Dutch Sole trader VAT number changed (2020). about vies HOT 16 CLOSED

I can disclose my VAT number: (removed since its not necessary anymore)

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Found it: http://kleineondernemer.nl/index.php/nieuw-btw-identificatienummer-vanaf-1-januari-2020-voor-eenmanszaken

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That's my plan . I will validate the TIN against both algo. If the first one fails -> will try the new one

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Waiting for code review ....

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Sorry being late at the game, this holiday time is for me the only time I can truly spend with my family in full. But now when they're off playing games themselves, I can have a look at this.

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Many thanks for the fast fix! 🥇

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@Smitsel Could You provide an example VAT number that validates in VIES and does not pass a checksum test ?

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Yes, for example: NL000099998B57

Taken from: https://www.belastingdienst.nl/wps/wcm/connect/nl/btw/content/gebruik-vanaf-1-januari-2020-uw-nieuwe-btw-identificatienummer

I can not disclose any real VAT numbers ofcourse.

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@MollieRick thx. Yep Your number validates in VIES but there is no information on the new algorithm to calculate the checksum. I'll try to google a little bit more but i may be easier to look for it in Dutch rather than in English ;)

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We've looked for it as well, could not find any information about calculating the checksum.
I will try to find a contact with the dutch government.

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Yep it works 23210023426721142 % 97 = 1 - so it's correct. Now I just need to find a way to distinguish old numbers from the new one's so that both validate ;)

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If they can't be differentiated you could do it by doing this check when the first one fails.
So in a failover kind of way.

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We can totally review your PR once you have something too btw.

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Awesome, i left some not super important comments. It works as intended, i've tried it out in our fork.

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@Smitsel should be fine now. I did add a regex expression to check if the number has only valid characters before calculating checksum [0-9A-Z+*]. This way it should be safer. In the older version there was a small % of chance that a number with an invalid char would validate :) It would then error on VIES but still :)

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Found this page in google, thanks to all participants for the information.
To those who also gets to this page, here is my version of this algorithm in C#. The simple conversion of VAT number to integer can cause overflow error, because the maximum length of converted VAT number is 26, while the max length of ulong type in C# is 20. Therefore I write converted VAT number to a string first and then subsequently convert the string to an integer and divide it by 97.

// Positions 1-2 must be NL, positions 13-14 must be digits. Positions 3-12 can contain digits, uppercase letters, '+' and '*'.
// Each letter is replaced by two-digit number, where 'A' = 10, 'B' = 11, ..., 'Z' = 35; '+' = 36, '*' = 37.
// Remainder of division the converted VAT number by 97 must be equal to 1.
// Example: NL123456789B13 is converted to 2321 123456789 11 13, i.e. to 23211234567891113 integer number. 23211234567891113 mod 97 = 1, it is a valid VAT number.

public void ValidateVatMod97Algorithm(string vat) {
    if (vat.Length != 14) {
        throw new System.Exception("VAT must have exactly 14 chars");
    }

    if (vat.Substring(0, 2) != "NL") {
        throw new System.FormatException("First two chars must be NL");
    }
    
    if (!char.IsDigit(vat[12]) || !char.IsDigit(vat[13])) {
        throw new System.FormatException("Last two chars must be digits");
    }
    // we can also check that the two-digit number in the end is between 02 and 98.
    
    var sb = new StringBuilder();
    foreach (char ch in vat) {
        int currNumber = 0;

        if (char.IsDigit(ch)) currNumber = ch - '0';
        else if (char.IsUpper(ch)) currNumber = ch - 55;
        else if (ch == '+') currNumber = 36;
        else if (ch == '*') currNumber = 37;
        else throw new System.FormatException("VAT contains incorrect chars");

        sb.Append(currNumber);
    }

    string vatDigitString = sb.ToString();
    int remainder = 0;
    foreach (char ch in vatDigitString) {
        int digit = ch - '0';
        remainder = (remainder * 10 + digit) % 97;
    }

    if (remainder != 1) {
        throw new System.FormatException("VAT is not valid according to the Modulus-97 checksum algorithm.");
    }
}

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