This book constitutes the proceedings of the 26th International Conference on Information Security, ISC 2023, which took place in Groningen, The Netherlands, in November 2023. The 29 full papers presented in this volume were carefully reviewed and selected from 90 submissions. The contributions were organized in topical sections as follows: privacy; intrusion detection and systems; machine learning; web security; mobile security and trusted execution; post-quantum cryptography; multiparty computation; symmetric cryptography; key management; functional and updatable encryption; and signatures, hashes, and cryptanalysis.