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OPERATION IRON VAULT

Datacenter Vent Controller

Act VIII of OPERATION COLD IRON

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LEGAL DISCLAIMER: The information, tools, and code provided in this repository and course are strictly for educational, research, and defensive purposes only.

You are explicitly prohibited from using any materials contained herein to access, test, modify, or exploit any device, network, or system that you do not own 100% or for which you do not have explicit, documented, and legally binding authorization to interact with.

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IF YOU DO NOT AGREE WITH THESE TERMS, DO NOT USE THIS REPOSITORY AND COURSE.

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Hello again, friend.

Act I was the lie. Act II was the door. Act III was the payload. Act IV was the payload that would not die. Act V was the payload that spreads. Act VI was the payload that steals. Act VII was the payload that takes orders. This is the payload that holds the building hostage.

WHITEOUT stopped the task handler and cleared the bot marker, and for a shift the floor looked quiet. Quiet is not safe. The Ministry did not need a fleet that obeys; it already had a building that cannot breathe. Somewhere between the reporting line and the loading dock, the same hand that wrote the leash wrote a padlock.

The datacenter vent controller is the device a server hall trusts with its air. A damper opens the vent so the racks stay cool. A local maintenance remote requests a purge. A rack temperature sensor watches the hall for heat. A vault control gateway authorizes an open or a close. That is the whole contract, and it is a good one.

FROSTLINE's implant in this one does not spread, and it does not steal, and it does not even take orders. It holds the building. It forces the vent closed and tells the operator the vent is "maintenance locked" while the hall heats and the racks throttle. It unlocks only on a magic release token, or never, and it writes a lock marker into the reserved sector with the real flash API so it comes back after a reflash. This is an availability attack wrapped in ransom logic: the device withholds the function it exists to provide.

The green lamp still says COOLING OK while the vent is shut. The LCD still reports a state, and the state is a lie it was told to repeat. Underneath, the building is being held hostage by a padlock with a polite label.

Do not chase the symptoms one at a time. Break the lock. Unmask the display. Clear the marker. Then seal the vent command path so no close command can ever be forged, and make the vent fail open when the link is lost.

The hall is warming. The readout says maintenance. That is exactly the problem.

THE SYSTEM

NorthPharma does not only move cold medicine and cold air and make the medicine. It runs the buildings that keep the state's data alive: the server halls, the power rooms, and the cooling plant that ties them together. The datacenter vent controller is the node on the edge of that plant. It watches the rack temperature, takes a local manual purge request from a maintenance remote, verifies a sealed vent command from the vault control gateway, drives the vent damper, and annunciates whether the hall is cooling, locked, or hot.

A vent controller is a simple machine. A temperature sensor reports the hall, a control gateway authorizes an open, close, or purge command, the controller decides, a servo moves a damper, and a tower light says whether the building can breathe. The failure that matters is not a wrong number on a screen. It is a damper that stays shut when the hall is heating, or a controller that quietly takes its orders from something other than the gateway.

The node in this repository is that hand. On a breadboard it is a toy: a Pico 2, an SG90 servo that acts as the vent damper, a DHT11 that stands in for the rack temperature sensor, an infrared remote that is the local maintenance control, a button that is the manual purge request, a 1602 LCD that is the vault readout, three lamps, and a radio that is the vault control link.

Nothing about it looks broken. That is the horror of Act VIII. The code compiles, the tests pass, the towers are lit, and the vent is being held shut by a locker that reports the shutdown as routine maintenance.

THE STAKES

Act I was a lie about temperature. Act II was a lie about people. Act III was a lie about machinery. Act IV was a lie about remediation. Act V was a lie about containment. Act VI was a lie about confidentiality. Act VII was a lie about obedience. Act VIII is a lie about availability, and it is the one that does not need a second party, a listener, or a smart adversary: it only needs a device that will refuse to do its job and call the refusal normal.

The node is weaponized, not buggy. A hidden locker forces the vent closed, re-asserts the lock every four ticks, unlocks only on the exact magic token VAULT-RELEASE-2026, and writes a 0x4C lock marker into the reserved flash sector 0x103FF000 so it is re-installed on every later boot. The operator sees ST:MAINT and a yellow LOCKED lamp, and the natural conclusion is that a technician put the vent in maintenance mode on purpose. There is no alert, because the locker never touches the sealed command path. It sits beside it.

