验证码的程序及原理

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 & "