网络安全技术手册，可以大体了解当前的信息安全的状况-Network security technical manual, you can generally understand the current state of information security

网络安全讲义：介绍网络安全最重要的知识。-Network Security Lecture: The most important knowledge of network security

A good document about network security : - Symmetric-Key Algorithms - Public-Key Algorithms - Digital Signatures - Key Management - Authentication Protocols