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Popular with Airbnb hosts, realtors and those looking for something better than a key under the mat – key lock boxes are not as secure as you might think.

This is the type of key lock box I’m talking about.

This is a metal box, usually fixed to the wall by the front door, with ten buttons on the front which you press to enter the secret combination.

If a four-digit combination was set, you would guess that it could be anything from 0000 to 9999 right? Wrong! Only one number can appear in any combination, so the first number that can be set is 0123 and the last number 6789.

Okay well that still means there are plenty of possible combinations right? Wrong! The number sequence can’t be set, so if you set 0123, the codes 0231, 0321, 3210, 2310 etcetera, would also work.

Shockingly, the total possible combinations when four digits are set is only 210.

Setting the Combination

The inside of the lock box lid has a cover which, when opened, exposes 10 screws corresponding to the numbered buttons on the keypad. The screws have arrows on them. A combination is set (or reset) by changing the direction of the arrows for numbers in the combination.

So you can only set a button on or off – you can’t set it more than once, nor can you set the combination order.

How can I improve security?

Best idea is to not use this type of key lock box! If you really need to continue using the box, don’t use a four-digit combination as that is the most common choice – set five as that gives you the maximum 252 combinations possible. See the formula later in this article.

Well five digits and 252 combinations isn’t so bad is it?

Sadly this isn’t the end of the story. I’ve seen reports that you can press hard on each button, while holding the opening latch and feel buttons that are set on.

That means a potential thief could press each button in turn, write down the ones that ‘give’ and enter those numbers in any order – because as we already worked out, the order doesn’t matter.

I’d almost be tempted to set all ten buttons to on. That way none would appear any different when pressed hard. Of course you could open the box by pressing each button in any order, but who would guess that? Plus the whole mechanism is quite clunky and even when you know the combination the cover doesn’t always come off.

Video

There is a video on how to open and close a lockbox on YouTube – not sure why a video was needed but if you’ve never seen one in action then now is your chance!

The Formula and Calculations

Here’s the formula:
(n!)/((n-r)! * (r!)) = number of combinations
where
n is the number of buttons on the lockbox – which is normally 10
r is the n is the number of buttons you enable

For those of you who don’t already know, the ! means factorial. So for example 5! = 5x4x3x2x1 although * seems preferred than x, but both means multiplied by/times.

Because n = 10 that means 10! = 3,628,800.

Here’s a table of results of 3,628,800 divided by ((10-r)! * (r!))

The last column is possible combinations.

r= 10-r= (n-r)! r! result of ((10-r)! * (r!)) 3,628,800 divided by ((10-r)! * (r!))
1 9 362,880 1 362,880 10
2 8 40,320 2 80,640 45
3 7 5,040 6 30,240 120
4 6 720 24 17,280 210
5 5 120 120 14,400 252
6 4 24 720 17,280 210
7 3 6 5,040 30,240 120
8 2 2 40,320 80,640 45
9 1 1 362,880 362,880 10

The formula doesn’t seem to work when r=10 as my calculation came out as 3,628,800 and I know that can’t be true as if you set all ten buttons, you could just press all ten in any order.

History

Oregon real estate agent, Delbert Williams, was annoyed by the amount of time it took to chase down keys of listed properties in the busy post-war real estate market of the early 1950s. His frustration compelled him to invent a secure key holder which combined a sand-casted metal container and a Yale bicycle lock.

The success of this product (which became the predecessor of the current day portable key box and eventually led to the development of the pushbutton lock) resulted in his forming Supra Products in 1955. Supra continued as a family managed company until 2002, when it was acquired by GE Security, a division of the General Electric Company.

How about boxes with a combination lock – are they better?

I would have hoped a combination lock would be better. But then I saw this video.

Final Advice

If you’re just hiding a key for emergencies, I’d maybe still use a lockbox, but hide it somewhere – or even bury it!

If you’re a realtor, you should be careful, that by using a lock box, you don’t become liable for any unauthorised entry into the property.

If you’re an Airbnb host, look at one of those keypad entry doors – I haven’t read about any hacks for those yet! Better still just meet your guests when they  arrive!