Book excerpt: This book constitutes revised selected papers from the 20th International Conference on Information Security and Cryptology, ICISC 2017, held in Seoul, South Korea, in November/December 2017. The total of 20 papers presented in this volume were carefully reviewed and selected from 70 submissions. The papers were organized in topical sections named: symmetric key encryption; homomorphic encryption, side channel analysis and implementation; broadcast encryption; elliptic curve; signature and protocol; and network and system security.