A Little Riddle for you.

[fawksie]

If a PIN Code consists of 4 numbers, and there are 10 numbers on a keypad (0,1,2,3,4,5,6,7,8,9) can there only be 9999 conceivable combinations of this number? EG: 0001, 0002, 0003, 0004, 0005........1111, 1112, 1113.......9997, 9998, 9999. And if so, lets say that there are more than 9999 people using the same bank as you, does this mean that someone who uses the same bank as me, has the same PIN Code for their card?? I'm not daft really, this is a question that my Dad asked me, and it stumps me!

[mikewall01]

It is obviously possible for a number of people to have the same PIN, not only from your own bank, but from anywhere in the world. Remember that most people can choose their own PIN. Also, most cards are accepted at most ATMs internationally. However, the PIN is a security measure to back up the other information contained in, or on, your card, which includes your account number. So, if you are worried about someone else having exactly the same identity as you in respect of a credit / debit card, then don't. If your question was as simple as "can someone have the same PIN as me" then the answer is 'yes' ......... thousands of people, probably.

[mikewall01]

I also think that there are 10,000 possible numbers because you missed 0000.

[lisaj]

Good security occurs by mixing security variables. A security variable is, for instance, "something you know", "something you have", or "something you are". Examples of these are a password, a swipe card, or a fingerprint respectively. By mixing them, this prevents someone who steals your card from just having access, or someone from cutting off your finger (in a biometrically accessed system). The PIN confirmation is only simple because you only have three attempts to get it right. More worrying is the recent research that showed that by certain offline attacks on the PIN algorithm a PIN could be guessed in as little as 12 tries. A good 'fix' that the banks could implement would be to expand and allow PINs from 4 to 8 digits. I currently have a 5-digit PIN on my phone, and probably the last thing a thief would guess successfully would be the number of digits, most likely probably that it was only 4.

[BenDToy]

Some excellent information here, but just to re-assure fawksie what all this means is that even if someone had your card, they would only have a 1 in 10,000 chance of guessing the correct PIN to go with it. Hopefully this should give a more secure feeling to your dad!!

[Midrac]

The number of combinations with 10 numbers, which are non-exclusive, to form a 4 digit number is 10 x 10 x 10 x 10 or 10000. If they were exclusive (not to have the same number in the combination twice) then it would be 10 x 9 x 8 x 7 or 5040.