We keep the key to our holiday cottage in a four-digit key safe by the door. Because of my poor eyesight, I move only one of the digit rotors when I leave, so it is easier to open next time. Most visitors rotate all rotors on leaving. Is my behaviour riskier?
鈥� If all the rotors were scrambled, a would-be intruder would have a one in 10,000 chance of success at each attempt using only trial and error. However, if they guess that only one rotor has been moved, then their chances improve dramatically. The safe-cracker would choose one rotor and test it at each of the nine other positions. If that didn鈥檛 work, they would return the rotor to its original position and repeat the process with the other three. The intruder would succeed after a maximum of 36 attempts.
We have a key safe and our children also tend to move only one rotor. It gets worse: they always move one of the end ones. I suspect this behaviour is quite common and it would be surprising if characters less scrupulously honest than myself haven鈥檛 already worked that out. Having thought this through, it looks like it is time for us to switch to a new key system.
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Tim McCulloch
Hurlstone Park, New South Wales, Australia
鈥� A potential thief will gain easy access to your property after just two visits. That is because moving only one rotor means the other three are left in the 鈥渙pen鈥� position. Of course, this assumes the thief knows you have only moved one rotor and indeed that they know which one you moved.
But it is you who tells them this. Let us say the thief sees the code is 8519 the first time they visit and 6519 the second. As three digits are identical, they now know that the first rotor is the one to focus on and that the correct digit is not 8 or 6.
If you change a different rotor when you leave the second time, you actually make it easier for the thief. Under this condition, they see 8519 the first time and 4517 the second. The clever thief knows not to bother with the middle two rotors, and also that the correct combination is either 8517 or 4519.
Sorry, but you need a better security system.
Pauline Keyne
Beaconsfield, Buckinghamshire, UK
鈥淚f you move just one rotor on the key safe, a thief could gain access to your cottage after two visits鈥�
鈥� The rotor type of key safe generally requires all the numbers to be aligned precisely before it will open. I would guess that if you are moving only one rotor and your eyesight is poor, you might not align it precisely to another number. A careful observer will spot this 鈥� so if they experiment with the poorly aligned rotor, they will soon be inside your cottage.
You may also wish to bear in mind that many people use the year of their birth, or the birth year of one of their children, as their security code. From 15 years of working in social services, I would estimate that about 90 per cent of people do this.
Liz Haigh
Cardiff, UK
鈥� A thoughtful intruder might apply a little psychology and realise that most people don鈥檛 scramble the rotors very efficiently. If we assume that each rotor is left within plus or minus two digits of its correct position, that leaves only five positions of each rotor to try. The potential intruder could be in the house within 15 minutes.
David Walmsley
Wokingham, Berkshire, UK
鈥� As a detective, formerly specialising in burglaries, I know that most burglars have their preferred modus operandi. This is the method they feel confident and competent at, roughly where the potential reward outweighs their perceived risk of getting caught. Some burglars might try rotating just one number, giving up and walking away if this is unsuccessful. If there is a burglar employing this method in your area, your behaviour certainly is riskier than moving all the rotors.
Adam Hewitt
Guildford, Surrey, UK
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