RogerBW's Blog

PIN security: is this a bad idea? 18 February 2015

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?

Tags: real life

  1. Posted by Owen Smith at 01:58pm on 18 February 2015

    Are you sure this is any better than simply changing the PINs on all your cards to be the same? Assuming you then wouldn't need to write it down anywhere.

  2. Posted by RogerBW at 02:14pm on 18 February 2015

    Is that a thing that happens these days? I haven't looked into it; it wasn't, when I first got PIN cards. At a casual glance, it's not offered through my on-line banking interfaces.

  3. Posted by Michael Cule at 06:24pm on 18 February 2015

    Yes it's a thing. You just put it into the machine enter the original code and then use the thing to reset to another number. It should even say you can do that on the letter the bank or credit card company sent you.

    Just don't use your birthday or anything like that.

  4. Posted by RogerBW at 12:29am on 19 February 2015

    Insert mildly offended look here.

  5. Posted by Owen Smith at 02:41pm on 19 February 2015

    I've been changing PINs on my cards for what feels like about twenty years. It's certainly not remotely recent.

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