Keyless quantum-secure file transfer across a multi-hop network. Team DoodleByte for Q-Hack India 2026, track Quantum Security & Cryptography.
QSDTS sends a file where the message itself is the quantum state, using the DL04 Quantum Secure Direct Communication protocol. No key is ever generated, stored or transmitted. Every link checks for an eavesdropper before the message is encoded on it, and when a link is tapped the network routes around it and the file still arrives, hash-verified.
It runs on Qiskit Aer. This is a simulation, not a hardware experiment.
- Today's key exchange (RSA, elliptic curves) is safe only while factoring and discrete logarithms stay slow. Shor's algorithm removes that on a large quantum computer, and traffic recorded now can be decrypted then.
- Quantum key distribution protects key agreement with physics, but the message still travels classically and keys still have to be stored and managed at every node.
- QSDC removes the key entirely, but it exists almost only in optics labs. A 2025 review of 11 quantum-network simulators found none that implement it [9].
QSDTS is an open, runnable QSDC network you can execute on a laptop.
Requires Python 3.10 or newer.
git clone https://github.com/gowtham472/qsdts
cd qsdts
python -m venv .venv
source .venv/bin/activate # Windows: .venv\Scripts\activate
pip install -e ".[dev]"Run the live demo (about 30 seconds):
python -m qsdts.demo1. DL04 on a clean fibre
check QBER 0.0% -> PROCEED
Bob decodes: 'no key was ever created' integrity OK
2. Same session, eavesdropper on the fibre
check QBER 23.5% on 85 sifted bits -> ABORT
message bits ever encoded: 0
3. File transfer across the network, attacked mid-transfer
414 bytes, 15 frames, eavesdropper switches on at frame 5
frame 5 ABORT R1-Bob QBER 24.4% ABORT: message never encoded
frame 5 REROUTE R1-Bob QBER 24.4% R1 routes around R1-Bob via R1-R3-Bob
...
path used: Alice > R1 > Bob
path used: Alice > R1 > R3 > Bob
SHA-256 received d663937a8cd13d42b013924c1972e40f... VERIFIED
aborts 1, reroutes 1, message bits exposed on aborted sessions: 0
Run the tests (40 tests, a couple of minutes):
pytestReproduce every number and figure:
python experiments/run_experiments.py # about 5 minutes, writes results/results.json
python experiments/figures.py # redraws results/*.pngSeeds are fixed, so a rerun reproduces the same measured values exactly.
Or skip installing: open the Colab notebook and choose Runtime > Run all.
All measured on Qiskit Aer. Fibre noise is a depolarizing channel with parameter 0.01 (about 0.5% bit flips per pass).
| What | Result |
|---|---|
| QBER under a full intercept-resend attack, DL04 | 25.27% over 16,652 check bits (theory 25%, within one standard error) |
| Same attack, BB84 on the same channel | 24.80% over 4,967 check bits |
| Message bits ever placed on a link that failed its check | 0, across 176 aborted sessions and transfers |
| Files delivered and SHA-256 verified under a mid-transfer attack | 8/8 with adaptive routing, 0/8 without |
| Message bits per qubit prepared | DL04 0.675, BB84 + one-time pad 0.374 (1.8x) |
| Message bits per fibre transmission | DL04 0.386, BB84 0.374 (about equal) |
DL04 is 1.8x more efficient per qubit prepared, but sends each qubit twice, so per fibre transmission the two are about equal. The advantage is that no key exists, not raw throughput.
How strong must an attack be to be caught within one block (30 blocks per point)?
| Qubits intercepted | 0% | 10% | 20% | 30% | 40% | 50% | 60% | 80% | 100% |
|---|---|---|---|---|---|---|---|---|---|
| Mean QBER | 0.4% | 2.9% | 5.8% | 7.6% | 10.2% | 13.7% | 15.7% | 20.9% | 24.5% |
| Blocks stopped | 0% | 0% | 20% | 53% | 63% | 100% | 93% | 100% | 100% |
An honest limit: strong attacks are stopped almost every time, but a weak attacker (20% of qubits or fewer) hides inside fibre noise within a single block. Detecting it across many blocks is the next research question.
- Prepare. Bob prepares N qubits, each randomly in |0>, |1>, |+> or |->, and sends them to Alice.
- Check. Alice measures a random subset in random bases and announces the results. Bob compares them with what he prepared and computes the QBER.
- Encode, only if the QBER is below the threshold. Alice encodes each message bit on a remaining qubit (I for 0, iY for 1) and sends the block back, mixed with a few return-check qubits.
- Decode. Bob measures each qubit in the basis he prepared it in. An unchanged state reads 0, a flipped one reads 1.
