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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