5 ms·

I suggested to my local supermarket that they change their alarm code: there were four digits that were white, the others were covered in grime. Knowing the on

I suggested to my local supermarket that they change their alarm code: there were four digits that were white, the others were covered in grime. Knowing the only possible digits makes it a cinch to work out the code just from watching movement from afar. Two years later, those same keys are still clean. The others are even dirtier.

antonb2011 14y ago
Not so. If it's a 4 digit combination, it's already n!, so 24 combinations, which is enough for a good system to lock up and automatically call the police. If it's more, then it's gonna grow as n!/k!l!m!... where, k, l, m... are the numbers of repeated digits, but still it's more than enough to know that someone's trying to brute force the system.
- FreeFull 14y ago
  The point is that the attacker watches from a distance, and using the rough hand movements he/she has seen can reduce that 24 combinations significantly.
  - sageikosa 14y ago
    Hence why I prefer the smaller keypads on ATM machines, so I can minimize my finger movements. Having one keycode for the supermarket is also a good way for disgruntled former employees to act malfeasantly.
- shawabawa3 14y ago
  > so 24 combinations, which is enough for a good system to lock up and automatically call the police 24 is nowhere enough 1. Having a roughly 10% chance of getting it after 2 tries isn't exactly secure. 2. An attacker could just try 1 or 2 combinations per day
  - stcredzero 14y ago
    http://mindyourdecisions.com/blog/2011/01/27/game-theory-and-probability-of-iphone-passwords/ http://mindyourdecisions.com/blog/2011/01/27/game-theory-and...
notatoad 14y ago
I'm not surprised they haven't changed it. At a normal supermarket, anything valuable (cigarettes, medicine, etc) is going to be locked up behind another layer of security during closing hours. Any cash left in the building overnight is going to be stored in a safe in a room with a separate lock and alarm system. The building itself will have a lock in addition to the alarm, cctv, and a security guard that comes by on a regular rotation. Nobody is going to take that risk in order to steal a couple armloads of vegetables. Overnight security is a very small concern to a grocery store compared to continuous shoplifting during the day.
- stcredzero 14y ago
  You can still have some security by repeating one digit. http://mindyourdecisions.com/blog/2011/01/27/game-theory-and-probability-of-iphone-passwords/ http://mindyourdecisions.com/blog/2011/01/27/game-theory-and...
greenyoda 14y ago
They don't even need to change the alarm code. They could just clean the keypad.

antonb2011 14y ago

Not so. If it's a 4 digit combination, it's already n!, so 24 combinations, which is enough for a good system to lock up and automatically call the police. If it's more, then it's gonna grow as n!/k!l!m!... where, k, l, m... are the numbers of repeated digits, but still it's more than enough to know that someone's trying to brute force the system.

FreeFull 14y ago
The point is that the attacker watches from a distance, and using the rough hand movements he/she has seen can reduce that 24 combinations significantly.
- sageikosa 14y ago
  Hence why I prefer the smaller keypads on ATM machines, so I can minimize my finger movements. Having one keycode for the supermarket is also a good way for disgruntled former employees to act malfeasantly.
shawabawa3 14y ago
> so 24 combinations, which is enough for a good system to lock up and automatically call the police 24 is nowhere enough 1. Having a roughly 10% chance of getting it after 2 tries isn't exactly secure. 2. An attacker could just try 1 or 2 combinations per day
- stcredzero 14y ago
  http://mindyourdecisions.com/blog/2011/01/27/game-theory-and-probability-of-iphone-passwords/ http://mindyourdecisions.com/blog/2011/01/27/game-theory-and...

notatoad 14y ago

I'm not surprised they haven't changed it. At a normal supermarket, anything valuable (cigarettes, medicine, etc) is going to be locked up behind another layer of security during closing hours. Any cash left in the building overnight is going to be stored in a safe in a room with a separate lock and alarm system. The building itself will have a lock in addition to the alarm, cctv, and a security guard that comes by on a regular rotation. Nobody is going to take that risk in order to steal a couple armloads of vegetables. Overnight security is a very small concern to a grocery store compared to continuous shoplifting during the day.

stcredzero 14y ago
You can still have some security by repeating one digit. http://mindyourdecisions.com/blog/2011/01/27/game-theory-and-probability-of-iphone-passwords/ http://mindyourdecisions.com/blog/2011/01/27/game-theory-and...

greenyoda 14y ago

They don't even need to change the alarm code. They could just clean the keypad.