The security-critical invariant is that step 3 never runs if step 2 failed. In
QSDTS that is structural, not a convention. Steps 1 and 2 run as one circuit execution;
Aer saves the quantum state at the end of it (save_stabilizer). Only if the classical
decision is PROCEED does a second circuit restore that state (set_stabilizer) and
apply the encoding gates. An aborted session returns before a single encoding gate
exists. tests/test_dl04.py::test_abort_precedes_encoding counts the gates to prove it.
- Every qubit is a real wire in a Qiskit circuit, simulated with Aer's stabilizer method. That is exact here: every operation (X, H, Y, measurement) is Clifford, and depolarizing noise is a Pauli channel.
- The eavesdropper is a mid-circuit measurement in a basis she picks at random. The errors she causes come from quantum mechanics, not from a formula. That is why the measured 25.27% is a real validation of the model.
- Fibre noise is attached only to an
idgate standing for the fibre, so every error in the simulation comes from the channel or from Eve.
-
Each hop is an independent DL04 session through a trusted relay, which decodes and re-encodes. This is the model every deployed quantum network uses today.
-
Every forward check measures that link's QBER. A monitor smooths it per link, and a policy acts on it:
QBER Action below 5% CONTINUE 5% to 8% HARDEN: check twice as many qubits 8% to 11% REROUTE: stop using the link 11% and above ABORT: beyond the provable-security bound -
Routing uses NetworkX with link cost
distance / (1 - QBER/11%)^2, so traffic leaves a degrading link before it has to be cut. Rerouting starts from the node holding the frame, so plaintext never crosses the tapped fibre. -
Frames use Hamming(7,4) with CRC-32 per hop (retransmit on failure) and SHA-256 end to end.
qsdts/
channel.py Qiskit Aer channel: fibre noise, eavesdropper, chunked stabilizer runs
dl04.py DL04 QSDC, check-before-encode enforced structurally
bb84.py BB84 baseline on the same channel
coding.py Hamming(7,4) + CRC-32 framing
network.py topology, QBER monitor, policy, router, file transfer
demo.py the live demo (python -m qsdts.demo)
tests/ 40 tests: protocol correctness, the invariant, policy, transfers
experiments/ run_experiments.py (all numbers), figures.py (all charts)
notebooks/ qsdts_demo.ipynb, the Colab demo, and the script that generates it
results/ results.json and every figure used in the presentation
deck/ builds the presentation from results.json (needs the organisers' template)
QSDTS implements published work. We did not invent:
- QSDC or the DL04 protocol [1], [2]
- multi-hop or relayed QSDC, demonstrated in hardware [6], [7]
- eavesdropper-triggered rerouting, demonstrated on the Tokyo QKD Network in 2010 [10]
- BB84 [3]
What we built is an open, runnable implementation of these ideas as a network: protocol, routing, policy, experiments and evidence, which until now existed in labs or on paper.
- Simulation, not hardware. Calibrating the channel on IBM Quantum hardware is the next step.
- Trusted relays see plaintext, as in every deployed quantum network. Removing that needs quantum repeaters.
- Idealised physics. Photon loss, detector dead time and multi-photon pulses are not modelled, so photon-number-splitting attacks are out of scope for now.
- Weak attackers on 20% of qubits or fewer are not reliably caught within one block.
- F.-G. Deng and G. L. Long, "Secure direct communication with a quantum one-time pad," Phys. Rev. A 69, 052319, 2004.
- G. L. Long and X. S. Liu, "Theoretically efficient high-capacity quantum-key-distribution scheme," Phys. Rev. A 65, 032302, 2002.
- C. H. Bennett and G. Brassard, "Quantum cryptography: Public key distribution and coin tossing," Proc. IEEE ICCSSP, Bangalore, 1984, pp. 175-179.
- P. W. Shor, "Algorithms for quantum computation: discrete logarithms and factoring," Proc. 35th FOCS, 1994, pp. 124-134.
- W. K. Wootters and W. H. Zurek, "A single quantum cannot be cloned," Nature 299, 802-803, 1982.
- Z. Qi et al., "A 15-user quantum secure direct communication network," Light: Sci. Appl. 10, 183, 2021.
- M. Wang et al., "Experimental demonstration of secure relay in quantum secure direct communication network," Entropy 25, 1548, 2023.
- S. Zhang and C. Zheng, "Quantum secure direct communication technology-enhanced time-sensitive networks," Entropy 27, 221, 2025.
- R. J. Hayek, J. Chung and R. Kettimuthu, "A review of software for designing and operating quantum networks," arXiv:2510.00203, 2025.
- M. Sasaki et al., "Field test of quantum key distribution in the Tokyo QKD Network," Opt. Express 19, 10387-10409, 2011.
Built with Qiskit, Qiskit Aer, NetworkX, NumPy and Matplotlib.
Gowtham K (team lead), Pranav A S, Jaya Suriya T R.
Built with AI assistance (Claude Code); design decisions, review and validation by the team.
Apache 2.0. See LICENSE.