Re: Credit card expiry date validation

["Joseph C. Sible" <josephcsible@...>]

On Mon, Jun 15, 2020 at 4:15 PM Wilmar Pérez <darioperezb@gmail.com> wrote:
>
> I have an application collecting credit card data. Before sending the information out to the payment entity I am trying to make sure the information entered is, at least, valid. I already worked out the card number and cvv numbers but I am not so sure about the expiry date. The format I get the info is MMYY.
>
> -- Extract the last two digits of the Year
> current_year = string.sub(current_year , 3, 4)
> (card_exp_year < current_year) or (card_exp_year == current_year and card_exp_month < current_month)
>
> I do not know if this is the most elegant solution but it works. However it has a huge bug: it will fail at the end of the century. Since I only know the last two digits of the expiry date year, if a card expires in 2102 for instance and we were in 2099 my logic would wrongly reject the date (02 is less than 99).
>

You need to make an assumption: one possibility is to assume that no
credit card will be valid for more than 50 years, and that nobody will
try to use a credit card that's been expired for more than 50 years.
You can then replace your "card_exp_year < current_year" check with
"(card_exp_year - current_year) % 100 < 50".

Joseph C. Sible