This document discusses secure communication in network systems. It defines network security as activities that protect a network's usability, reliability, integrity and safety. Secure communication ensures confidentiality by encrypting messages so only the recipient can understand them, authentication to verify identities, and message integrity to ensure messages are not altered. It discusses how authentication can be based on what users know (passwords), have (tokens, cards) or are (biometrics). MACs (message authentication codes) using hash functions with a secret key also ensure message integrity and non-repudiation. The document outlines some principles of implementing network security like layering, limiting access, obscurity and simplicity.