Four Of These Keys Are Shinier Than The Others

Ten thousand possible codes, until somebody looks at the keypad. Four keys are worn shiny and eight are not, so the code is those four digits and the only thing left to work out is the order. There are twenty four ways to arrange four digits, which is a minute of somebody standing at your door. The code has not been changed since the person who chose it left.

The arithmetic on the keypad

10,000

Four digits, ten options each. That's the number people have in mind when they say a keypad is secure.

4

The keys worn shiny, which tells anybody standing at the door which digits are in the code.

24

Once the worn keys have narrowed it to four digits, that is every order they could go in. A person can work through that standing at the door.

0

The number of those attempts anybody will ever hear about, because the keypad has nothing to report to and nobody to report it to.

What a shared code cannot do

• Tell you who came in, because it is the same four digits for everybody.
• Stop one person, because changing it stops everybody until they are all told the new one.
• Get changed often, for the same reason, so in practice it never is.
• Survive somebody leaving, or being told it by a contractor, or writing it on the back of a badge.

What a door ought to be able to tell you

• Who opened it and when, without anybody having to remember to write it down.
• That the person who left in March cannot still get in, because one credential was switched off rather than a code being changed for everybody at once.
• Which doors somebody can open and which they cannot, changed in one place instead of at twelve separate keypads.
• A record you could hand to an insurer or to the police that is more than one person's recollection of the evening.

How this goes

1. We walk the doors and write it down

For every entrance we record what's on it now and who can currently get through it. That list is almost always longer than anybody expects, and the document is yours whatever you decide.

2. It changes at your pace

Usually the doors nobody has thought about in years go first, and the rest follow on a schedule you set.