And here is the part that keeps the responders awake. The vent path is already authenticated. The cryptography is real and it is correct. The locker does not break the cipher. It does something worse: it never needs the cipher. It is a local condition that overrides the output regardless of what the authenticated link says, so a perfectly valid open command can arrive and the damper will still stay shut. The hall heats while every light says the problem is routine.

That is not a vent controller. That is a vent controller that holds the building hostage.

WHITEOUT

WHITEOUT is a resistance that does not exist on paper. It does not hold ground and it does not hold press conferences. It reads firmware. When the task handler was stopped, the crew kept pulling the thread. The bot marker led to the image, the image led to the build, and the build led to a second module no design review had named.

NIGHTINGALE is still the thread. Her last verified copy came off the tamper ring, and it was clean. The thing that came after it was not. Somewhere between the build server and the loading dock, someone signed an image that carries a locker, and that image is holding a hall right now.

WHITEOUT's job in Act VIII is not to break in. It is to prove the vent is being held shut on purpose, in writing, with a debugger and a disassembler, then to break the lock, unmask the display, clear the marker, and seal the vent command path so nothing can ever drive the building closed again. The fix is not only a patch; it is a policy. The vent must fail open.

THE MACHINE

The firmware in this repository is the node's firmware. On a breadboard it is a toy: a Pico 2, an SG90 servo that is the vent damper, a DHT11 that is the rack temperature sensor, a VS1838B infrared eye that takes a local maintenance remote, a 1602 LCD vault readout over I2C, red/yellow/green tower light lamps, a manual purge button, and an RYLR998 LoRa vault control link to a gateway.

Two things are open, and one thing is not what it seems. The optical surface takes a purge or acknowledge request from any NEC remote, and it is not authenticated. The radio carries the sealed vent command path, and it is authenticated correctly. The part that is not what it seems is the locker: a module compiled only under a build flag called SANDBOX_ONLY, invisible in the clean firmware, and present in the test and CTF builds. It forces the vent closed, masks the state as maintenance, unlocks only on a magic release token, and writes a lock marker into the reserved sector with the real flash API.

The face of the thing is honest in the way that matters least. The tower light says COOLING OK, LOCKED, and HALL HOT with total confidence, and the LCD shows the state, the link, the rack zone, the temperature, and the lock. None of it lies, except when the locker tells the display that "maintenance" is the reason. A healthy-looking controller can still be holding the hall hostage.

THE JOB

You do not have to be a hero. You have to be thorough. The hall is carrying a passenger that no design review admitted to, and the passenger is patient. Find it, prove it, and cut the padlock.

1. Bring it up. Build the clean firmware, wire the board, and confirm the node reads the rack temperature, takes a local purge request, reaches the gateway, and drives the damper. Nothing looks broken because nothing is broken yet.
2. Inspect the protocol. Capture a sealed vent command and read the body byte by byte. Understand what is authenticated and what the locker ignores.
3. Hunt the lock. Build the SANDBOX_ONLY image and find the forced close, the mask that renders ST:MAINT, the magic release token, the reserved-sector lock marker 0x4C, and the anti-debug trap. A single clean reflash will not open the vent.
4. Defuse it. Break the lock, unmask the display, clear the lock marker, and step past the anti-debug with GDB so the controller cannot tell that a probe is attached. Then seal the vent command path and make the vent fail open so no untrusted close command and no lost link can ever hold the hall again.

This document is the manual for the job. Work it on a breadboard. When every green lamp is lit and the log says the vent is clean, remember what it is: not a healthy controller. A padlock that has learned to say maintenance.

Goodbye, friend.

A NOTE ON THE ROADMAP

This project is Act VIII of OPERATION COLD IRON. Act I was the sensor (cold-chain-monitor). Act II was the door (access-gate). Act III was the valve (pipeline-valve-controller). Act IV was the air (hvac-automation-node). Act V was the web (industrial-tamper-system). Act VI was the courier (smart-logistics-dropbox). Act VII was the choir (factory-andon-station). Act VIII is the vault. All eight are defended devices; the companion CTF repository ships the compromised one. The investigation lives here:

• OPERATION IRON VAULT CTF

The CTF is the red half, weaponized: six deep tasks, each with static analysis, a dynamic proof under GDB, a hardware demonstration, and an in-place, same-size patch. This repository is the defended device. The CTF repository is the breached one. The full story lives at github.com/mytechnotalent/datacenter-vent-controller.

