These course materials from Dr Siaw-Lynn Ng's information security module introduce fundamental concepts in cryptography, exploring both historical and modern approaches to secure communication. The initial document outlines core terminology like cryptosystems and adversarial models, alongside classical ciphers such as Caesar and Vigenère, highlighting their vulnerabilities. A subsequent excerpt details the mechanics of permutation ciphers through definitions and examples. Finally, another segment demonstrates the cryptanalysis of substitution ciphers, illustrating techniques like frequency analysis to break encrypted messages.