Noncesense Encryption

Technical Walkthrough

Writeup

Challenge

• Name: Noncesense-Encryption
• Category: Crypto
• Difficulty: Medium
• Mode: hybrid

Summary

The service exposes chosen-plaintext encryption for a custom stream-cipher-like

scheme. The keystream is not random: each query uses the flag integer reduced

modulo 0x13373 * (nonce + counter), then expands that small residue through an

invertible linear bit transform. Because the attacker controls plaintext, every

ciphertext reveals one expanded keystream value.

The solve inverts the key expansion to recover several modular residues of the

flag, searches the small timestamp nonce window, and uses CRT to reconstruct the

flag integer.

Artifact Inventory

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

• files/a12c7366-3ed9-4c61-bdcf-b385e875b951.zip: original HTB ZIP.
• analysis/extracted/crypto_noncesense_encryption/server.py: provided source

for the encryption service.

• analysis/server-numbered.txt: numbered source view used during audit.
• analysis/local-simulation.txt: local proof that the residue recovery and CRT

reconstruction work against fake flags.

• analysis/remote-recovery-summary.json: sanitized remote recovery metadata.
• analysis/remote-solve-run.txt: gated remote solve transcript without raw

flag output.

Analysis

The source initializes:

• generator = bytes_to_long(FLAG.encode())
• k = 0x13373
• nonce = int(time())
• counter = 0

For each encryption, the base key is:

base = FLAG_integer mod ((nonce + counter) * k)

Then counter increments. The base key is split into two 25-bit halves and run

through 250 total rounds of the state update:

kh, kl = kl, kh ^ kl

The resulting ten 50-bit states are concatenated into the final XOR key. This

update is invertible: given a later (kh, kl), the previous state is

(kh ^ kl, kh). Therefore the low 50 bits of the expanded key can be inverted

250 rounds to recover the original base key.

Since encryption is ciphertext = plaintext xor expanded_key, querying a known

plaintext reveals the expanded key. Repeating this for consecutive counters

gives residues:

FLAG_integer ≡ residue_i mod (0x13373 * (nonce + i))

The nonce is a Unix timestamp from service startup. The solver searches a small

window around the observed connection time and applies CRT for each candidate

nonce. With 14 residues, the combined modulus is large enough to recover the

full flag integer directly. The local simulation succeeded in 20/20 fake-flag

runs before remote execution.

Solve

The solver is solve/solve.py. It supports a local simulation and remote mode:

cd <local workspace>
python3 Crypto/Noncesense-Encryption/solve/solve.py --simulate --simulate-runs 20
python3 Crypto/Noncesense-Encryption/solve/solve.py \
  --host <TARGET> \
  --port 31089 \
  --queries 14 \
  --window 1800 \
  --output Crypto/Noncesense-Encryption/loot/flag-candidate.txt

The candidate flag was captured through the harness:

python3 scripts/challenge_harness.py capture-flag Crypto/Noncesense-Encryption --from loot/flag-candidate.txt

Flag

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

Lessons

Custom stream ciphers fail quickly when their keystream is deterministic and

recoverable under chosen plaintext. Nonces must be unique unpredictable inputs

to secure keystream generation; using int(time()) plus a counter around secret

material gave enough structured modular equations to reconstruct the secret.

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: Noncesense-Encryption
• Category: Crypto
• Difficulty: Medium
• Mode: hybrid
• Remote instance: <TARGET>:31089
• Start time: 2026-06-11T08:22:29Z
• 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

I've mastered Stream Ciphers, and i decided to create one myself!

Operator Question

HTB Crypto Medium Noncesense Encryption custom stream cipher nonce misuse source remote solve path

Memory Summary

Metadata

• Platform: HackTheBox Challenges
• Category: Crypto
• Challenge: Noncesense-Encryption
• 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: Crypto
• Challenge: Noncesense-Encryption
• 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: Noncesense-Encryption
• Category: Crypto
• Difficulty: Medium
• Mode: hybrid
• Remote instance: <TARGET>:31089
• Start time: 2026-06-11T08:22:29Z
• 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

I've mastered Stream Ciphers, and i decided to create one myself!

Operator Question

HTB Crypto Medium Noncesense Encryption custom stream cipher nonce misuse source remote solve path