This book includes extended and revised versions of a set of selected papers presented at the 20th International Conference on Security and Cryptography, SECRYPT 2023, which took place in Rome, Italy, during July 10–12, 2023; and  the 21st International Conference on Security and Cryptography, SECRYPT 2024, which was held in Dijon, France, during July 8-10, 2024.This book includes 5 papers from SECRYPT 2023 which has received a total of 169 submissions, for SECRYPT 2024 a total of 175 submission has been received of which 7 papers are included in this book. They deal with all theoretical and practical asepcts of data protection, privacy, security and applied cryptography, focusing on the application of security technology, the implementation of systems and lessons learned.