🔒 Quantum Key Distribution (QKD) & BB84 Protocol

🧠 What is QKD?

Quantum Key Distribution (QKD) is a method for two parties (usually called Alice and Bob) to share a secret cryptographic key using the principles of quantum mechanics. Unlike classical key exchange (e.g., RSA or Diffie-Hellman), QKD offers unconditional security — any eavesdropping attempt by an attacker (Eve) will introduce detectable anomalies due to quantum measurement disturbance.

🔬 The BB84 Protocol

The BB84 protocol, developed by Charles Bennett and Gilles Brassard in 1984, is the first and most well-known QKD protocol. It uses quantum properties of photons — particularly their polarization — to encode binary information.

📦 How It Works

1. Alice encodes each bit (0 or 1) in one of two bases — Rectilinear (+) or Diagonal (×).
2. She sends a stream of polarized photons to Bob over a quantum channel.
3. Bob measures each photon randomly in one of the two bases.
4. Later, Alice and Bob publicly compare which bases they used — they keep only the bits where their bases matched.
5. This shared subset becomes their raw key.
6. To check for eavesdropping, they publicly compare a small part of the key. If too many discrepancies are found, they discard the key.
7. If the test passes, they use privacy amplification and error correction to derive a final secure key.

Polarization Bases:
+ (Rectilinear):
  0 = Horizontal (→)
  1 = Vertical   (↑)

× (Diagonal):
  0 = 45°    (↗)
  1 = 135°   (↖)

🚀 Advantages of QKD

• Unbreakable by quantum computers — not based on mathematical assumptions
• Detection of eavesdropping is guaranteed due to quantum properties
• Forward secrecy — past communications remain secure even if keys are exposed later

📉 Limitations & Challenges

• Expensive hardware: requires single-photon sources and detectors
• Limited distance: quantum signals degrade over long fiber or free space
• Doesn’t encrypt data — only distributes keys (must be combined with AES, etc.)

🔗 Applications of BB84 & QKD

• Secure military and government communications
• Banking infrastructure and stock exchanges
• Quantum-safe networks and data centers
• Satellite-to-ground quantum links (e.g., China’s Micius satellite)

🧩 Summary

1. QKD uses quantum mechanics to securely distribute encryption keys.
2. BB84 is the foundational protocol using polarized photons.
3. Eavesdropping introduces detectable anomalies.
4. QKD doesn't encrypt messages — it creates secure keys for AES, etc.
5. Used in critical, high-security communication systems.