The act of secretly listening to or intercepting private communications.Eavesdropping is the act of secretly listening to or intercepting private communications, whether spoken, written, or electronic. In physics, the concept is central to the field of information security and quantum cryptography, where the goal is to detect or prevent an unauthorized party (often called 'Eve' in cryptographic literature) from gaining access to transmitted data. The term is used metaphorically to describe any attempt to extract information from a channel without the sender's knowledge.
The term 'eavesdropping' dates back to the 16th century, referring to someone standing under the eaves of a house to listen to conversations inside. The modern scientific use in cryptography emerged in the 20th century with the development of information theory and secure communication.
In classical communication, eavesdropping can be as simple as tapping a wire or intercepting a radio signal. In quantum communication, however, the act of eavesdropping is fundamentally different: according to the no-cloning theorem, an unknown quantum state cannot be perfectly copied, and any measurement that extracts information inevitably disturbs the state. This disturbance can be detected by the legitimate parties, who can then discard the compromised key. Thus, quantum eavesdropping is not just a security threat but also a physical phenomenon that reveals itself through measurable changes in the transmitted photons.
Eavesdropping detection is a core principle in quantum key distribution (QKD), which is used in secure communication networks for banks, governments, and military applications.
In quantum cryptography, the eavesdropper is traditionally named 'Eve' (from 'eavesdropper'), while the legitimate parties are 'Alice' and 'Bob' — a naming convention introduced by Ron Rivest in 1978.
