Function to encrypt/decrypt a string

Try :

<%
Function MyASC(OneChar)
      If OneChar = "" Then MyASC = 0 Else MyASC = Asc(OneChar)
End Function

Function Base64Encode(inData)
      Const Base64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
      Dim cOut, sOut, I
      For I = 1 To Len(inData) Step 3
            Dim nGroup, pOut, sGroup
            nGroup = &H10000 * Asc(Mid(inData, I, 1)) + &H100 * MyAsc(Mid(inData, I + 1, 1)) + MyAsc(Mid(inData, I + 2, 1))
            nGroup = Oct(nGroup)
            nGroup = String(8 - Len(nGroup), "0") & nGroup
            pOut = Mid(Base64, CLng("&o" & Mid(nGroup, 1, 2)) + 1, 1) + Mid(Base64, CLng("&o" & Mid(nGroup, 3, 2)) + 1, 1) + Mid(Base64, CLng("&o" & Mid(nGroup, 5, 2)) + 1, 1) + Mid(Base64, CLng("&o" & Mid(nGroup, 7, 2)) + 1, 1)
            sOut = sOut + pOut
      Next
      Select Case Len(inData) Mod 3
            Case 1: '8 bit final
                  sOut = Left(sOut, Len(sOut) - 2) + "=="
            Case 2: '16 bit final
                  sOut = Left(sOut, Len(sOut) - 1) + "="
      End Select
      Base64Encode = sOut
End Function

Function Base64Decode(base64String)
  'rfc1521
  '1999 Antonin Foller, Motobit Software, http://Motobit.cz
  Const Base64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
  Dim dataLength, sOut, groupBegin
 
  'remove white spaces, If any
  base64String = Replace(base64String, vbCrLf, "")
  base64String = Replace(base64String, vbTab, "")
  base64String = Replace(base64String, " ", "")
 
  'The source must consists from groups with Len of 4 chars
  dataLength = Len(base64String)
  If dataLength Mod 4 <> 0 Then
    Err.Raise 1, "Base64Decode", "Bad Base64 string."
    Exit Function
  End If

 
  ' Now decode each group:
  For groupBegin = 1 To dataLength Step 4
    Dim numDataBytes, CharCounter, thisChar, thisData, nGroup, pOut
    ' Each data group encodes up To 3 actual bytes.
    numDataBytes = 3
    nGroup = 0

    For CharCounter = 0 To 3
      ' Convert each character into 6 bits of data, And add it To
      ' an integer For temporary storage.  If a character is a '=', there
      ' is one fewer data byte.  (There can only be a maximum of 2 '=' In
      ' the whole string.)

      thisChar = Mid(base64String, groupBegin + CharCounter, 1)

      If thisChar = "=" Then
        numDataBytes = numDataBytes - 1
        thisData = 0
      Else
        thisData = InStr(1, Base64, thisChar, vbBinaryCompare) - 1
      End If
      If thisData = -1 Then
        Err.Raise 2, "Base64Decode", "Bad character In Base64 string."
        Exit Function
      End If

      nGroup = 64 * nGroup + thisData
    Next
   
    'Hex splits the long To 6 groups with 4 bits
    nGroup = Hex(nGroup)
   
    'Add leading zeros
    nGroup = String(6 - Len(nGroup), "0") & nGroup
   
    'Convert the 3 byte hex integer (6 chars) To 3 characters
    pOut = Chr(CByte("&H" & Mid(nGroup, 1, 2))) + _
      Chr(CByte("&H" & Mid(nGroup, 3, 2))) + _
      Chr(CByte("&H" & Mid(nGroup, 5, 2)))
   
    'add numDataBytes characters To out string
    sOut = sOut & Left(pOut, numDataBytes)
  Next

  Base64Decode = sOut
End Function

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