Untrusted Node

Technical Walkthrough

Writeup

Challenge

• Name: Untrusted-Node
• Category: Quantum
• Difficulty: Medium
• Mode: hybrid

Summary

The service implements a BB84-like key negotiation with an intentionally

compromised trusted node. The transmitter sends multiple identical qubits for

each logical key bit and prints a sync_signal that reveals the chunk sizes.

The trusted node can measure selected copies, the receiver later prints its

measurement bases, and the node can still choose the final basis list sent back

to the transmitter for match calculation.

That order breaks the protocol. By measuring sacrificial copies in known bases,

leaving other copies for the receiver, then forging the final basis list after

seeing the receiver bases, the node can make the transmitter and receiver agree

on a key that the node also knows. The recovered shared key decrypts/encrypts

the receiver command channel and allows a request for the secret.

Artifact Inventory

Reference analysis/artifact-inventory.json and summarize the relevant files or remote surface.

• files/a12c73b2-3fe5-4b00-b65d-53626f395ad9.zip: original HTB ZIP.
• analysis/extracted/quantum_untrusted_node/server.py: terminal protocol and

trusted-node interception flow.

• analysis/extracted/quantum_untrusted_node/transmitter.py: duplicated-qubit

generation, sync signal, match selection, entropy check, and command XOR.

• analysis/extracted/quantum_untrusted_node/receiver.py: receiver measurement,

filtering, command processing, and secret command branch.

• analysis/local-simulation.txt: local protocol simulation showing the attack

succeeds reliably before remote execution.

• analysis/remote-solve-run.txt: gated remote solve transcript without raw

flag exposure.

Analysis

The transmitter creates 128 logical key bits. For each bit it draws

k = poisson(lambda) + 2, then emits k identical one-qubit circuits prepared

with the same bit and basis. Those k values are printed as the sync signal in

server.py, so the compromised node knows how to group physical qubits back

into logical key bits.

The trusted node has two chances to interfere:

• Before receiver measurement, it may measure chosen physical qubits in a Z or X

basis. Measured qubits are replaced with None, so the receiver treats them

as lost.

• After receiver measurement, the receiver's bases are printed, and the node

supplies the basis list that Transmitter.get_matches() trusts for key

filtering.

The exploit uses that ordering. For each chunk, it measures one sacrificial copy

in Z and, when available, one in X. The remaining copies are left for the

receiver. Once receiver bases are printed, the exploit chooses final transmitter

gates so that the selected match is a receiver-measured copy whose basis is also

known from a sacrificial measurement. If both Z and X receiver copies exist, the

bit is recovered regardless of the transmitter's hidden basis. If only one

usable receiver basis exists, the bit is included only when that basis matches.

The source entropy check requires at least 64 balanced bits before hashing.

The local simulation in analysis/local-simulation.txt showed 49 successes in

50 trials, with known matched key lengths comfortably above the threshold. The

remote solve then used the derived SHA256 key to encrypt TX|FETCH|SECRET for

Receiver.process_command().

Solve

The solver is solve/solve.py. It has a local simulator and a remote mode:

cd <local workspace>
python3 Quantum/Untrusted-Node/solve/solve.py --simulate --attempts 50
python3 Quantum/Untrusted-Node/solve/solve.py \
  --host <TARGET> \
  --port 31565 \
  --output Quantum/Untrusted-Node/loot/flag-candidate.txt \
  --attempts 8

The remote command saves the candidate flag under loot/. It was captured with

the harness:

python3 scripts/challenge_harness.py capture-flag Quantum/Untrusted-Node --from loot/flag-candidate.txt

Flag

Raw flag is stored in loot/flag.txt and intentionally not reproduced here.

Lessons

Repeated qubits make a BB84-style protocol vulnerable when an intermediate node

can selectively measure copies and later influence basis reconciliation. The

important weakness was not quantum math complexity; it was protocol ordering:

the compromised node saw the receiver bases before submitting the gate list used

by the transmitter to decide which measurements form the final key.

Source-Backed Dossier

The sections below are merged from companion Markdown notes for the same case. They are rendered after sanitization so the article stays precise without publishing raw flags, credentials, or target-specific secrets.

