>> Really? Hmm... sounds physically implausible to me, but OK.
> 
> That is because you don't know how a lock and key work. They are very
> simple physical devices.

The shape of the key aligns a series of pins. By design, exactly one 
shape of key does this. It's difficult to see how you'd make two 
unrelated shapes both have this property.

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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Invisible <voi### [at] devnull> wrote:
> The shape of the key aligns a series of pins. By design, exactly one 
> shape of key does this. It's difficult to see how you'd make two 
> unrelated shapes both have this property.

  The simplest lock type is the pin lock: the shape of the key raises each
pin a different amount. Each pin is cut in two parts. If the pin is raised
the correct amount, it will free the pin (as the cut will coincide with the
edge of the lock cylinder). If all the pins are raised the correct amount,
the entire lock will be freed and can be turned.

  In pin locks designed for master keys some of the pins are cut in three
parts instead of two. This means that two differently-shaped keys can open
the lock. When all the locks designed for a given master key have the same
secondary cuts, all of them can be opened with the master key. An individual
such lock can also be opened with the individual key assigned to that lock
(but can't be opened with any other key, except the master key, of course).

  Thus a series of locks with a master keys have all the same pin cut
assignment, plus some of the pins have a second cut, for the lock-specific
key.

  (Of course pin locks are quickly falling out of use, at least in Europe,
because they are so easy to lockpick. Other, more secure lock types are
usually used, but the master key idea is basically the same.)

-- 
                                                          - Warp

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On Fri, 20 Jun 2008 12:00:33 +0100, Invisible <voi### [at] devnull> wrote:

>>> Really? Hmm... sounds physically implausible to me, but OK.
>> 
>> That is because you don't know how a lock and key work. They are very
>> simple physical devices.
>
>The shape of the key aligns a series of pins. By design, exactly one 
>shape of key does this. It's difficult to see how you'd make two 
>unrelated shapes both have this property.

Think of a house key, a Yale. The zig zag shape fits into a mask of a
similar shape allowing the key to turn. As you say.
But if on the key one of the high points is reduced then the key will
still turn, as the key will still fit through the mask. With me so
far? Once through the mask the zig zags have to engage a mechanism to
turn the lock. So if you flattened the key to the lowest point it
would turn but not engage the lock. A master key has the relevant zig
zags to engage the lock but not enough to be blocked by the various
masks on different locks.
This is reduced to being too simple.
-- 

Regards
     Stephen

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Warp wrote:

>   In pin locks designed for master keys some of the pins are cut in three
> parts instead of two. This means that two differently-shaped keys can open
> the lock.

I see...

Actually, I wonder. They call them "pins", but just how strong are they? 
Couldn't you open the lock just by breaking all the pins?

>   (Of course pin locks are quickly falling out of use, at least in Europe,
> because they are so easy to lockpick. Other, more secure lock types are
> usually used, but the master key idea is basically the same.)

Yeah, they all involve the shape of the key moving the internal 
components of the lock around...

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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Invisible <voi### [at] devnull> wrote:
> Actually, I wonder. They call them "pins", but just how strong are they? 
> Couldn't you open the lock just by breaking all the pins?

  Depends on the quality and width of the pins. There are, in fact, locks
(even on the main doors of houses at some places) which can be broken by
simply turning then with enough force. A friend of mine who has lived in
Vancouver says that most locks there were of that type. They were completely
insecure. You could open a door by simply turning the knob with enough
force.

  More high-quality pin locks cannot be broken so easily, but they are
still so easy to lockpick that it's better to use a safer design (for
example a disc tumbler lock, which is common here, and which is extremely
hard to lockpick, and while not impossible, requires very sophisticated
tools).

-- 
                                                          - Warp

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>> Actually, I wonder. They call them "pins", but just how strong are they? 
>> Couldn't you open the lock just by breaking all the pins?
> 
>   Depends on the quality and width of the pins. There are, in fact, locks
> (even on the main doors of houses at some places) which can be broken by
> simply turning then with enough force.

Oh, that's cute.

>   More high-quality pin locks cannot be broken so easily, but they are
> still so easy to lockpick that it's better to use a safer design (for
> example a disc tumbler lock, which is common here, and which is extremely
> hard to lockpick, and while not impossible, requires very sophisticated
> tools).

Yeah, well, devise an unpickable lock and somebody will just rip the 
door off its hinges, or break in through a window or something. ;-) 
Weakest point and all that... All you really need is for the lock not to 
be the weakest part of the system.

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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scott wrote:
> Locks can be designed to be opened by two different keys (usually a 
> unique key and a master key common to several locks).

We have cool locks on the doors of our houses, designed especially for 
builders. Basically, all the locks can be opened with the master key, 
until the first time you use the lock-specific key that the builder 
gives to the new homeowner.  So, the first time you unlock your own 
door, you lock the builder's master key out. I have only the vaguest 
idea of how they accomplish that.

-- 
Darren New / San Diego, CA, USA (PST)
  Helpful housekeeping hints:
   Check your feather pillows for holes
    before putting them in the washing machine.

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And lo on Fri, 20 Jun 2008 16:39:56 +0100, Darren New <dne### [at] sanrrcom>  
did spake, saying:

> scott wrote:
>> Locks can be designed to be opened by two different keys (usually a  
>> unique key and a master key common to several locks).
>
> We have cool locks on the doors of our houses, designed especially for  
> builders. Basically, all the locks can be opened with the master key,  
> until the first time you use the lock-specific key that the builder  
> gives to the new homeowner.  So, the first time you unlock your own  
> door, you lock the builder's master key out. I have only the vaguest  
> idea of how they accomplish that.

They use little ball-bearings between the pins, when the builders use  
their key they shear at the top of the bearings and the bearings stay in  
the plug. When you use your key you raise the pins so the shear line is at  
the bottom of the bearings and when you turn they fall into holes set in  
the plug and are thus removed.

-- 
Phil Cook

--
I once tried to be apathetic, but I just couldn't be bothered
http://flipc.blogspot.com

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Phil Cook wrote:
> They use little ball-bearings between the pins, when the builders use 

Well. Now I know. Thanks! :-)

-- 
Darren New / San Diego, CA, USA (PST)
  Helpful housekeeping hints:
   Check your feather pillows for holes
    before putting them in the washing machine.

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Invisible a écrit :
> Yeah, well, devise an unpickable lock and somebody will just rip the 
> door off its hinges, or break in through a window or something. ;-) 
> Weakest point and all that... All you really need is for the lock not to 
> be the weakest part of the system.
> 

And do not lose your keys :-)

It's all nice and well to have an unbreakable door and lock until you 
have to destroy your own wall to go into your own house because the keys 
were lost somehow...

-- 
Vincent

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