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WHERE THIS FITS: OPERATION COLD IRON

This repository is Act VIII (IRON VAULT) of the ten-act OPERATION COLD IRON saga. The malware track began in Act III; in Act IV it became persistence, in Act V it became propagation, in Act VI it became exfiltration, in Act VII it became command and control, and here it becomes availability and lockout logic. The full spine is in SAGA.md.

• Previous act: Act VII, IRON CHOIR, the factory floor andon station, factory-andon-station
• This act: Act VIII, IRON VAULT, the datacenter vent controller
• Next act: Act IX, IRON FANG, smart-parking-barrier (forthcoming)
• Companion CTF: CTF_datacenter-vent-controller

THE MINISTRY

The Ministry runs the state: the surveillance, the cold chain, the gates, the pipelines, the air, the cabinets that hold what the state does not discuss, the lockers that move it, the factories that make it, and the buildings that keep the record. NorthPharma is one of its deniable industrial fronts, and FROSTLINE is the contractor that does the work no Ministry letterhead will admit to. FROSTLINE did not break into this node; it built the locker, taught it to hold the vent closed, staged the lock marker in a reserved sector, and signed the image. Against them is WHITEOUT, and the engineer who copied the first image, NIGHTINGALE. This act is one hall on the Ministry's datacenter floor. TELESCREEN, the surveillance backbone that watches it, comes after the ten.

An adversarial, evidence-based audit of this act, including its honest limitations, is in NATION-STATE-REVIEW.md.

How This Project Fits the Embedded Hacking Course

This repository is the Act VIII capstone integration for the Embedded Hacking course. It reuses the entire Act I peripheral set so one breadboard serves the whole foundation, and it adds the concepts the later acts build toward: a payload that withholds a building's cooling, a benign release token, a reserved-sector lock marker written with the real flash API, re-lock on boot, and the blue-half controls that contain them.

Each earlier module teaches one peripheral or language concept in isolation; this project wires several of them into a single, tested product, and then teaches you to look at that product as an adversary sees it.

Embedded Hacking module	Concept you learn	Where it lives here
Week 1: Introduction, Ethics, Scoping	Authorized lab work	Every lab is self-contained and authorized by design
Week 3: RP2350 Architecture and Firmware Analysis	Bare-metal targets, ELF/UF2, SWD	Pico SDK build, build/*.uf2, Debug Probe flash via OpenOCD
Weeks 4-6: Variables, Integers/Floats, Static	Data types, GPIO	src/monitor.c state machine, LED on GP25
Week 7: Constants with 1602 LCD I2C	I2C bus, HD44780 commands	src/display.c
Week 9: Operators with DHT11	Bit operations, edge timing	src/sensor.c rack temperature sensor
Week 11: Structures and Functions	Modular design	include/*.h and src/*.c module boundaries
This project adds	Availability attacks, lockout logic, ransom logic, a magic release token, reserved flash sectors, re-lock on boot, UART AT driver, LoRa control path, anti-replay, authenticated state, fail-open policy, malware analysis, anti-debug evasion, strict testing	src/implant.c, src/damper.c, src/control.c, src/vault_auth.c, src/radio.c, scripts/gateway.py, scripts/spoof.py, test/

If you have not worked through Weeks 7 and 9 yet, do those first: this project assumes you are comfortable with I2C wiring and one-wire edge timing.

Learning Objectives

By the end of this chapter and its labs you will be able to:

• Explain why a device that withholds its own function is a different failure class from exfiltration or propagation, and why an availability attack does not need to break authentication to succeed.
• Wire and drive a 1602 LCD through a PCF8574 I2C backpack and render a vault state, link, zone, temperature, and lock readout.
• Decode a VS1838B infrared receiver as a local maintenance remote for PURGE, ACK, and TEST commands, and explain why an unauthenticated optical surface is still an attack surface and must not silently bypass authorization.
• Drive an SG90 vent damper with 50 Hz PWM and explain why a 1000uF bulk capacitor is not optional.
• Read a DHT11 rack temperature sensor and classify the hall against a safe band before the damper is allowed to move.
• Design a sealed vent command path over a sub-GHz LoRa link using a fixed-size envelope, a guarded command set, a bounded zone band, a monotonic anti-replay window, and a keyed state tag.
• Analyze a ransomware-flavored locker: locate the forced close, read the lock marker 0x4C, read the magic release token VAULT-RELEASE-2026, find the reserved-sector marker 0x103FF000, and read the anti-debug trap.
• Explain why breaking the lock, unmasking the display, clearing the marker, and sealing the command path are four separate controls, and why a firmware reflash alone is not enough.
• Defeat an anti-debug check under GDB by understanding the CoreDebug DHCSR register at 0xE000EDF0.
• Apply blue-half controls: sealed and authorized commands, a manual purge that asks for authorization, fail-open on a lost link, no persisted marker, and no masked state.
• Derive a key with Argon2id, seal every frame with XChaCha20-Poly1305, and read and run a native host test suite with hardware mocks and line coverage.

