Traffic Flow Security
So far, we have looked at controls that cover the most common network threats: cryptography for eavesdropping, authentication methods for impersonation, intrusion detection systems for attacks in progress, architecture for structural flaws. Earlier in this chapter, we listed threats, including a threat of traffic flow inference. If the attacker can detect an exceptional volume of traffic between two points, the attacker may infer the location of an event about to occur.
The countermeasure to traffic flow threats is to disguise the traffic flow. One way to disguise traffic flow, albeit costly and perhaps crude, is to ensure a steady volume of traffic between two points. If traffic between A and B is encrypted so that the attacker can detect only the number of packets flowing, A and B can agree to pass recognizable (to them) but meaningless encrypted traffic. When A has much to communicate to B, there will be few meaningless packets; when communication is light, A will pad the traffic stream with many spurious packets.
A more sophisticated approach to traffic flow security is called onion routing [SYV97]. Consider a message that is covered in multiple layers, like the layers of an onion. A wants to send a message to B but doesn't want anyone in or intercepting traffic on the network to know A is communicating with B. So A takes the message to B, wraps it in a package for D to send to B. Then, A wraps that package in another package for C to send to D. Finally, A sends this package to C. This process is shown in Figure 7-33. The internal wrappings are all encrypted under a key appropriate for the intermediate recipient.
Receiving the package, C knows it came from A, although C does not know if A is the originator or an intermediate point. C then unwraps the outer layer and sees it should be sent to D. At this point, C cannot know if D is the final recipient or merely an intermediary. C sends the message to D, who unwraps the next layer. D knows neither where the package originally came from nor where its final destination is. D forwards the package to B, its ultimate recipient.
With this scheme, any intermediate recipientsthose other than the original sender and ultimate receiverknow neither where the package originated nor where it will end up. This scheme provides confidentiality of content, source, destination, and routing.
At the end of our earlier discussion on threats in networks, we listed in Table 7-4 many of the vulnerabilities present in networks. Now that we have surveyed the controls available for networks, we repeat that table as Table 7-7, adding a column to show the controls that can protect against each vulnerability. (Note: This table is not exhaustive; other controls can be used against some of the vulnerabilities.)
As Table 7-7 shows, network security designers have many successful tools at their disposal. Some of these, such as encryption, access control and authentication, and programming controls, are familiar from previous chapters in this book.
But three are specific to networked settings, and we explore them now in greater depth: firewalls, intrusion detection systems, and encrypted e-mail. Firewalls control traffic flow into and out of protected network segments. Intrusion detection systems monitor traffic within a network to spot potential attacks under way or about to occur. And encrypted email uses encryption to enhance the confidentiality or authenticity of e-mail messages.