This book constitutes the refereed proceedings of the 18th International Workshop on Security, IWSEC 2023, which took place as a hybrid event in Yokohama, Japan, during August 29-31, 2023. This event was held in hybrid mode. The 14 full papers presented in this book were carefully reviewed and selected from 47 submissions. They were organized in topical sections as follows: System and Hardware Security; Symmetric Key Cryptography; Public Key Cryptography; Zero Knowledge Proofs; and Card Based Cryptography.