Cryptography and Security Systems is designed to provide a clear and comprehensive introduction to the principles, techniques, and real-world applications of modern information security. As digital technologies increasingly govern communication, commerce, and critical infrastructure, protecting data and systems from unauthorized access and malicious attacks has become essential. This book presents foundational concepts such as encryption, authentication, integrity, and key management, while also exploring how these concepts are implemented in practical security systems. The content is structured to bridge theory and practice, making it suitable for students, educators, and professionals in computer science and information technology. Emphasis is placed on understanding the rationale behind cryptographic algorithms, common security protocols, and emerging challenges such as cyber threats and post-quantum security. By the end of this book, readers will gain the analytical skills and technical awareness required to design, evaluate, and apply secure systems in real-world scenarios.