💾 Archived View for log.pfad.fr › 2023 › iban-adventures captured on 2023-12-28 at 15:05:00. Gemini links have been rewritten to link to archived content
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I have been hired to develop free software to refund traveling costs for Boy and Girl Scouts. Wire transfer being quite widespread, the participants are asked for an IBAN (International Bank Account Number) where the money will be transferred.
This post will first take a glance at the mathematics inside such numbers, then look at the existing options that were at my disposal, before finally re-inventing the wheel and contacting the Liechtenstein Bankers Association...
An IBAN is composed of:
The two check digits are computed from the rest of the IBAN, to ensure that the number does not contain "simple" errors (missing or swapped characters). This allows for a local/offline sanity check, which should prevent most mistakes. I won't go on explaining the algorithm, which is nicely described on Wikipedia.
To go further, one can also perform checks on the BBAN, depending on the country. Each country has a specific format and length. Some countries also have a specific checksum.
Fun fact: because the BBAN checksum of a French account is very similar to the IBAN checksum, all numerical French IBANs start with FR76.
Usually, to make a transfer, the BIC is also required. The BIC identifies the Bank of the recipient. However, within the BBAN, the first characters already indicate a "Bank Code" for most countries. Unfortunately, there is no mathematical relation between the Bank Code and the BIC. So to deduce the BIC from the IBAN, one must compile mapping for each country.
It would have been too boring if such lists were easily available. Here is an overview of what I ended up parsing per country:
I wanted a Go package with would make the mathematical validation of the IBAN as well as the BIC deduction for some countries (DE & FR at least). Searching on pkg.go.dev yielded some results, but (obviously) I found none of them satisfying (and I probably have an NIH syndrome):
So I created a Go module that validates the IBAN checksum as well as provides an automatic deduction of the BIC for the countries listed above (the BIC mapping is in the subpackage "bic"):
code.pfad.fr/swift - check IBAN validity and deduce BIC
To ensure ease of update, I generate a good part of the code based on remote files. In this quest, I identified a couple of mistakes...
I am using this package with success in my commissioned software and proposed adding it to the Codeberg registration server. Let's see how it goes (to be polite, my first PR for this project consisted of repairing the tests. If it gets accepted, I will create a PR to use my swift package :).
📅 2023-11-21