[cap-talk] Authority vs. Information Flow

[Jack Lloyd]

On Wed, Feb 13, 2008 at 04:19:23PM +0000, Toby Murray wrote:

> Both Alice and Dave now have authority to cause Bob to be able to see
> the light turn on. But can either of them pass information to Bob?
> 
> This question seems tricky because when Bob sees the light turn on, he
> can't know for sure who turned it on (Alice or Dave). Hence, it seems
> possible to argue that neither can pass information to Bob.

Consider an iterated example, where both Dave and Alice can turn the
light on or off at any time - then the usual channel coding mechanisms
can be applied. How fast they could communicate would depend on how
'noisy' Dave was (for instance if he never touches it, compared to him
flipping the light on and off constantly), but it seems it would
always be possible for Alice to send information to Bob at some
non-zero rate (if perhaps very small). If Dave and Alice are working
in concert, then they could even both transmit to Bob without
interfering with the other (ala Ethernet or TDMA), for instance by
Alice signalling on every odd second and Dave signalling every even
one.

I think you could make a reasonable analogy between your situation and
covert channels.

-Jack