[Enigmail] Sender under duress

[Derek L. VerLee]

On Sun, 2008-09-21 at 11:03 -0500, Robert J. Hansen wrote:
> enigmail at devek.us wrote:
> > Is there some way that a person signing a message with enigmail can
> > indicate that they are under duress so that only the recipient will know?
> 
> It generally requires having a pre-arranged signal.  For instance, using
> the word "wacky" might be the "I'm under duress" signal -- the word
> needs to be common enough that the person applying pressure won't think
> it's strange, but sufficiently uncommon that it's not something you'd
> normally use in everyday communications.
> 
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This is an interesting question that reminds me of several situations in
fiction I have read and no doubt comes up in real live.  The
pre-arranged signal is an optimal solution.  
A more interesting question is, can this be done without any prearranged
signal?  
I think so, but there introduces some risk of misinterpretation.  It
requires that both the sender and the receiver share knowledge that the
adversary does not have.  An example would be referring to a fact (such
as a previous event between the senders) that did not take place, and
about which neither of you is likely to be mistaken, but which the
adversary can not know did not take place.  The adversary will not be
suspicious if the reference is mundane and plausible. It would help if
the receiver of the message, however, is privy to the possibility of the
sender's duress, thus having received the message, would think the error
is some sort of hidden meaning, instead of the worst case scenario,
sending a message back with a correction or request for clarification,
which would notify the adversary of the attempt to send the message.

It might be more difficult still if the content of the message were to
be prescribed.  For example, the message is given to the sender by the
adversary and they are told to sign it.  In this case it would be
impossible I think, if the adversary verifies the signature is correct
before sending the message.  

_derek