This book constitutes the proceedings of the 20th International Workshop on Security on Advances in Information and Computer Security, IWSEC 2025, held in Fukuoka, Japan, in November 25, 2025.The 27 full papers were carefully reviewed and selected from 95 submissions. These papers were categorized into the following sections: Public-Key Encryption; Signature Schemes; Security and Analysis of Post-Quantum Cryptography; Card-Based Cryptography; Protocols; Privacy-Preserving Techniques; AI Attacks and Mitigations; Vulnerability Detection; Cache & Code-Execution: Attacks and Mitigations; Cyber Attack & Threat Analysis; Wireless Communication Security and Human Factors in Cyber Security.