IBAN Validator Swift

Question

I am writing an algorithm to validate IBAN (International Bank Account Number) in Swift 3 and not able to figure one of the validation.

Example IBAN - BE68539007547034

Here are the rules to validate -

1. Input number should be of length 16.
2. First 2 characters are country code (not numeric).
3. Last 14 are numeric.
4. Last 2 characters are the modulo 97 result of the previous 12 numeric characters.

While #1 - #3 are clear I need clarity on #4. If anyone have done this before and know about it then please let me know.

Answer 1

The validation algorithm is rather simple if you follow the algorithm on wikipedia:

extension String {
    private func mod97() -> Int {
        let symbols: [Character] = Array(self)
        let swapped = symbols.dropFirst(4) + symbols.prefix(4)

        let mod: Int = swapped.reduce(0) { (previousMod, char) in
            let value = Int(String(char), radix: 36)! // "0" => 0, "A" => 10, "Z" => 35
            let factor = value < 10 ? 10 : 100          
            return (factor * previousMod + value) % 97
        }

        return mod
    }    

    func passesMod97Check() -> Bool {
        guard self.characters.count >= 4 else {
            return false
        }

        let uppercase = self.uppercased()

        guard uppercase.range(of: "^[0-9A-Z]*$", options: .regularExpression) != nil else {
            return false
        }

        return (uppercase.mod97() == 1)
    }
}

Usage:

let iban = "XX0000000..."
let valid = iban.passesMod97Check()

If you want to validate the format for a specific country, just modify the regular expression, e.g.

"^[A-Z]{2}[0-9]{14}$"

or directly

"^BE\\d{14}$"

Answer 2

From Wikipedia

let IBAN = "GB82WEST12345698765432" // uppercase, no whitespace !!!!
var a = IBAN.utf8.map{ $0 }
while a.count < 4 {
    a.append(0)
}
let b = a[4..<a.count] + a[0..<4]
let c = b.reduce(0) { (r, u) -> Int in
    let i = Int(u)
    return i > 64 ? (100 * r + i - 55) % 97: (10 * r + i - 48) % 97
}
print( "IBAN \(IBAN) is", c == 1 ? "valid": "invalid")

prints

IBAN GB82WEST12345698765432 is valid

With IBAN from your question it prints

IBAN BE68539007547034 is valid