The field of cryptography and security has become increasingly important in our modern digital age. This book provides an introduction to information and network security, addressing the challenges and risks associated with security attacks. The various types of security attacks are discussed, including passive attacks, active attacks, and malware, along with their potential consequences. The importance of security is emphasized, as is the need for security measures in a variety of settings. The book also explores the components and policies necessary for effective security, including confidentiality, integrity, and access control. Finally, the book covers the basics of cryptography, including classical and modern techniques, symmetric-key cryptography, and public-key cryptography. Real-world applications of cryptography, including authentication, digital signatures, and encryption, are also discussed. This book serves as a valuable resource for anyone interested in the fundamentals of cryptography and security. Keywords: Cryptography, Security fundamentals, Confidentiality services, Integrity services, Email security, Access control mechanism, Modern cryptographic techniques, Symmetric-key cryptography, Feistel block ciphers, Authentication, Digital signatures