Page 207 - Handout Computer Network.

Computer Network 2026

(possibly stealing passwords and data), impersonating another entity, hijacking an
ongoing session, denying service to legitimate network users by overloading system
resources, and so on. A summary of reported attacks is maintained at the CERT
Coordination Center [CERT 2020].

Having established that there are indeed real threats loose in the Internet, what are the
Internet equivalents of Alice and Bob, our friends who need to communicate securely?
Certainly, Bob and Alice might be human users at two end systems, for example, a real
Alice and a real Bob who really do want to exchange secure e-mail.

They might also be participants in an electronic commerce transaction. For example, a
real Bob might want to transfer his credit card number securely to a Web server to
purchase an item online.

Similarly, a real Alice might want to interact with her bank online.

The parties needing secure communication might themselves also be part of the network
infrastructure.

Recall that the domain name system or routing daemons that exchange routing
information require secure communication between two parties. The same is true for
network management applications, a topic we examined.

An intruder that could actively interfere with DNS lookups, routing computations, or
network management functions could wreak havoc in the Internet.

Having now established the framework, a few of the most important definitions, and the
need for network security, let us next delve into cryptography.

While the use of cryptography in providing confidentiality is self-evident, we’ll see shortly
that it is also central to providing end-point authentication and message integrity—
making cryptography a cornerstone of network security.

7.2 Principles of Cryptography
Although cryptography has a long history dating back at least as far as Julius Caesar,
modern cryptographic techniques, including many of those used in the Internet, are
based on advances made in the past 3️0 years. Kahn’s book, The Codebreakers [Kahn
1967], and Singh’s book, The Code Book: The Science of Secrecy from Ancient Egypt to
Quantum Cryptography [Singh 1999], provide a fascinating look at the

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