N. Anthapadmanabhan, A. Barg, and I. Dumer. (2006)cite arxiv:cs/0612073
Comment: final version, 12 pages, 2 figures, to appear in IEEE Trans. on
Inform. Theory - Special Issue on Information-theoretic Security, Jun 2008,
simplified proofs in Sections II and III, changes in Theorem 4.1.