The signature guarantees the source and integrity of the message. Digital signature is one of the great innovations in the history of cryptography. The recipient of a signed message knows that the message is in fact from the alleged source and that the message hasn't been altered in any way after it was signed. Public-key encryption is used to authenticate the public key. Impersonation 1 and 2 are the worst problems.
Intruders can put rogue code onto a Web server by overrunning a software buffer.
Intruders can embed a Unix command into a Web request so that the Web server is tricked into passing that command on to the operating system. Eavesdropping can be done between Web server and client. They allow client and server to negotiate acceptable levels of security for a particular transaction or session. 20.6 Virtual Private Networks and IPSec VPN - the use of encryption and authentication in the lower protocol layers to provide a secure connection through an otherwise insecure network, typically the Internet. VPNs are generally cheaper than real private networks using private lines but rely on having the same encryption and authentication system at both ends.