Notes

Scope

• Challenge: Untrusted-Node
• Category: Quantum
• Difficulty: Medium
• Mode: hybrid
• Remote instance: <TARGET>:31565
• Start time: 2026-06-11T08:08:38Z
• Operator: harness
• State file: challenge-state.json

Harness Status

• Current phase: see challenge-state.json
• Next allowed actions: see next-action.json
• Raw flags and sensitive material stay in loot/ only. Do not paste them here.

Artifact Inventory

Evidence Ledger

Key Findings

-

RAG / Advisory Memory

RAG output is advisory only. Record evaluated retrievals with:

scripts/challenge_harness.py rag-record <workspace> --query "..." --tag MATCHED|PARTIAL|MISSING|<secret redacted>|GENERIC --validation "..."

Secrets/Flags

Raw flags and sensitive material stay in loot/ only. Use scripts/challenge_harness.py capture-flag to validate and record flag capture without printing the value.

Scenario

At the heart of Qubitrix's empire lies their quantum key distribution network. Normally unbreakable — except one relay was compromised. It is the weak link. Impersonate the trusted node, insert yourself between transmitter and receiver, and hijack their negotiation. If you succeed, you'll hold their crown-jewel keys in your hands. This is where the real war begins.

Operator Question

HTB Quantum Medium Untrusted Node QKD compromised relay trusted node impersonation hybrid source remote solve path

Memory Summary

Metadata

• Platform: HackTheBox Challenges
• Category: Quantum
• Challenge: Untrusted-Node
• Difficulty: Medium
• Source workspace: <local workspace>

Validated Solve Chain

Concepts only. Do not include raw flags, reusable credentials, tokens, cookies, private keys, or live secrets.

1.

Reusable Lessons

-

Dead Ends

-

Tool Quirks

-

Evidence Paths

-

Ingestion Decision

• Proposed for LightRAG: yes/no
• Requires user approval before ingestion: yes

Hypothesis Board

Keep no more than 3 active hypotheses on Easy/Medium and 5 on Hard unless the user explicitly asks for breadth.

Closed Branches

Memory Summary

approval_required: true

Sanitized Memory Summary

Metadata

• Platform: HackTheBox Challenges
• Category: Quantum
• Challenge: Untrusted-Node
• Difficulty: Medium
• Source workspace: <local workspace>

Validated Solve Chain

Concepts only. Do not include raw flags, reusable credentials, tokens, cookies, private keys, or live secrets.

1.

Reusable Lessons

-

Dead Ends

-

Tool Quirks

-

Evidence Paths

-

Ingestion Decision

• Proposed for LightRAG: yes/no
• Requires user approval before ingestion: yes

Notes

Notes

Scope

• Challenge: Untrusted-Node
• Category: Quantum
• Difficulty: Medium
• Mode: hybrid
• Remote instance: <TARGET>:31565
• Start time: 2026-06-11T08:08:38Z
• Operator: harness
• State file: challenge-state.json

Harness Status

• Current phase: see challenge-state.json
• Next allowed actions: see next-action.json
• Raw flags and sensitive material stay in loot/ only. Do not paste them here.

Artifact Inventory

Evidence Ledger

Key Findings

-

RAG / Advisory Memory

RAG output is advisory only. Record evaluated retrievals with:

scripts/challenge_harness.py rag-record <workspace> --query "..." --tag MATCHED|PARTIAL|MISSING|<secret redacted>|GENERIC --validation "..."

Secrets/Flags

Raw flags and sensitive material stay in loot/ only. Use scripts/challenge_harness.py capture-flag to validate and record flag capture without printing the value.

Scenario

At the heart of Qubitrix's empire lies their quantum key distribution network. Normally unbreakable — except one relay was compromised. It is the weak link. Impersonate the trusted node, insert yourself between transmitter and receiver, and hijack their negotiation. If you succeed, you'll hold their crown-jewel keys in your hands. This is where the real war begins.

Operator Question

HTB Quantum Medium Untrusted Node QKD compromised relay trusted node impersonation hybrid source remote solve path