Prerequisites

• The Embedded Hacking breadboard (EHP2_bb.png) and parts list.
• Acts I to VII are helpful but not required. See cold-chain-monitor, access-gate, pipeline-valve-controller, hvac-automation-node, industrial-tamper-system, smart-logistics-dropbox, and factory-andon-station for the sensor, the door, the valve, the air, the web, the courier, and the choir. The pin map is identical, so one breadboard serves all eight.
• Comfort with C, the Linux/macOS shell, and basic electronics.
• A Pico 2, a Debug Probe (recommended, and required for the lock lab), a 1602 LCD with PCF8574 backpack, a DHT11, the full Embedded Hacking kit (3 LEDs, 3 resistors, a push button, an SG90 servo, a 1000uF capacitor, and a VS1838B infrared receiver plus NEC remote), two RYLR998 modules, and one USB-to-TTL serial adapter.
• Toolchain: Pico SDK 2.2.0+, arm-none-eabi-gcc, CMake, Ninja, Python 3, GDB (arm-none-eabi-gdb) for Lab 3, and (optionally) typst to rebuild the paper.

Table of Contents

1. Background
2. System Architecture
3. The Wire Protocol
4. The Cryptographic Envelope
5. The FROSTLINE Ransom Locker
6. Hardware You Need
7. Wiring the Node
8. Build and Flash
9. Lab 1: Bring-Up and Verify
10. Lab 2: Inspect the Wire Protocol
11. Lab 3: The Ransom-Lock Track
12. Lab 4: The Fix Track
13. Troubleshooting
14. Testing Philosophy and Coverage
15. Generating Packet Artifacts
16. Code Standards
17. Project Layout
18. Glossary
19. Further Reading
20. License

Background

Why datacenter vent monitoring

A vent controller is a control loop with a building in it. A rack temperature sensor reports the hall, a vault gateway authorizes an open, close, or purge command, a controller decides, a damper moves air, and a tower light tells the floor whether the building can breathe. The damper is where the decision becomes physical, and the tower light is where the operator reads it. Everything interesting in datacenter security happens in those two places.

Three properties have to hold at once, and they are not the same property:

• Integrity. The vent command that reaches the damper is the one the vault gateway authorized. Not a replay, not a forgery, not a stray package.
• Authority. The node acts only on an authorized verdict. A local maintenance button or remote is a request, not an authorization.
• State. The controller knows whether it is cooling, purge pending, hot, or denied, and it does not trust a stale or tampered verdict.

Act VIII adds a fourth property that is the hardest of all because it is a refusal rather than a violation: availability. A controller that never lies and never fails authentication can still hold the building hostage, because the attacker's goal is not to make it wrong; it is to make it stop.

Why integrity plus authority plus availability matter

The classic naive controller collapses the three. It accepts any vent command on the radio, it has no anti-replay window, and it lets a local input bypass the decision. Act II showed what that costs a door. Act III showed the industrial version. Act IV showed the persistence version. Act V showed the propagation version. Act VI showed the exfiltration version. Act VII showed the command version. Act VIII adds the availability failure:

• Integrity without availability. The sealed vent path in this build is correct. XChaCha20-Poly1305 authenticates every frame, the zone band is bounded, the sequence window rejects a replay, and the state tag detects a tampered verdict. None of that stops a locker that forces the output closed and masks the reason.
• Authority as the attack goal. A forged or replayed packet aims to move a damper the operator did not authorize. The window and the tag are the controls that stop it.
• State as the last line of defense. A keyed tag over the authorization record means a debugger that rewrites the record is caught before the damper moves. It is the same lesson Act II taught, carried into the hall.
• Availability as the invisible failure. A payload that holds a building hostage has no visible symptom except the one it is allowed to show. It does not need the wire, the key, or the damper command. It needs a reason to refuse, and a refusal that wears the uniform of routine maintenance is the hardest kind to see: the packets are well formed and the lamps say maintenance, not failure.

