VbScript validate IBAN with mod97 (number is too big)

Question

So I am writing an application in VbScript and I am trying to validate an IBAN number. The problem though, the number I have to mod with, is too big.

For example: 734027177486111478 mod 97

that is what I want to do, but I can't seem to find a type to use for doing this. Underneath is the code I am using at the moment. In the case of the example, vCheckCode at the last point is 734027177486111478.

Function checkIBAN(ByVal vIban)
vLandcode = Mid(vIban, 1, 2)    

Select Case vLandcode
    Case "BE"
        vIban = Replace(vIban, " ", "")

        If Len(vIban) = 16 Then
            vPrefix = Mid(vIban, 1, 4)
            vCheckCode = Replace(vIban, vPrefix, "")
            vSuffix = Replace(vPrefix, "BE", "1114")
            vCheckCode = vCheckCode + vSuffix
            vCheckCode = CDbl(vCheckCode)
            vResult = vCheckCode Mod 97             
        End If
End Select
End Function

Any idea on how to do this?

Thanks in advance!

Answer 1

734027177486111478 is too big to be represented as a 32-bit integer, so you can't just use Mod 97 like this.

A simple solution is to use some basic math :

• (a + b) mod n = ((a mod n) + (b mod n)) mod n
• (a * b) mod n = ((a mod n) * (b mod n)) mod n

And from that you can compute your modulo digit by digit:

Function Mod97(ByVal vIban)
    Dim i, m, digit
    m = 0
    For i = 1 To Len(vIban)
        digit = CInt(Mid(vIban, i, 1))
        m = (10*m + digit) Mod 97
    Next
    Mod97 = m
End Function

Mod97("734027177486111478") returns 1, which looks correct.

Answer 2

In case you need the function to be a bit broader (inspired by Schnouki):

Public Function VALIDATEIBAN(ByVal IBAN As String) As String

' Created by : Koen Rijnsent (www.castoro.nl)
' Inspired by : Chris Fannin (AbbydonKrafts)
' Inspired by : bonsvr (http://stackoverflow.com/users/872583/bonsvr)
' Inspired by : schnouki http://stackoverflow.com/users/113325/schnouki

On Error GoTo CatchError

Dim objRegExp As Object
Dim IBANformat As Boolean
Dim IBANNR As String
Dim ReplaceChr As String
Dim ReplaceBy As String
Dim LeftOver As Long
Dim digit As Integer
Dim i As Long

'Base format clean - uppercase and remove spaces
IBAN = Replace(UCase(IBAN), " ", "")
'Check format, length 15-31 characters, first countrycode, then two digits, then number
Set objRegExp = CreateObject("vbscript.regexp")
objRegExp.IgnoreCase = True
objRegExp.Global = True
objRegExp.Pattern = "[a-zA-Z]{2}[0-9]{2}[a-zA-Z0-9]{11}([a-zA-Z0-9]?){0,16}"
IBANformat = objRegExp.Test(IBAN)

'Validity of country code will not be checked!
If IBANformat = False Then
    VALIDATEIBAN = "FORMAT NOT RECOGNIZED"
Else
    'Flip first 4 characters to the back
    IBANNR = Right(IBAN, Len(IBAN) - 4) & Left(IBAN, 4)

    'Replace letters by the right numbers
    For i = 10 To 35
        ReplaceChr = Chr(i + 55)
        ReplaceBy = Trim(str(i))
        IBANNR = Replace(IBANNR, ReplaceChr, ReplaceBy)
    Next i

    'Loop through the IBAN, as it is too long to calculate at one go
    LeftOver = 0
    For i = 1 To Len(IBANNR)
        digit = CInt(Mid(IBANNR, i, 1))
        LeftOver = (10 * LeftOver + digit) Mod 97
    Next

    If LeftOver = 1 Then
        If Len(IBAN) = IBANLEN(Left(IBAN, 2)) Then
            VALIDATEIBAN = "IBAN OK"
        ElseIf IBANLEN(Left(IBAN, 2)) = 0 Then
            VALIDATEIBAN = "COUNTRYCODE UNKNOWN, 97 CHECK OK"
        Else
            VALIDATEIBAN = "LENGTH INVALID WITH COUNTRYCODE"
        End If
    Else
        VALIDATEIBAN = "97 CHECK FAILED"
    End If
End If

