Curriculum / Quantum Cryptography / Building a Quantum-Secure Communication System
Building a Quantum-Secure Communication System
Combine QKD and one-time pad for provably secure message transmission.
Building a Quantum-Secure Communication System
This lesson integrates the key components of quantum-safe communication into a single pipeline. You will wire together quantum key distribution (BB84), classical XOR encryption (the one-time pad), and eavesdropping detection to build a functional end-to-end quantum-secure messaging system.
System Architecture
A complete quantum-secure communication system has these layers:
Layer 1: Quantum Key Distribution (BB84) Alice and Bob exchange qubits over a quantum channel. After sifting, they share a raw key: a stream of random bits correlated by quantum physics. This is the only part requiring quantum hardware.
Layer 2: Classical Post-Processing
- •Error correction: Alice and Bob reconcile differences in their sifted keys (caused by channel noise, not just eavesdropping)
- •Privacy amplification: A hash function compresses the key to remove any partial information Eve may hold
- •Authentication: The classical channel uses an information-theoretically secure authentication code to prevent man-in-the-middle attacks
Layer 3: One-Time Pad Encryption Using the shared key from Layer 2, Alice encrypts her message by XOR-ing each bit with the corresponding key bit. Bob decrypts by XOR-ing the ciphertext with the same key. Since XOR is its own inverse: decrypt(encrypt(m, k), k) = m.
Layer 4: Eavesdropping Detection Before using the key for encryption, Alice and Bob compare a sample of sifted key bits over the authenticated classical channel. A QBER below the threshold (11%) indicates no significant eavesdropping.
The XOR Encryption Primitive
The one-time pad's encryption and decryption operations are identical:
E(m, k) = m XOR k D(c, k) = c XOR k = (m XOR k) XOR k = m
In Python with integer lists:
- •
[m ^ k for m, k in zip(message_bits, key_bits)]
The key must be at least as long as the message. In a real QKD system, the shared key rate limits the message throughput.
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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