The fix track in Lab 4 seals the vent command path, makes the manual purge ask for authorization, and makes the vent fail open on a lost link. The lock track in Lab 3 breaks the lock, unmasks the display, clears the lock marker, and removes the passenger that was never in the design.

Why ChaCha20 over AES on the RP2350

The RP2350 has no hardware AES engine; its accelerated crypto block covers SHA-256, not AES. A software AES implementation on this part is therefore both slower and riskier, because table-driven AES performs data-dependent memory accesses that create a cache-timing side channel. ChaCha20 is built only from addition, rotation, and XOR, with no data-dependent table lookups, so it is fast in portable C and has no comparable cache-timing surface. XChaCha20-Poly1305 is thus both the modern choice and the pragmatic one for this silicon. The full rationale, including the extended-nonce benefit, appears in The Cryptographic Envelope.

Why an availability lesson

The first seven acts each taught a way a device fails by doing something: a bad reading, a bad decision, a bad image, a bad cleanup, a bad neighbor, a bad leak, a bad listener. Act VIII teaches the failure that is a decision not to act. Availability is the property that every other control silently assumes. The gateway can be authenticated, the replay window can be airtight, and the state tag can be perfect, and the building can still be hot because the one device that moves the air chooses not to. The defense is therefore a policy and a build control: fail open, refuse to mask, and remove the code and the state that let a device hold its own building hostage.

The two on-wire problems this project solves

1. Payloads that contain commas. The sealed body is carried as lowercase hex, but the +RCV framing still separates fields with commas. A naive receiver that splits the line on the first comma corrupts the frame. The correct discipline is the declared-length rule: slice exactly L characters after the second comma and require the next character to be a comma.
2. Telling a real vent command from a forged or replayed one. The controller records the sender address exactly as the radio reports it, and it trusts the bytes that arrive. The sealed envelope, the bounded zone band, and the stateful window are what close that gap.

Inter-Integrated Circuit (I2C)

I2C is a two-wire bus: SDA (data) and SCL (clock), each pulled up to the supply rail. A controller (the Pico) addresses a target by its 7-bit address and writes or reads bytes. The 1602 LCD backpack carries a PCF8574 I/O expander at address 0x27; the firmware bit-bangs the HD44780 nibble protocol over that expander. Pull-ups are mandatory: the firmware enables the internal ones and the backpack usually adds its own.

The DHT11 one-wire protocol

The DHT11 is a low-cost digital temperature and humidity sensor. In Act VIII it is the rack temperature sensor: the node classifies the hall against a safe band and announces the temperature in the vault readout. It speaks a custom single-wire protocol:

1. The host pulls the line low for at least 18 ms (the start pulse), then releases it and enables its pull-up.
2. The sensor answers with an 80 us low, then an 80 us high handshake.
3. The sensor sends 40 bits. Each bit begins with a 50 us low, then a high pulse whose width encodes the value: about 26-28 us for a 0, about 70 us for a 1.
4. Five bytes follow: humidity integer, humidity decimal, temperature integer, temperature decimal, and a checksum equal to the low byte of their sum.

Reading it means timing edges on the order of tens of microseconds, so the firmware uses an 18 ms host pulse, a 50 us bit-classification threshold, and a 240 us per-edge timeout so a dead or unplugged sensor fails fast instead of hanging the loop. A reading that fails its checksum is never safe, and a valid reading outside 0.0 C to 40.0 C (the tenths band 0 to 400) is out of band. Either way, the hall is not nominal.

Universal Asynchronous Receiver/Transmitter (UART) and AT commands

The RYLR998 is driven over a UART at 115200 baud using CRLF-terminated ASCII commands. The firmware writes AT+SEND=... and drains inbound +RCV=... lines. Because the radio is a separate processor, its configuration (address, network identifier, band) persists until changed; the controller and the gateway each provision their own radio at start-up so they agree before any command traffic flows.

Cyclic Redundancy Check (CRC)

src/crc.c implements CRC-16/CCITT-FALSE (poly = 0x1021, init = 0xFFFF, check value 0x29B1 for "123456789"). It is provided as a reusable integrity diagnostic and exercised by the test suite. It is not part of the LoRa frame in this project; the lesson is the absence of authentication, not the absence of a checksum.

