Encryption can be performed at different communication levels, each with different types of protection and implications. Two general modes of encryption implementation are link encryption and end-to-end encryption. Link encryption encrypts all the data along a specific communication path, as in a satellite link, T3 line, or telephone circuit. Not only is the user information encrypted, but the header, trailers, addresses, and routing data that are part of the packets are also encrypted. The only traffic not encrypted in this technology is the data link control messaging information, which includes instructions and parameters that the different link devices use to synchronize communication methods. Link encryption provides protection against packet sniffers and eavesdroppers. In end-to-end encryption, the headers, addresses, routing, and trailer information are not encrypted, enabling attackers to learn more about a captured packet and where it is headed.
Link encryption, which is sometimes called online encryption, is usually provided by service providers and is incorporated into network protocols. All of the information is encrypted, and the packets must be decrypted at each hop so the router, or other intermediate device, knows where to send the packet next. The router must decrypt the header portion of the packet, read the routing and address information within the header, and then re-encrypt it and send it on its way.
With end-to-end encryption, the packets do not need to be decrypted and then encrypted again at each hop, because the headers and trailers are not encrypted. The devices in between the origin and destination just read the necessary routing information and pass the packets on their way.
End-to-end encryption is usually initiated by the user of the originating computer. It provides more flexibility for the user to be able to determine whether or not certain messages will get encrypted. It is called “end-to-end encryption” because the message stays encrypted from one end of its journey to the other. Link encryption has to decrypt the packets at every device between the two ends.
Link encryption occurs at the data link and physical layers. Hardware encryption devices interface with the physical layer and encrypt all data that pass through them. Because no part of the data is available to an attacker, the attacker cannot learn basic information about how data flows through the environment. This is referred to as traffic-flow security.
The following list outlines the advantages and disadvantages of end-to-end and link encryption methods.
Advantages of end-to-end encryption include the following:
• It provides more flexibility to the user in choosing what gets encrypted and how.
• Higher granularity of functionality is available because each application or user can choose specific configurations.
• Each hop computer on the network does not need to have a key to decrypt each packet.
Disadvantages of end-to-end encryption include the following:
• Headers, addresses, and routing information are not encrypted, and therefore not protected.
Advantages of link encryption include the following:
• All data are encrypted, including headers, addresses, and routing information.
• Users do not need to do anything to initiate it. It works at a lower layer in the OSI model.
Disadvantages of link encryption include the following:
• Key distribution and management are more complex because each hop device must receive a key, and when the keys change, each must be updated.
• Packets are decrypted at each hop; thus, more points of vulnerability exist.