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Unformatted text preview: ust be protected, since it processes unencrypted data. If all the network’s users trust one another, and all nodes are in secure locations, this may be tolerable. But this is unlikely. Even in a single corporation, information might have to be kept secret within a department. If the network accidentally misroutes information, anyone can read it. Table 10.2 summarizes the pros and cons of link-by-link encryption. End-to-End Encryption
Another approach is to put encryption equipment between the network layer and the transport layer. The encryption device must understand the data according to the protocols up to layer three and encrypt only the transport data units, which are then recombined with the unencrypted routing information and sent to lower layers for transmission. This approach avoids the encryption/decryption problem at the physical layer. By providing end-to-end encryption, the data remains encrypted until it reaches its final destination (see Figure 10.2). The primary problem with end-to-end encryption is that the routing information for the data is not encrypted; a good cryptanalyst can learn much from who is talking to whom, at what times and for how long, without ever knowing the contents of those conversations. Key management is also more difficult, since individual users must make sure they have common keys. Table 10.2 Link-by-Link Encryption: Advantages and Disadvantages Advantages: Easier operation, since it can be made transparent to the user. That is, everything is encrypted before being sent over the link. Only one set of keys per link is required. Provides traffic-flow security, since any routing information is encrypted. Encryption is online. Disadvantages: Data is exposed in the intermediate nodes. Figure 10.2 End-to-end encryption. Building end-to-end encryption equipment is difficult. Each particular communications system has its own protocols. Sometimes the interfaces between the levels are not well-defined, making the task even more difficult. If encryption takes place at a high layer of the communications architecture, like the applications layer or the present...
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- Fall '10