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.