Selecting a secure PIN for building access

Question

What are the general criteria for rejecting an insecure PIN to access a building?

There has been a lot of discussion about password selection security. Minimum length requirements, mandatory classes of characters, etc. I have not found much concerning PIN selection security. Furthermore, there are several types of access that a PIN protects (phone, credit card, building). Each may have their own particular vulnerability characteristics.

I am in the loop on the installation of a new security system which allows personnel to enter a building by entering a PIN on a keypad by the door. Individuals are allowed to select their own PIN which is entered into the system under their name. I am aware of some insecure PINs, such as 12345 (or any consecutive digits sequence), the location's zip code or part of the company's phone number or address.

Anyone with a legitimate need may request access to the building. This includes everyone from executives to employees and and perhaps occasionally, guests. This particular building is a church, so there are no national secrets to protect. I would characterize the security requirements to be similar to a house. Interior offices are protected with physical keys.

Edit: I appreciate the information presented, however, they don't answer the question. I am looking for PIN patterns that attackers are known to use (because users have selected bad PINs).

Answer 1

You're setting yourself up for an endless game of whack-a-mole. You listed a few ... No matter how many you think of, lazy users will come up with more guessable PINs. It's a church, so think of any Christian influenced PINs. The date of Christmas, the priest's favorite Bible verse, the patron saint's birth-date, maybe the Church's construction date... so on and so on. Consider patterns too if you want to go that far. For more possible PINs, here's a blogpost where the writer analyzed PINs leaked in various data breaches. May be helpful.

I know you're asking how to make your PINs more secure, but I feel the answer here is not to use standalone individual PINs. Consider that you're working with a small set of possible passwords already. Then you're reducing it by eliminating guessable ones. Then you're making multiple PINs work at once. Every step makes random guesses more and more likely to succeed.

Answer 2

PINs are usually accompanied by some other authentication factor, like biometrics or a physical token. So, the PIN is not a lone factor in authentication.

But you appear to describe a situation where people can gain access to a secured building by punching in a code. Let me rephrase that: by using static, difficult-to-change sequence of numbers that can be easily copied and shared, the public gains access to the inside of your building.

The control against that threat is to make it very difficult to record and guess or use the physical appearance of the pin pad to deduce the code. That means making the PINs long and putting physical controls in place to make recording more difficult and to place layers of additional authentication protection on the inside of the building.

Because you are asking about the rejection process, my suggestion would be to use a random number generator instead of people choosing their own, and make it long. This bypasses people choosing PIN pad patterns or other pattern-based PINs which can be easy to guess or observe.

Answer 3

There are only 100000 potential values if you have a 5 digit PIN. That means that a single PIN can be brute forced in a relatively small length of time, unless there is some form of secondary restriction against a person standing next to the door and trying each number. This gets worse if there are multiple people with distinct codes for access to the system - assuming each person selects a random code, an attacker entering a single code has a n/100000 chance to find a working code, where n is the number of people with access.

It would probably be reasonable to expect a dedicated attacker to manage 1000 tries per hour - it doesn't take very long to type a 5 digit number. That would give an upper bound of 100 hours, with a single code. That's 3 (pretty boring) weekends with some breaks for food, which, depending on what is in the building, may well be worth it. The system as described can't implement account lockout - the only way to determine which user is trying to access is through the code.

So, how to solve this?

1. Use the PIN as a secondary factor - have an access card, and a PIN, for example. It is then possible to have account lockout after some failed PIN entries.
2. Enforce limits on the PIN in other ways - have someone guarding the door who prevents too many tries, have a CCTV camera pointing at the door which is monitored for unusual activity (someone standing there trying each possible code)
3. Have a much longer PIN, making it impractical to keep trying codes

You would also probably want to ensure that the codes being entered cannot be monitored in other ways - for example, by pointing a video camera at the key pad.

Answer 4

Full disclosure: I worked for several years as the software architect for a very large, Fortune 500 company that manufactures and programs these sort of electronic locks, including government and homeland security applications. Our biggest customer, though, was the National Association of Realtors. There is a very good chance you are using a product from my former company.

Our engineers had many discussions about PIN black listing, and they were all very fun while they lasted, but always ended the same way:

1. We are way overthinking this.
2. We just have to make it harder to guess the PIN than to throw a rock through a window. Physical security trumps all.

Also:

3. When there is a limited number of digits, it is safer to provide a single PIN to all users and rotate it frequently (once a month is typical). If you allow each user their own PIN, it reduces the search space dramatically. For example, if you have 100 users and there are only 1000 possible codes, with a single guess you have about a 1 in 10 chance of getting through that door. If everyone shared a single code it would be 1 in 1000.

4. All of our electronic products did ship with a blacklist of ten codes: all 1's, all 2's, all 3's, all 4's, etc. Any other code was selectable by the customer. The black list was configurable by the customer but honestly nobody ever did it.

Answer 5

Check out the 20 most popular 4-digit pins:

Those are the combinations that the attacker will try first. These include:

• repeated digits / pairs
• incrementing/decrementing sequences
• geometrical patterns on the keypad

Excluding these (extrapolated to your 5-digit pins) will provide a tangible increase in security. I wouldn't bother going further than that (that is, excluding dates / zip codes / etc.), because an attacker which is determined enough to learn your coworker's birthday will be determined enough to get a valid pin through social engineering, or simply try enough pins to get in. Mind you that with 100000 combinations and 100 valid pins (assuming you have 100 coworkers), every 1000th pin will be valid even if the attacker goes for simple bruteforce.

If the fact that a pin can eventually be guessed sounds insecure in the environment where the lock will be used, you should choose a different kind of lock and/or additional security measures. Something as simple as a short alarm sound after 10 wrong pins will help a lot.