Mod 97

For my current project we need to validate IBAN numbers. IBAN numbers are validated with an ISO 7064 mod-97-10 calculation where the remainder must equal 1

static public bool Validate(string iban)
{
    // pre-conditions
    if ( string.IsNullOrEmpty( iban ) || (!Regex.IsMatch(iban, "^[A-Z0-9]") ) )
    {
        return false;
    }

    // clean-up IBAN
    iban = iban.Replace(" ", String.Empty);

    // 1.Move the four initial characters to the end of the string
    string iban2 = iban.Substring(4, iban.Length - 4) + iban.Substring(0, 4);

    // 2.Replace the letters in the string with digits, expanding the string as necessary, such that A=10, B=11 and Z=35.
    const int asciiShift = 55;
    var sb = new StringBuilder();
    foreach (char c in iban2)
    {
        int x;
        if (Char.IsLetter(c)) {
            x = c - asciiShift;
        }
        else {
            x = int.Parse(c.ToString(), CultureInfo.InvariantCulture);
        }
        sb.Append(x);
    }

    // 3.Convert the string to an integer and mod-97 the entire number
    string checkSumString = sb.ToString();
    int checksum = int.Parse( checkSumString.Substring(0, 1), CultureInfo.InvariantCulture );
    for (var i = 1; i < checkSumString.Length; i++)
    {
        int v = int.Parse(checkSumString.Substring(i, 1), CultureInfo.InvariantCulture);
        checksum *= 10;
        checksum += v;
        checksum %= 97;
    }
    return ( checksum == 1 );
}

Since I didn’t expect we would be the first to want to implement this in C# we first did a quick search on the web for an existing implementation. The following resources have been used to come to the above implementation:

International Bank Account Number.
ISO/IEC 7064:2003.
IBAN validation, C# Implementation (this is the implementation we went for).