This book constitutes the proceedings of the 21st International Conference on Information Security, ISC 2018, held in Guildford, UK, in September 2018. The 26 full papers presented in this volume were carefully reviewed and selected from 59 submissions. The book also includes one invited talk in full-paper length. The papers were organized in topical sections named: software security; symmetric ciphers and cryptanalysis; data privacy and anonymization; outsourcing and assisted computing; advanced encryption; privacy-preserving applications; advanced signatures; and network security.