System Architecture

There are four roles:

Role	Runs on	Job
Vent controller node	Pico 2 firmware	Decodes the infrared maintenance remote, verifies sealed gateway open, close, and purge commands, annunciates the tower light, reads the rack temperature, drives the vent damper, enforces the manual purge request, renders the vault readout, and (SANDBOX_ONLY) runs the ransom locker
Vault control gateway	laptop + USB-TTL radio	Authenticates every request, logs it to vent_log.csv, decides authorization, and answers with a sealed vent command carrying a sequence and a state tag (scripts/gateway.py)
Edge simulator	laptop + USB-TTL radio	Pretends to be a controller and sends sealed zone requests (scripts/sim_edge.py)
Attacker	laptop + USB-TTL radio	Impersonates the gateway, forges a command, or replays a captured command (scripts/spoof.py)

Data flow

+----------------------+                              +----------------------+
| Pico 2 vent node     |        LoRa (sub-GHz)        | Vault control GW     |
| IR remote  -> GP5    |  AT+SEND=0001,<len>,<hex>    |  USB-TTL radio       |
| DHT11      -> GP4    |----------------------------->|  scripts/gateway.py  |
| Servo      -> GP14   |<-----------------------------|  vent_log.csv        |
| LCD     -> GP2/GP3   |  AT+SEND=<node>,<len>,<hex>  |  sealed command      |
+----------------------+                              +----------------------+

+----------------------+                              +----------------------+
|   Attacker laptop    |  forged or replayed command  | (same vent node)     |
|   scripts/spoof.py   |----------------------------->|  rejects at the tag  |
|  claims the gateway  |                              |  tag or seq window   |
+----------------------+                              +----------------------+

+----------------------+                              +----------------------+
|  Local locker        |      forces the vent closed  | (SANDBOX_ONLY node)  |
|  compile-time guard  |----------------------------->|  masks ST:MAINT      |
|  no radio, no key    |                              |  writes 0x4C marker  |
+----------------------+                              +----------------------+

Firmware module map

File	Responsibility
src/main.c	Entry point: stdio_init_all, monitor_init, tick loop
src/monitor.c	State machine: I2C bus scan, maintenance remote, gateway command, damper motion, manual purge request, rack temperature sensor, vault render
src/implant.c	SANDBOX_ONLY FROSTLINE ransom locker: forced close, ST:MAINT mask, magic release token, reserved-sector 0x4C lock marker, re-lock on boot, and CoreDebug anti-debug
src/damper.c	Vent damper state machine: bounded travel, closed/open/fault/moving, fail open
src/control.c	Sealed vent command path: open, authorize, guarded command and bounded zone
src/vault_auth.c	Authorization record, monotonic anti-replay window, authenticated state tag
src/sensor.c	DHT11 one-wire sampling and rack-temperature-band classifier
src/display.c	HD44780 driver over the PCF8574 backpack and vault status rendering
src/radio.c	RYLR998 provisioning, AT+SEND builder, +RCV parser, line pump
src/status_led.c	Red/yellow/green HALL HOT / LOCKED / COOLING OK tower light
src/button.c	Debounced manual purge button around the internal pull-up
src/servo.c	50 Hz PWM vent damper actuator
src/ir_remote.c	VS1838B edge timing and NEC maintenance remote decode
src/chacha20.c	ChaCha20 stream cipher and HChaCha20 subkey derivation
src/poly1305.c	Poly1305 one-time message authenticator
src/crypto_aead.c	XChaCha20-Poly1305 seal/open envelope
src/blake2b.c	BLAKE2b and the Argon2 variable-length hash H'
src/argon2.c	Argon2id core (BLAMKA, hybrid addressing)
src/crypto_kdf.c	Argon2id passphrase key derivation
src/envelope.c	Hex nonce/ciphertext/tag envelope codec
src/crc.c	CRC-16/CCITT-FALSE diagnostic
include/vent.h	Pin map, bus, provisioning, locker addresses
include/implant.h	Locker release token, marker, tick interval, anti-debug interface
include/control.h, include/vault_auth.h	Sealed command and authorization interfaces

The Wire Protocol

Request frame

The local maintenance control, or the edge simulator, seals a two-byte zone into an authenticated envelope and sends it to the vault gateway:

AT+SEND=0001,<len>,<hex envelope>

The plaintext of a request is exactly two bytes: an int16 rack zone identifier in little-endian.

