1. Department of Applied Science, Faculty of Engineering; Yamaguchi University; Tokiwadai Ube 755-8611 Japan
2. Indian Statistical Institute (ISI), Chennai Centre, SETS (Society for Electronic Transactions and Security),MGR Knowledge City, CIT Campus, Taramani, Chennai 600 113, India;; On leave from the Indian Institute ofTechnology Madras
3. Department of Mathematics; Indian Institute of Technology Kharagpur; Kharagpur-721 302 West Bengal India