PIN codes for credit cards. It's hard to remember. Can't forget. That's why I write them directly on the cards themselves.

When a person has one credit/debit card, I think he will be able to remember its PIN code. And when there are two too. When there are three or more of them, you can write them down just in case. But if there are more than 5 of them? And you don't use them every day? What if there are also corporate ones? Then you need to somehow remember the PIN codes. And in case of loss, it’s a good idea to erase CVV2 on the card and remember it somehow.

I solved this problem for myself. You just need to write the codes with a permanent marker directly on the card. If you exchanged it for a new one, then cross it out and write the new one on the card.

Of course, many will say that it is dumb to write code directly onto the card. Exactly. But in order to avoid incurring costs if you lose your card, you need to come up with some kind of hash function and write on the card not the code value itself, but a certain number, which also consists of 4 characters, but is calculated using some algorithm from the real PIN code. Only you know the algorithm, and therefore, if the card is lost, the finder will receive a card with a random set of numbers. And they will help him no more than any random numbers that he can make up himself.

What kind of algorithms could these be?
I use two methods.

1. Rearranging numbers. I change the first with the third place, and the second with the fourth.

2. Adding a number to a digit after (or before) execution of p 1. For example, I add 2 to all digits. When passing through 10, we write the least significant digit.

3. For MIR, Mastercard and Visa, use different algorithms.

That's all. Same thing with CVV2. Only there are 3 numbers.

After such an action, any 4-digit number becomes equal to the attacker. Whether he steals the phone with these recorded numbers or the card itself, he will have to go through all the PIN codes.

One additional tip. It is better to write the code on the front side and on the part without a chip. Because the code is also needed when you have already inserted the card somewhere. And it would be better if it could be read. Therefore, it is necessary to write in such a way that it ends up on the part sticking out from the device.