Quantum Key Distribution (QKD) is a method for securely sharing encryption keys between two parties, using the fundamental principles of quantum mechanics. Unlike classical encryption, which relies on mathematical complexity, QKD guarantees security based on the laws of physics.
Core Principle:
QKD uses quantum states of particles (like photons) to encode key bits. Because of the no-cloning theorem and quantum measurement disturbance, any attempt to intercept or measure these particles inevitably alters their state, alerting the communicating parties to eavesdropping.
How It Works (BB84 Protocol Example):
- Key Encoding: A sender (Alice) sends photons to a receiver (Bob), each polarized in one of four states (e.g., vertical, horizontal, +45°, or −45°).
- Random Measurement: Bob randomly chooses measurement bases. He later compares choices with Alice over a public channel (without revealing the actual bits).
- Key Sifting: They discard mismatched measurements and keep the rest, forming a raw key.
- Error Checking: They test a portion of the key for errors. If too many discrepancies exist, it suggests eavesdropping, and the key is discarded.
- Final Key: After error correction and privacy amplification, a secure, shared encryption key remains.
Key Advantages:
- Unconditional security based on quantum laws.
- Detects any eavesdropping attempt.
- Enables forward secrecy, even against future quantum computers.
QKD is already used in secure government and banking networks and is a cornerstone of future quantum-secure communication systems.