Chapter 8: Network Security

It is true greatness to have in one the frailty of a man and the security of a god. —Seneca

Problem: Security Attacks

Computer networks are typically a shared resource used by many applications representing different interests. The Internet is particularly widely shared, being used by competing businesses, mutually antagonistic governments, and opportunistic criminals. Unless security measures are taken, a network conversation or a distributed application may be compromised by an adversary.

Consider, for example, some threats to secure use of the web. Suppose you are a customer using a credit card to order an item from a website. An obvious threat is that an adversary would eavesdrop on your network communication, reading your messages to obtain your credit card information. How might that eavesdropping be accomplished? It is trivial on a broadcast network such as an Ethernet or Wi-Fi, where any node can be configured to receive all the message traffic on that network. More elaborate approaches include wiretapping and planting spy software on any of the chain of nodes involved. Only in the most extreme cases (e.g.,national security) are serious measures taken to prevent such monitoring, and the Internet is not one of those cases. It is possible and practical, however, to encrypt messages so as to prevent an adversary from understanding the message contents. A protocol that does so is said to provide confidentiality. Taking the concept a step farther, concealing the quantity or destination of communication is called traffic confidentiality—because merely knowing how much communication is going where can be useful to an adversary in some situations.

Even with confidentiality there still remains threats for the website customer. An adversary who can't read the contents of your encrypted message might still be able to change a few bits in it, resulting in a valid order for, say, a completely different item or perhaps 1000 units of the item. There are techniques to detect, if not prevent, such tampering. A protocol that detects such message tampering is said to provide integrity.

Another threat to the customer is unknowingly being directed to a false website. This can result from a Domain Name System (DNS) attack, in which false information is entered in a DNS server or the name service cache of the customer's computer. This leads to translating a correct URL into an incorrect IP address—the address of a false website. A protocol that ensures that you really are talking to whom you think you're talking is said to provide authentication. Authentication entails integrity, since it is meaningless to say that a message came from a certain participant if it is no longer the same message.

The owner of the website can be attacked as well. Some websites have been defaced; the files that make up the website content have been remotely accessed and modified without authorization. That is an issue of access control: enforcing the rules regarding who is allowed to do what. Websites have also been subject to denial of service (DoS) attacks, during which would-be customers are unable to access the website because it is being overwhelmed by bogus requests. Ensuring a degree of access is called availability.

In addition to these issues, the Internet has notably been used as a means for deploying malicious code, generally called malware, that exploits vulnerabilities in end systems. Worms, pieces of self-replicating code that spread over networks, have been known for several decades and continue to cause problems, as do their relatives, viruses, which are spread by the transmission of infected files. Infected machines can then be arranged into botnets, which can be used to inflict further harm, such as launching DoS attacks.

Although the Internet was designed with the redundancy to survive problems such as the disruption of a link or router, it was not originally designed to provide the kind of security we have been discussing. Internet security mechanisms have essentially been patches. If a comprehensive redesign of the Internet were to take place, integrating security would likely be the foremost driving factor. That possibility makes this chapter all the more pertinent.

There are many tools today for securing networked systems, ranging from various forms of cryptography to specialized devices such as firewalls. This chapter introduces these tools with a particular focus on the use of cryptographic methods to improve network security. Improving the security of networks continues to be a field of rapid change and considerable research effort.

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