNumberData(10) = "1111011111111101111111101011111110101111111010111111101011111100000111110111011111011101111000100011"
 vNumberData(11) = "1000000111110111101111011110111101110111110000111111011101111101111011110111101111011110111000000111"
 vNumberData(12) = "1110000011110111101110111110111011111111101111111110111111111011111111101111101111011101111110001111"
 vNumberData(13) = "1000001111110111011111011110111101111011110111101111011110111101111011110111101111011101111000001111"
 vNumberData(14) = "1000000111110111101111011011111101101111110000111111011011111101101111110111111111011110111000000111"
 vNumberData(15) = "1000000111110111101111011011111101101111110000111111011011111101101111110111111111011111111000111111"
 vNumberData(16) = "1110000111110111011110111101111011111111101111111110111111111011100011101111011111011101111110001111"
 vNumberData(17) = "1000100011110111011111011101111101110111110000011111011101111101110111110111011111011101111000100011"
 vNumberData(18) = "1100000111111101111111110111111111011111111101111111110111111111011111111101111111110111111100000111"
 vNumberData(19) = "1110000011111110111111111011111111101111111110111111111011111111101111111110111110111011111000011111"
 vNumberData(20) = "1000100011110111011111011011111101011111110001111111010111111101101111110110111111011101111000100011"
 vNumberData(21) = "1000111111110111111111011111111101111111110111111111011111111101111111110111111111011110111000000011"
 vNumberData(22) = "1000100011110010011111001001111100100111110101011111010101111101010111110101011111010101111001010011"
 vNumberData(23) = "1000100011110011011111001101111101010111110101011111010101111101100111110110011111011001111000110111"
 vNumberData(24) = "1110001111110111011110111110111011111011101111101110111110111011111011101111101111011101111110001111"
 vNumberData(25) = "1000000111110111101111011110111101111011110000011111011111111101111111110111111111011111111000111111"
 vNumberData(26) = "1110001111110111011110111110111011111011101111101110111110111011111011101001101111011001111110001011"
 vNumberData(27) = "1000001111110111011111011101111101110111110000111111010111111101101111110110111111011101111000110011"
 vNumberData(28) = "1110000011110111101111011110111101111111111001111111111001111111111011110111101111011110111100000111"
 vNumberData(29) = "1000000011101101101111110111111111011111111101111111110111111111011111111101111111110111111110001111"
 vNumberData(30) = "1000100011110111011111011101111101110111110111011111011101111101110111110111011111011101111110001111"
 vNumberData(31) = "1000100011110111011111011101111101110111111010111111101011111110101111111010111111110111111111011111"
 vNumberData(32) = "1001010011110101011111010101111101010111110101011111001001111110101111111010111111101011111110101111"
 vNumberData(33) = "1000100011110111011111101011111110101111111101111111110111111110101111111010111111011101111000100011"
 vNumberData(34) = "1000100011110111011111011101111110101111111010111111110111111111011111111101111111110111111110001111"
 vNumberData(35) = "1100000011110111011111111101111111101111111110111111110111111111011111111011111111101110111100000011"
  ' 输出图像文件头
 Response.BinaryWrite ChrB(66) & ChrB(77) & ChrB(230) & ChrB(4) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(54) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(40) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(40) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(10) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(1) & ChrB(0)
  ' 输出图像信息头
 Response.BinaryWrite ChrB(24) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(176) & ChrB(4) & ChrB(0) & ChrB(0) & ChrB(18) & ChrB(11) & ChrB(0) & ChrB(0) & ChrB(18) & ChrB(11) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(0) & ChrB(0)
 For i=9 To 0 Step -1 ' 历经所有行
 For ii=0 To 3 ' 历经所有字
 For iii=1 To 10 ' 历经所有像素
 If Rnd * 99 + 1 < cOdds Then ' 随机生成杂点
 Response.BinaryWrite vColorData(0)
 Else
 Response.BinaryWrite vColorData(Mid(vNumberData(vCode(ii)), i*10+iii, 1))
 End If
 Next
 Next
 Next
 End Sub

Call ImageValidCode("ValidCode")
%>
验证码使用方法：比较表单输入的验证码字符是否和下面的值一致。

Session("ValidCode")