This book constitutes the thoroughly refereed proceedings of the 21st International Conference on Information Security Applications, WISA 2020, held in Jeju Island, South Korea, in August 2020. The 30 full research papers included in this book were carefully reviewed and selected from 89 submissions. They are organized in the following topical sections: AI Security and Intrusion Detection; Steganography and Malware; Application, System, and Hardware Security; Cryptography; Advances in Network Security and Attack Defense; and Cyber Security.