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What is Link Encryption?
Link encryption is a security method used in communications networks to transmit encrypted data between individual computers. With this method, data is encrypted and decrypted by each piece of hardware along the path, such as a network router or other specialized device.
When the communication link is encrypted in this way, the entire data transmission is hidden, unlike other encryption schemes where the transmission can still be intercepted. The method may also be referred to as link layer encryption or link layer encryption. This is because everything happens at the bottom layer of the Open Systems Interconnect (OSI) model, known as the data link layer.
As data packets leave the network interface, the entire packet, the bundle of data sent over the network connection, is encrypted. Link encryption is thus unique because the packet's header information, which contains source and destination address information, is encrypted along with the actual data payload.
Secure packets are then sent across the line until they encounter another device along the way, where the header is decrypted and checked for address information. If the packets do not reach their destination, they are re-encrypted and sent on their way.
This is useful for securing transmissions against someone trying to sneak onto the line or capture packets for analysis. An attacker cannot know who the data came from, where it is going, and the path it took. The process is usually free of human error because it all happens automatically, saving the user from having to remember to encrypt his communications, facilitating large, regular data transmissions that need to be secure.
The method has some drawbacks. Link encryption suffers greatly on public networks such as the Internet. Many who use this method will only use it on dedicated, leased lines, where greater control over the hardware can be achieved along the way. This means that the keys used to encrypt and decrypt data must be maintained across multiple devices, making each point along the path potentially vulnerable if an attacker gains access to one of the devices along the route.
Another solution is a method known as superencryption, which is used by the user to encrypt the data payload at the application layer and then encrypt the rest of the header information as it travels across the wider network. An additional method of superencryption is known as end-to-end encryption.
The primary difference between a link encryption, then, is that the end-to-end method allows data to traverse an insecure network for some time because the encryption and decryption keys are known at each end of the transmission.
The address and routing information in the headers is still visible to an eavesdropper, but the initial data payload remains secure. In the case of super encryption, however, where both end-to-end and link encryption are used, data transport rarely has to travel further than the local router before entering the encrypted link.