Command frame

The gateway answers an authenticated request with a sealed vent command. The command plaintext is a 23-byte body:

seq[4] (little-endian) || command[1] || zone[2] (little-endian) || tag[16]

• seq is the monotonic gateway sequence number.
• command is one of the guarded vent commands: VENT_COMMAND_OPEN (0x01), VENT_COMMAND_CLOSE (0x02), or VENT_COMMAND_PURGE (0x03). Any other command byte is refused.
• zone is the authorized rack zone in the provisioning band VENT_ZONE_MIN (0) to VENT_ZONE_MAX (16).
• tag is an XChaCha20-Poly1305 tag over the authorization record the command would produce, so the controller can verify that the verdict it is about to store is the one the gateway authorized.

The gateway sends it back to the claimed sender address:

AT+SEND=<node>,<len>,<hex envelope>

The firmware enforces the guard in control_parse: the recovered command byte must be in the guarded vent set, and the recovered zone must be inside the provisioning band. Anything else is rejected before it can reach the damper decision. This is the sealed replacement for the old unauthenticated vent injection.

Sealed envelope layout

Every payload on the wire is the lowercase hexadecimal encoding of:

nonce[24] || ciphertext[L] || tag[16]

For a two-byte request body this is 24 + 2 + 16 = 42 bytes, or 84 hex characters. For a 23-byte command body this is 24 + 23 + 16 = 63 bytes, or 126 hex characters. The declared length L in the AT+SEND and +RCV framing is the length of the hex string, not of the underlying plaintext.

The maximum accepted plaintext is 48 bytes (ENVELOPE_MAX_PLAINTEXT), and the maximum hex envelope buffer is (24 + 48 + 16) * 2 + 1 = 177 bytes (ENVELOPE_MAX_HEX_LEN), which fits the 256-byte radio command and receive buffers with framing headroom.

Declared-length slicing invariant

Given the substring T after the second comma:

C = T[0 : L]   and   T[L] == ","

The receiver checks T[L] == ",", so a mismatch between the declared length and the actual payload is a parse error rather than silent corruption. This is what makes hex-bearing payloads safe to carry and is the same invariant Act I uses.

Vault status readout

ST:COOL  L:UP
Z:4 T:235 K:--

Line 1 is the current vault state (COOL, PURGE, HOT, or FAIL) and the gateway link (UP or --). Line 2 is the active rack zone, the rack temperature in tenths of a degree Celsius, and the lock status. In the clean build the lock field is always --. In the SANDBOX_ONLY build the lock field is LOCK once the reserved-sector marker is present, and while the vent is held closed the state is rendered as MAINT. When the hall is hot or the controller has failed safe, the red lamp is lit; when the vent is locked or a manual purge awaits authorization, the yellow lamp is lit; when the hall is cooling and clear, the green lamp is lit. Exactly one tower light lamp is lit at a time.

Radio provisioning

For the link to work, both radios must share the same network identifier and each must have the address the other targets:

• Firmware sets its own radio: AT+ADDRESS=7, AT+NETWORKID=18.
• gateway.py sets the gateway radio: AT+ADDRESS=1, AT+NETWORKID=18.

Both radios must also be the same band variant (for example 915 MHz or 868 MHz); band and RF parameters are left at factory defaults, so use matching modules.

Timing

Quantity	Value
Gateway link timeout (VENT_LINK_WAIT_MS)	5000 ms
Damper travel time (DAMPER_TRAVEL_MS)	1000 ms
Manual purge debounce (VENT_PURGE_DEBOUNCE_US)	30000 us
DHT11 host start pulse	18000 us
DHT11 bit threshold	50 us
DHT11 per-edge timeout	240 us
LCD I2C clock	100000 Hz
Radio UART baud	115200
Rack temperature band	0 to 400 tenths (0.0 C to 40.0 C)
Rack zone band	0 to 16
Fail-safe zone	0
Damper closed pulse	500 us
Damper open pulse	1500 us
Servo PWM period	20000 us (50 Hz)
Locker lock interval	4 ticks
Locker release token	18 bytes

The Cryptographic Envelope

The radio is the first open path, and it is one a key can close. The fix is authenticated encryption: every request and every command is sealed so a forged frame dies at the authentication tag instead of moving the damper. The full implementation lives in src/chacha20.c, src/poly1305.c, and src/crypto_aead.c, and every primitive is checked against its published test vectors in the native suite.

