This document provides an overview of cipher techniques in networks. It discusses end-to-end encryption versus link-layer encryption and the keys used in each. End-to-end encryption only encrypts at the source and destination, while link-layer encryption encrypts at each hop. It also introduces the concept of traffic analysis and analyzing traffic patterns to deduce information. The document directs the reader to later chapters for more information on specific network security topics like firewalls, intrusion detection, and authentication.