The preliminary version of this paper was presented in OPODIS 2004.
The following changes were made in this journal version

* A tighter bound is achieved for securing of protection zone of
  arbitrary shape.  The necessary number of verifiers decreases from
  O(S+P) to O(S) where S and P are the area and perimeter of the
  the protection zone. The discussion as to how this bound is achieved
  is added.

* An algorithm for placing the verifiers such that the ambiguity
  gap is arbitrarily small is added to the logarithmic verification
  time subsection. This section was rather abstract in the preliminary
  version and lacked concrete implementation details.

* A discussion of rejector jamming and disabling attack and ways of
  countering it is added to the implementation consideration section.

* added discussion of directional and sector antenns
  as well as verififer co-location defense.

* Overall, the structure of the paper is changed to include the basic
  verification protocol and its proprieties in one section and various
  extensions in the other.

* The whole paper underwent a major editing effort, the writing is
  streamlined, wording improved, the typographical errors fixed,
  explanations and definitions added throughout.