This book constitutes the refereed proceedings of the 20th International Conference on Information Security, ISC 2017, held in Ho Chi Minh City, Vietnam, in November 2017.  The 25 revised full papers presented were carefully reviewed and selected from 97 submissions. The papers are organized in topical sections on symmetric cryptography, post-quantum cryptography, public-key cryptography, authentication, attacks, privacy, mobile security, software security, and network and system security.