Curriculum / Quantum Cryptography / Eavesdropping Strategies and Detection
Eavesdropping Strategies and Detection
Explore different attack strategies on QKD and how they are detected.
Eavesdropping Strategies and Detection
In BB84, Eve has several attack strategies, each with different tradeoffs between information gain and error introduction. Understanding these attacks concretely, by simulating them, deepens intuition for why the QBER threshold matters and how privacy amplification bounds Eve's information.
The Intercept-Resend Attack
The simplest attack: Eve intercepts Alice's qubit, measures it in a random basis, then resends a new qubit based on her result.
When Eve's basis matches Alice's (50% probability):
- •Eve gets the correct bit with certainty
- •She resends the same state Alice sent
- •Bob's measurement gives the correct result
- •No error introduced: Eve gains 1 bit of information
When Eve's basis differs (50% probability):
- •Eve's measurement result is random (50% correct, 50% wrong)
- •She resends a qubit in her (wrong) basis
- •Bob's measurement gives the correct result only 50% of the time
- •Error probability: 50% × 50% = 25% in this case
Net QBER from full intercept-resend: 50% × 0% + 50% × 50% = 25%
This 25% threshold is why BB84 protocols abort when QBER > 11% (the practical security threshold accounting for channel noise and privacy amplification).
The Cloning Attack
Eve cannot clone the quantum state (no-cloning theorem), but she can make an imperfect copy using a universal quantum cloning machine (UQCM). The optimal UQCM produces a copy with fidelity 5/6 ≈ 0.833: the clone is correct 83.3% of the time.
A cloning attack introduces a QBER of approximately 1/6 ≈ 16.7%: less than the intercept-resend attack, but still above the 11% abort threshold.
The Coherent Attack
In a coherent attack, Eve entangles her probe qubit with Alice's transmitted qubit without measuring it immediately. She stores her probe qubit, waits for Alice and Bob to complete sifting and announce bases, then measures her probe in the optimal basis to extract maximum information.
This is the opening of the lesson. The full walkthrough, the interactive circuit, and the graded challenge continue inside myqubit.
How this lesson works
A hands-on coding challenge. You write Qiskit-compatible Python in the browser editor, run it instantly via WebAssembly, watch the circuit and Bloch sphere react, and pass automatic output checks. The AI tutor Qubitus gives Socratic hints if you get stuck.
Part of: Quantum Cryptography
Master quantum key distribution, the threat quantum computers pose to classical cryptography, and post-quantum cryptographic standards.
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