Before I can begin discussing VPN implementations such as Internet Protocol Security (IPsec), Layer 2 Tunnel Protocol (L2TP), Point-to-Point Tunneling Protocol (PPTP), and Secure Socket Layer (SSL), you first need an understanding of the technologies that VPNs can use to provide protection for traffic. I'm sure you have already heard of terms such as keys, DES, 3DES, MD5, pre-shared keys, and the like; however, an in-depth understanding of these protocols, algorithms, functions, and processes will help you determine the pros and cons of VPN technologies. Use this information to pick the optimal VPN implementation based on the type of technologies you'll need to protect your traffic.
I've broken this chapter into five sections: keys, encryption, packet authentication, key exchange, and authentication methods. This chapter will discuss these technologies and how they are related to VPNs, including the advantages and disadvantages of the technologies for a particular category, for example, using a pre-shared key or keys for authentication versus digital certificates.