So I have 1 secret 4 digit number that I use to encrypt all my other pin numbers by adding it to them using modulo 10 addition. I then write the resultant 4 digit pin on the back of the card. I've been doing this for well over a decade since seeing it in a book about encryption. Without it I'd have needed to have been issued with new pin numbers many times

Interesting. If you chose a pin for your card that was the result of modulo arithmetic with some part of the card number (last four digits, for example, but perhaps something more obscure), you would have to write on the card at all.

That's a very clever suggestion.

The only thing is that if you're going to pick the pin you might as well use the secret one.

You probably wouldn't want to do that: If the pin was somehow compromised, then the attacker would likely try the same compromised pin number in other places. Using different pins for each service avoids this attack.

Ooh, that's neat. I use a similar system, though mine permits several possible ways to encrypt a number and I usually pick an encryption with 5 or 6 digits. It doesn't require me to remember anything but the algorithm, so I don't feel comfortable with sharing it. I know it might be less secure than encrypting the pin with another pin, but I can't trust myself to remember any pin.

Errr ... You have a secret pin.

You modulo that with the pin your bank gives you.

You write the result on the back of the card.

If you forget the pin you can work backwards to the bank issued pin?

Ok I get it now. Sounds crazy.