Simple IBAN (checksum) validation in Go

iban.go

package iban

import (
	"math/big"
	"strconv"
)

// IsValid ensures bank account numbers are valid to prevent processing errors
// in financial transactions and comply with international banking standards.
// It implements the ISO 13616 standard for IBAN validation, verifying the 
// checksum using the MOD97 algorithm to catch transcription errors.
func IsValid(iban string) bool {
	// Most IBANs should be between 15-34 characters (varies by country)
	if len(iban) < 15 || len(iban) > 34 {
		return false
	}

	// Apply the MOD97 algorithm required by ISO 13616:
	// 1. Move country code and check digits to end to ensure uniform processing
	rearranged := iban[4:] + iban[:4]

	// 2. Convert letters to numbers per ISO standard to create a single large integer
	result := ""
	for _, character := range rearranged {
		if character >= 'A' && character <= 'Z' {
			// Convert letter to number (A=10, B=11, ..., Z=35)
			result += strconv.Itoa(int(character-'A') + 10)
		} else if character >= '0' && character <= '9' {
			result += string(character)
		} else {
			// Invalid character
			return false
		}
	}

	// 3. Calculate modulo 97 - valid IBANs must give remainder of 1
	// Using big.Int because IBAN numbers converted to integers exceed standard int sizes
	num := new(big.Int)
	num.SetString(result, 10)

	mod := new(big.Int)
	mod.Mod(num, big.NewInt(97))
	return mod.Int64() == 1
}