How easy would it be to fool the secret knock detector?

Discussion in 'Math' started by strantor, Feb 23, 2012.

  1. strantor

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    http://www.youtube.com/watch?v=zE5PGeh2K9k

    I love this lock. It could be integrated with a solenoid and used on a safe or a bunker, anything. No more need for a door handle on the outside. No external dial for a safecracker to manipulate. No digital keypad for a hacker to hack. No way to get in without knocking on the door in the correct pattern. So, I'm wondering how easy it would be to defeat it. With a 4 digit rolling padlock, you can simply go through as many of the 10,000 possible combinations needed until you get the right one. This secret knock detector confuses me when I try to think of it in the same way, because there are not a set number of possible combinations - it all depends on the beat (I think). It could be as simple as a single knock, or it could be a drumroll. And the dead time between knocks counts too as well (I think). So, how would you go about estimating the chances of unauthorized entry? (yes, I realize someone could record the knock, or mow the door down with a truck, I'm not asking about that)
     
  2. praondevou

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    But that's the way it can be done. You can record the knock by various means, don't even have to be near the door.

    Video or sound recording, with laser listening systems or just a tiny hidden audiorecorder... There are many ways and I think it would be quite easy to fool it... unless it changes the knock code after every knock and only you know in what sequence...

    Electronics can fail so you still need a normal lock with key. If you have a key then you can also carry around a transponder, it would be easier to install a near range transponder system to automatically unlock the door. Plus it's much harder to hack.
     
  3. strantor

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    I've read (chain email) that transponder signals can be recorded and replayed. I'm not sure if it's accurate, but what I read was that people can hide in a parking lot with some kind of device that captures the signal from you car's key fob when you unlock it; then they follow you home, wait for you to go to bed, and then open your car and steal all your stuff and/or your car. The scenario seems a little far fetched (too much work for limited return/ too sophisticated for the common criminal who steals things from cars), but my limited understanding of the workings of these things tells me that it is very likely possible. Is it?

    This mathematical question of "what are the odds of guessing the code" is becoming or has already become more of a technical curiosity than the big picture. It seems to defeat the "maximum number of possible combinations" logic. The there are 20 knocks maximum but it all depends on the beat. It seems there could potentially be an infinite number of beats using 20 knocks - how do you define a beat? with 1 knock, there could be only one beat (one combination). With 2 knocks, there could be only one beat, because the time between knocks does not matter. With 3 knocks, I think there could be 3 beats. With 4 knocks, I lose track because it could go:
    knock . knock . knock . knock, or
    knock ... knock . knock . knock, or
    knock . knock ... knock . knock, or
    knock . knock . knock ... knock,
    that seems to be the end, but what about
    knock . knock . knock ............. knock, and so forth?
    ... getting to 20 seems like I need a formula for it.
     
    Last edited: Feb 23, 2012
  4. praondevou

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    Let's take an example. A common transponder system is like this:

    A magnetic field at 125kHz is being "transmitted" via a coil. The range of this device is very short, like a 1 or 2 feet. The passive transponder gets into the range of the transmitter and by drawing energy from the transmitter in a certain pattern according to the code of the transponder it modulates the signal that comes from the transmitter, this in turn detects that modulation. I would find it very hard to hack this thing. First you have to put the listening device very near to the transmitter (magnetic field decays much faster than an RF wave). ANd you need the necessary equipment to decrypt the transponders messages. Very difficult to intercept in my opinion, because of invisibility of the signal and no possibility to read it from any distance. (before someone tries this I guess he will prefer the truck method you mentioned)

    RF unlock devices (active transmitters) may be easier to intercept.

    The knocking device could be seen, heard and recorded and copied more easily from a distance.
     
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  5. praondevou

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    What if you forget todays knock code?

    Was it ...knock knock... knock or was it knockknock... knockknock:D?

    Imagine that with 20 knocks:rolleyes:
     
  6. strantor

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    yeah, and If my thinking is correct (that there is potentially infinite combinations) then if you forgot, you are SCREWED. Especially if it's to you underground bunker where your entire zombie combat arsenal and life's savings worth post-apocalypse supplies are :).
     
  7. justtrying

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    Also, what if your knock is not very consistent, how forgiving is the system? I have terrible sense of beat and rhythm and bad memory so it sounds like I would be screwed with this system.

    I like keys. Buttons break and need batteries, its difficult to break a key, although I did it once...

    just watched the video: so all your neighbours will have access to the apartment then :cool:
     
  8. strantor

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    it has a sensitivity adjustment apparently. I browsed through the guy's website; he said with the adjustment all the way up he can't open his own door - his knock is not consistent enough. He mentioned instead of using a knock sensor you could use a door bell. I think that's better - if the door bell were not connected to an actual bell, there would be no sound, nobody could record it.

    key locks can be easily picked I hear.
     
  9. THE_RB

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    If you want something that can't be beaten make a viewport the person puts his eye up against, and turn a knob etc to enter the code, which can only be seen by the person.

    Anyone recording can only see a person turning a knob back and forth.

    Add an exponential timeout for wrong codes, so nobody can find the code by repeat attempts. Bingo, unhackable doorlock.
     
  10. uknow

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    Apr 2, 2012
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    That would be interesting to walk up to a flat wall make your knocks and then enter. Sounds like it would be easy to hack though if someone was interested but who would know it was the knocks that let you enter in the first place?
     
  11. steveb

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    ... A curious neighbor who notices that you always knock before entering your own house. Once a person's curiosity is aroused, it's just a matter of time before they figure it out.
     
  12. THE_RB

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    Yep, we live in a world full of $20 digital recording devices, dictaphones etc many are sound activated. You just hide one near their door and it records the "secret knock".

    It still sounds like a fun kids level secret security project though. :)
     
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