IISC-hackathon

🧠 BB84 Quantum Key Distribution Simulation

This project simulates the Bennett-Brassard 1984 (BB84) Quantum Key Distribution (QKD) protocol using Qiskit.
It demonstrates how two parties — Alice and Bob — can securely share a cryptographic key using the principles of quantum mechanics, while detecting any eavesdropper (Eve) attempting to intercept their communication.

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📘 Overview

The BB84 protocol leverages quantum physics — particularly the no-cloning theorem and measurement disturbance — to ensure that any attempt to intercept quantum information introduces detectable errors.

This simulation includes:

• A secure communication channel (no eavesdropping).
• An insecure channel where Eve performs an intercept-resend attack.
• QBER (Quantum Bit Error Rate) analysis for eavesdropper detection.

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⚙️ Features

• Full implementation of BB84 key distribution in Qiskit.
• Configurable simulation parameters (key size, eavesdropping, etc.).
• Batch execution optimization for improved performance.
• Automatic QBER calculation and key validation.
• Comparison between secure and insecure scenarios.

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🧩 BB84 Protocol Workflow

1. Quantum Channel (Encoding & Measurement)

• Alice randomly generates bit and basis strings.
• She encodes each bit into a qubit using:
  • Z-basis (rectilinear): 0 → |0⟩, 1 → |1⟩
  • X-basis (diagonal): 0 → |+⟩, 1 → |−⟩
• Bob measures each qubit using his own random basis choices.

2. Public Channel (Key Sifting)

• Bob announces his bases.
• Alice tells which bases match.
• Both discard bits where bases differ.
• The remaining bits form the sifted key.

3. Eavesdropper Detection (QBER)

• A random subset of the sifted key is compared publicly.
• The Quantum Bit Error Rate (QBER) is computed:
  • If QBER ≤ threshold (10%), the key is secure.
  • If QBER > threshold, eavesdropping detected — key is discarded.

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🧠 Simulation Design

🧍 Eve: Intercept-Resend Attack

• Eve measures each qubit using a random basis.
• Resends a new qubit to Bob using her measured value.
• When Eve’s basis ≠ Alice’s, random errors occur — raising QBER.

🚀 Performance Optimization

• All circuits are generated, transpiled, and executed in batch mode using Qiskit’s AerSimulator for speed and efficiency.

🔐 Security Parameter

• QBER_THRESHOLD = 0.10
  Keys exceeding this threshold are rejected as insecure.

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📊 Results Summary

Metric	Secure Channel (No Eve)	Insecure Channel (With Eve)
Sifted Key Length	≈ 500 bits	≈ 500 bits
Errors Detected	0	≈ 125
QBER	0.00	≈ 0.25
Security Threshold	0.10	0.10
Outcome	✅ Secure	❌ Insecure (Key Discarded)

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🧾 Conclusion

The Qiskit simulation validates the core security promise of the BB84 protocol:

• Secure Channel: QBER = 0%, key accepted.
• Insecure Channel: QBER ≈ 25%, eavesdropping detected, key discarded.

BB84’s security arises not from preventing eavesdropping — but from guaranteeing its detection.

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🧠 Requirements

• Python 3.8+
• Qiskit
• NumPy
• Matplotlib (optional for visualization)

Install dependencies:

pip install qiskit numpy matplotlib