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一共4个页面：form.asp; chk.asp; num.asp; count.asp
得到一个随即数字。加密！
解密后成成XBM图片
利用session 判断

form.asp


<%
     '### To encrypt/decrypt include this code in your page
     '### strMyEncryptedString = EncryptString(strString)
     '### strMyDecryptedString = DeCryptString(strMyEncryptedString)
     '### You are free to use this code as long as credits remain in place
     '### also if you improve this code let me know.
     
     Private Function EncryptString(strString)
     '####################################################################
     '### Crypt Function (C) 2001 by Slavic Kozyuk grindkore@yahoo.com ###
     '### Arguments: strString <--- String you wish to encrypt ###
     '### Output: Encrypted HEX string ###
     '####################################################################
     
     Dim CharHexSet, intStringLen, strTemp, strRAW, i, intKey, intOffSet
     Randomize Timer
     
     intKey = Round((RND * 1000000) + 1000000) '##### Key Bitsize
     intOffSet = Round((RND * 1000000) + 1000000) '##### KeyOffSet Bitsize
     
     If IsNull(strString) = False Then
     strRAW = strString
     intStringLen = Len(strRAW)
     
     For i = 0 to intStringLen - 1
     strTemp = Left(strRAW, 1)
     strRAW = Right(strRAW, Len(strRAW) - 1)
     CharHexSet = CharHexSet & Hex(Asc(strTemp) * intKey)& Hex(intKey)
     Next
     
     EncryptString = CharHexSet & "|" & Hex(intOffSet + intKey) & "|" & Hex(intOffSet)
     Else
     EncryptString = ""
     End If
     End Function
     
     
     
     
     Private Function DeCryptString(strCryptString)
     '####################################################################
     '### Crypt Function (C) 2001 by Slavic Kozyuk grindkore@yahoo.com ###
     '### Arguments: Encrypted HEX stringt ###
     '### Output: Decrypted ASCII string ###
     '####################################################################
     '### Note this function uses HexConv() and get_hxno() functions ###
     '### so make sure they are not removed ###
     '####################################################################
     
     Dim strRAW, arHexCharSet, i, intKey, intOffSet, strRawKey, strHexCrypData
     
     
     strRawKey = Right(strCryptString, Len(strCryptString) - InStr(strCryptString, "|"))
     intOffSet = Right(strRawKey, Len(strRawKey) - InStr(strRawKey,"|"))
     intKey = HexConv(Left(strRawKey, InStr(strRawKey, "|") - 1)) - HexConv(intOffSet)
     strHexCrypData = Left(strCryptString, Len(strCryptString) - (Len(strRawKey) + 1))
     
     
     arHexCharSet = Split(strHexCrypData, Hex(intKey))
     
     For i=0 to UBound(arHexCharSet)
     strRAW = strRAW & Chr(HexConv(arHexCharSet(i))/intKey)
     Next
     
     DeCryptString = strRAW
     End Function
     
     
     
     Private Function HexConv(hexVar)
     Dim hxx, hxx_var, multiply
     IF hexVar <> "" THEN
     hexVar = UCASE(hexVar)
     hexVar = StrReverse(hexVar)
     DIM hx()
     REDIM hx(LEN(hexVar))
     hxx = 0
     hxx_var = 0
     FOR hxx = 1 TO LEN(hexVar)
     IF multiply = "" THEN multiply = 1
     hx(hxx) = mid(hexVar,hxx,1)
     hxx_var = (get_hxno(hx(hxx)) * multiply) + hxx_var
     multiply = (multiply * 16)
     NEXT
     hexVar = hxx_var
     HexConv = hexVar
     END IF
     End Function
     
     Private Function get_hxno(ghx)
     If ghx = "A" Then
     ghx = 10
     ElseIf ghx = "B" Then
     ghx = 11
     ElseIf ghx = "C" Then
     ghx = 12
     ElseIf ghx = "D" Then
     ghx = 13
     ElseIf ghx = "E" Then
     ghx = 14
     ElseIf ghx = "F" Then
     ghx = 15
     End If
     get_hxno = ghx
     End Function
     
     
     %>


<%
randomize
num = int(7999*rnd+2000) '计数器的值
num2 = EncryptString(num)
session("pwdt")=num
%>
<form action="chk.asp" method=post>
请输入验证码： <input type="text" name="pwds">
<img src="count.asp?sksid=<%=num2%>"> <input type=submit value=提交>
</form>


chk.asp
<%
if trim(request.form("pwds"))<>trim(session("pwdt")) then
%>
输入错误： 应该为：<%=session("pwdt")%>，可你输入的是:<%=request.form("pwds")%>
<%
else
%>
输入正确
<%end if%>