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"  It's a good idea to force your opponent to have to solve multiple
   problems in order to mount a successful attack.  Some examples of
   widely differing problems we might like to present him with are:
   Stealing a shared secret from one of the parties, being present on
   the very first session and every subsequent session to carry out an
   active MitM attack, and solving the discrete log problem.  We want to
   force the opponent to solve more than one of these problems to
   succeed.

   The protocol can make use different kinds of shared secrets.  Each
   type of shared secret is determined by a different method.  All of
   the shared secrets are hashed together to form a session key to
   encrypt the call.  An attacker must defeat all of the methods in
   order to determine the session key."