"Foundations of Cryptographic Systems and Network Defence" serves as a comprehensive introductory text for students and practitioners entering the field of cybersecurity. The book is anchored in the "CIA Triad"—Confidentiality, Integrity, and Availability—and systematically explores the mechanisms required to maintain these pillars over insecure channels. The narrative begins with a rigorous examination of the threat landscape, classifying various cyber-attacks and security models. It then pivots to the mathematical and algorithmic heart of the subject: cryptography. Readers are guided through the evolution of ciphers, from classical substitution to modern symmetric standards like DES and IDEA, and into the sophisticated realm of Asymmetric Key Cryptography (RSA). Beyond encryption, the text addresses the vital components of digital trust, including hash functions, digital signatures, and biometric authentication. The final sections bridge theory and practice by analyzing internet security protocols (SSL/TLS), email protection (PGP, S/MIME), and the architectural deployment of firewalls. By the conclusion, the reader will possess the analytical tools to identify protocol weaknesses and the technical knowledge to design secure systems.