[e-lang] Card game based on covert channels

[David Hopwood]

Constantine Plotnikov wrote:
> Chris Hibbert wrote:
> 
>>> I have just remembered that there is a card game that is based on
>>> covert channels. It is "Contract Bridge". Covert channel is
>>> established and used in bidding process. There is also a way to
>>> introduce a noise in the channel to confuse opponents covert channel.
>>>
>>> This game might be a good sample to explain what is a covert channel
>>> in the protocol in security books or articles.
>>
>> That's an interesting idea, but the fact that the bidders are required
>> to explain their (partners') bids makes it more overt than covert.  I
>> think it may make a good opening example, but if you try to explain it
>> in any depth, the analogy will get pretty strained.
> 
> This is a covert channel because the formal surface meaning of the
> protocol is making bids. But partners interpreting it as information
> about state of their hands.

I disagree that this is a good example; bidding is an overt channel.

As the Wikipedia article correctly says:

    * Information may only be passed by the calls made and later by the cards
      played, and not by any other means.
    * The agreed-upon *meaning* [emphasis added] of all information passed
      must be available to the opponents.

Use of covert channels in Contract Bridge -- that is, communication channels
other than bids and the playing of cards -- is normally considered to be cheating.

How much effort is expended on preventing covert channels depends on how seriously
the game is being played. Again quoting from the Wikipedia article:

# In tournaments, "bidding boxes" are frequently used. A bidding box is a box
# of cards, each bearing the name of one of the legal calls in bridge. A player
# wishing to make a call displays the appropriate card from the box, rather
# than making a verbal declaration. This prevents unauthorized information from
# being conveyed via voice inflection. In top national and international events,
# "bidding screens" are used. These are diagonal screens which are placed across
# the table, preventing a player from seeing his partner during the game.

> I have not played game myself, so it is
> interesting to learn that there are even rules for using bids as covert
> channel ;).
>
>> And most bidding systems discourage injecting noise into the
>> opponents' channel, since your partner is expected to decode that as a
>> signal to her.
>>
> I had a friend that played the game quite much and he told me that some
> bid values are often reserved in the protocol to make empty shots that
> answers using standard systems more difficult if usage of standard
> system is detected. He also did not use standard systems for some reasons.

I haven't played bridge at tournament level, but I have played with some friends
who play Duplicate Bridge (<http://www.pagat.com/boston/bridge.html#duplicate>).
In that case the systems used by each pair were explained before the game, and
relying on conventions not known to other pairs was considered at best impolite.

-- 
David Hopwood <david.nospam.hopwood at blueyonder.co.uk>