This book features extended versions of selected papers that were presented and discussed at the 6th International Doctoral Symposium on Applied Computation and Security Systems (ACSS 2019) held in Kolkata, India on 12-13 March, 2019. Organized by the Departments of Computer Science & Engineering and A.K. Choudhury School of Information Technology, both from the University of Calcutta, the symposium's international partners were Ca' Foscari University of Venice, Italy and Bialystok University of Technology, Poland. The chapters cover topics such as biometrics, image processing, pattern recognition, algorithms, cloud computing, wireless sensor networks and security systems, reflecting the various symposium sessions.