Link encryption is an approach to communications security

Communications security

Communications security is the discipline of preventing unauthorized interceptors from accessing telecommunications in an intelligible form, while still delivering content to the intended recipients. In the United States Department of Defense culture, it is often referred to by the abbreviation...

 that encrypts

Encryption

In cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...

 and decrypts all traffic at each end of a communications line (e.g. a teleprinter

Teleprinter

A teleprinter is a electromechanical typewriter that can be used to communicate typed messages from point to point and point to multipoint over a variety of communication channels that range from a simple electrical connection, such as a pair of wires, to the use of radio and microwave as the...

 circuit or the line between two network switches

Network switch

A network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...

). It contrasts with end-to-end encryption

End-to-end encryption

End-to-end encryption encrypts clear data at source with knowledge of the intended recipient, allowing the encrypted data to travel safely through vulnerable channels End-to-end encryption (E2EE) encrypts clear (red) data at source with knowledge of the intended recipient, allowing the encrypted...

where messages are encrypted by the sender at the point of origin and only decrypted by the intended receiver. Link encryption offers a couple of advantages:

• encryption is automatic so there is less opportunity for human error.
• if the communications link operates continuously and carries an unvarying level of traffic, link encryption defeats traffic analysis
  Traffic analysis
  Traffic analysis is the process of intercepting and examining messages in order to deduce information from patterns in communication. It can be performed even when the messages are encrypted and cannot be decrypted. In general, the greater the number of messages observed, or even intercepted and...
  
  .

On the other hand, end-to-end encryption ensures only authorized persons see the plaintext

Plaintext

In cryptography, plaintext is information a sender wishes to transmit to a receiver. Cleartext is often used as a synonym. Before the computer era, plaintext most commonly meant message text in the language of the communicating parties....

.

Link encryption can be used with end-to-end systems by superencrypting

Superencryption

Multiple encryption is the process of encrypting an already encrypted message one or more times, either using the same or a different algorithm. The terms cascade encryption, cascade ciphering, multiple encryption, multiple ciphering, and superencipherment are used with the same meaning...

 the messages.

Bulk encryption refers to encrypting a large number of circuits at once, after they have been multiplexed

Multiplexing

The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...

.