I have a number of bank accounts with cards that need PINs to use, but
I use them very rarely. I think I've come up with a way of marking
them on the cards with some degree of security. Does this make sense
to other people?
Let's say I have PINs 6309, 7440, and 8118 that I don't use very often.
I need a single four-digit number that I can reliably remember (the
"key number"). Let's say that's 6604.
For each digit in the PIN, I subtract the corresponding digit of the
key number, modulo ten (i.e. if the result would be less than zero,
add ten until it isn't, and don't worry about carry digits).
6 - 6 = 0
3 - 6 = 13 - 6 = 7
0 - 0 = 0
9 - 4 = 5
So I get 0705, and by the same process 1846 and 2514. I write those
numbers on the back of each card (being ready to say "don't worry,
that's not my PIN" if I ever have to present them to a cashier).
When I want to use the PIN, I just reverse the process, mentally
adding 6604 to the number on the card.
As far as I can see, the only problem is that if one PIN is revealed
it's possible to work out the key number and so the PINs on my other
cards can be decoded. This strikes me as an acceptably small risk.
What have I missed?
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