However it is not impossible, and there is still no way to prevent someone who knows the password from accessing the resource. The FTP protocol Figure 9 illustrates this simple access control mechanism for the FTP protocol. Oscar can obtain the password, since all Internet communication between Alice and Bob passes through him. He can then access Bob s FTP server as Alice. Even without the password, he could masquerade as Bob s FTP server to Alice.
Cryptography promises to secure Internet communications.
We will use the email and FTP examples in the following chapters to illustrate the link between an infrastructure s characteristics and its effects on real systems.