Exit Function

CatchError:
    VALIDATEIBAN = "ERROR: " & Err.Description
    MsgBox "Module: MODULE - VALIDATEIBAN function" & vbCrLf & vbCrLf _
    & "Error#:  " & Err.Number & vbCrLf & vbCrLf & Err.Description

End Function

And a function for the countrycode/length:

Function IBANLEN(CountryCode As String) As Integer

    ' Created by : Koen Rijnsent (www.castoro.nl)
    If Len(CountryCode) <> 2 Then
        IBANLEN = 0
    Else

        'List of country codes, based on http://en.wikipedia.org/wiki/International_Bank_Account_Number
        Select Case CountryCode
            Case "AL": IBANLEN = 28
            Case "AD": IBANLEN = 24
            Case "AE": IBANLEN = 23
            Case "AO": IBANLEN = 25
            Case "AT": IBANLEN = 20
            Case "AZ": IBANLEN = 28
            Case "BH": IBANLEN = 22
            Case "BE": IBANLEN = 16
            Case "BA": IBANLEN = 20
            Case "BF": IBANLEN = 27
            Case "BI": IBANLEN = 16
            Case "BJ": IBANLEN = 28
            Case "BR": IBANLEN = 29
            Case "BG": IBANLEN = 22
            Case "CH": IBANLEN = 21
            Case "CI": IBANLEN = 28
            Case "CM": IBANLEN = 27
            Case "CR": IBANLEN = 21
            Case "CV": IBANLEN = 25
            Case "CY": IBANLEN = 28
            Case "CZ": IBANLEN = 24
            Case "DE": IBANLEN = 22
            Case "DK": IBANLEN = 18
            Case "DO": IBANLEN = 28
            Case "EE": IBANLEN = 20
            Case "ES": IBANLEN = 24
            Case "FO": IBANLEN = 18
            Case "FI": IBANLEN = 18
            Case "FR": IBANLEN = 27
            Case "GB": IBANLEN = 22
            Case "GE": IBANLEN = 22
            Case "GI": IBANLEN = 23
            Case "GR": IBANLEN = 27
            Case "GL": IBANLEN = 18
            Case "GT": IBANLEN = 28
            Case "HR": IBANLEN = 21
            Case "HU": IBANLEN = 28
            Case "IE": IBANLEN = 22
            Case "IL": IBANLEN = 23
            Case "IR": IBANLEN = 26
            Case "IS": IBANLEN = 26
            Case "IT": IBANLEN = 27
            Case "KZ": IBANLEN = 20
            Case "KW": IBANLEN = 30
            Case "LB": IBANLEN = 28
            Case "LI": IBANLEN = 21
            Case "LT": IBANLEN = 20
            Case "LU": IBANLEN = 20
            Case "LV": IBANLEN = 21
            Case "MC": IBANLEN = 27
            Case "MD": IBANLEN = 24
            Case "ME": IBANLEN = 22
            Case "MG": IBANLEN = 27
            Case "MK": IBANLEN = 19
            Case "ML": IBANLEN = 28
            Case "MT": IBANLEN = 31
            Case "MR": IBANLEN = 27
            Case "MU": IBANLEN = 30
            Case "MZ": IBANLEN = 25
            Case "NL": IBANLEN = 18
            Case "NO": IBANLEN = 15
            Case "PK": IBANLEN = 24
            Case "PS": IBANLEN = 29
            Case "PL": IBANLEN = 28
            Case "PT": IBANLEN = 25
            Case "RO": IBANLEN = 24
            Case "RS": IBANLEN = 22
            Case "SA": IBANLEN = 24
            Case "SE": IBANLEN = 24
            Case "SI": IBANLEN = 19
            Case "SK": IBANLEN = 24
            Case "SM": IBANLEN = 27
            Case "SN": IBANLEN = 28
            Case "TN": IBANLEN = 24
            Case "TR": IBANLEN = 26
            Case "UA": IBANLEN = 29
            Case "VG": IBANLEN = 24
            
            Case Else: IBANLEN = 0
        End Select
    
    End If

End Function