VBScript Adding Large Hex Strings

This function is an example of how to add two large hex strings together keeping them as string values:

Function HexAdd(ByVal strNum1, ByVal strNum2)
   size1 = Len(strNum1)
   size2 = Len(strNum2)
   If (size1 <> size2) Then
       If (size1 > size2) Then
           strNum2 = String(size1 - size2, "0") + strNum2
           size2 = size1
       Else
           strNum1 = String(size2 - size1, "0") + strNum1
           size1 = size2
       End If
   End If
   ' Begin adding
   Dim i
   Dim ret
   Dim carry
   Dim sum
   ret = ""
   i = CDbl(0)
   carry = 0
   For i = 1 To size1 '199999999999999999999998
       a = "&H" & CStr(Mid(strNum1, size1 - i + 1, 1))
       b = "&H" & CStr(Mid(strNum2, size1 - i + 1, 1))
       sum = CInt(a) + CInt(b) + carry
       ret = Hex(CStr(sum Mod 16)) & ret
       carry = Fix(sum / 16)
   Next
   If (carry > 0) Then
       ret = CStr(Hex(carry)) & ret
   End If
   HexAdd = ret
End Function

Call it like this:

ret = HexAdd("FFFFF", "123456789ABCDEF")