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")

Handling Large Hex in VBScript

VBScript doesn't inherently support hex constants over 8 bits in length so I made a little function that attempts to handle large Hex strings and convert them to Decimals. It should be noted that Doubles lose precision in the E15 range of magnitude so beware using anything too large:

Function HexToDec(strHex)
    Dim i
    Dim size
    Dim ret
    size = Len(strHex) - 1
    ret = CDbl(0)
    For i = 0 To size
        ret = ret + CDbl("&H" & Mid(strHex, size - i + 1, 1)) * (CDbl(16) ^ CDbl(i))
    Next
    HexToDec = ret
End Function
Function DecToHex(dblNumber)
    Dim Q
    Dim ret

    ret = ""
    Q = CDbl(Fix(dblNumber))
    While Q > 0
        ret = Hex(Q - Fix(Q / 16) * 16) & ret
        Q = Fix(Q / CDbl(16))
    Wend
    DecToHex = ret
End Function

Called like this:

ret = HexToDec("0C0BA715B2223D47A8CAC87BE5D9679")
ret = DecToHex(CDBL(16843216856846468461654862))