magic boxes and secret knocks

[Kragen Sitaker]

Beatrice has a little wooden box with a lockable sliding top.  There
are no movable parts evident from the outside, other than the top
itself; the box is unlocked by tapping it against your palm in one
direction, and locked again by tapping it against your palm in another
direction.

In spy movies and novels and such nonsense, there is sometimes a
secret knock that serves to authenticate one requesting entry.  But
this is my first experience with a mechanical device that responds to
a "secret knock".

This seems like it could be a useful computer-human interface
technique, in limited circumstances.  It provides an input channel
whose existence is not evident if you have not seen it used, and with
a little misdirection, it may not be evident even while it is in use.

This could be useful, for example, at an office door.  A secret knock
could provide the something-you-know and difficult-to-pick properties
of a keypad lock, but could be used in places where a keypad would be
expensive to install or would attract undue attention.

If I wanted my laptop to respond to secret knocks, I would probably
use its microphone to collect the knocks.  This could be useful for
allowing me to issue covert commands --- for example, with tongue
clicks --- even when the laptop is closed and across the room from me.

You could build a little computer rather like the Dallas Semiconductor
iButton --- but instead of communicating with the outside world by
electrical signals, it would produce auditory output and accept input
by shocks or sounds.  It could look like the Bao Ding iron balls
you're supposed to use to exercise your hands.

-- 
<kragen@pobox.com>       Kragen Sitaker     <http://www.pobox.com/~kragen/>
Irony and sarcasm deflate seriousness, and when your seriousness becomes detum-
escent, you're not held responsible for your thoughts. Irony beats thinking like
rock beats scissors. -- http://www.hyperorg.com/backissues/joho-june2-98.html