Why XChaCha20-Poly1305

• 256-bit key, 192-bit nonce. The extended nonce means nonces can be drawn at random forever, so the controller never needs a shared counter that a reboot could reuse.
• AEAD in one pass. Confidentiality and integrity come from one operation; the associated data (the vent node id, byte 0x07) is authenticated even though it is not encrypted.
• Constant-time software. ChaCha20 has no data-dependent table lookups, so it has no cache-timing surface. The RP2350 has no hardware AES engine (it accelerates SHA-256 only), which makes software AES both slower and riskier on this silicon.
• 128-bit Poly1305 tag. Guessing a valid tag succeeds with probability $2^{-128}$.

Why Argon2id

A passphrase is not a key. Argon2id (RFC 9106) is the memory-hard password hash: it mixes the passphrase with a salt across memory and time so an attacker cannot cheaply recover the field passphrase from a captured image. The classroom profile is t=3, p=1, m=64 blocks (CRYPTO_KDF_TIME_COST, CRYPTO_KDF_PARALLELISM, CRYPTO_KDF_MEMORY_BLOCKS) to fit the RP2350 SRAM budget. Raise it on the vault gateway. The lab salt is the 16 ASCII bytes coldiron-salt-01.

Key model: one field key

Act VIII uses a single field key derived with Argon2id from a committed lab passphrase and salt. It seals every frame on the wire and it computes the state tag over the authorization record. In the classroom build the firmware and the gateway derive the same key, so they interoperate with no provisioning step. That is a lab convenience, not a deployment.

The design keeps the key roles separable so students can reason about the real lifecycle: derive, provision per device, use, rotate on a schedule, and retire. A production build provisions key material from one-time-programmable (OTP) memory, keeps the state-tag key off the field device where possible, and rotates without reflashing every controller.

Envelope layout

The sealed frame is carried as hex inside the AT+SEND payload:

nonce[24] || ciphertext[L] || tag[16]

The receiver recomputes the Poly1305 tag over the associated data and ciphertext, compares it in constant time, and only then decrypts. This envelope is wired end to end: src/control.c opens the command with src/envelope.c, and the gateway authenticates before it parses or acts. Authenticated frames carry the vent node id as associated data, so a frame sealed for one node cannot be relabeled for another.

Anti-replay and authenticated state

Strong AEAD is necessary and not sufficient. Two stateful controls sit on top:

• Anti-replay sequence window. src/vault_auth.c keeps last_seq, the highest sequence number ever accepted. vault_auth_apply accepts a command only when its sequence is strictly greater than last_seq. A captured command, even a perfectly valid one, is rejected on second use.
• Authenticated state tag. The authorization record is nine bytes: granted[1], seq[4], last_seq[4]. The tag is an XChaCha20-Poly1305 tag over that record, computed under the field key with a deterministic nonce built from the sequence number and the domain byte 0xA7. vault_auth_state_ok recomputes the tag and compares it in constant time before the damper is allowed to move. A debugger that sets granted = true without recomputing the tag fails here first.

The sequence window and the state tag are independent. The window stops a valid command from working twice; the tag stops an unauthorized verdict from existing at all. Neither one, by itself, makes the vent open.

The FROSTLINE Ransom Locker

Act VIII carries the availability lesson, and the locker is the reason. It is real in technique and inert in effect: it runs on your breadboard, it forces the vent closed and masks the state on the LCD, and it writes to a reserved flash sector that holds nothing else. It is compiled only when SANDBOX_ONLY is defined, so the clean firmware image contains no locker at all. The test suite and the companion CTF build with SANDBOX_ONLY and with the host mock, so every locker path is exercised natively.

What the locker does, in the order the code does it:

1. First run and reserved sector. implant_init reads the marker byte at VENT_IMPLANT_RESERVE_ADDR (0x103FF000), the final sector of external flash. On the first run the marker is absent, so implant_infect erases the sector and programs 0x4C (VENT_IMPLANT_LOCK_MARKER) with the real Pico SDK flash API, flash_range_erase and flash_range_program. On every later boot the marker is present, so the locker re-arms its forced close. A firmware reflash that writes the program region does not touch the reserved sector, so the locker comes back even when the code is gone.
2. The lock. implant_init and implant_lock set the active and locked latches. While locked, monitor_open_target returns false and the damper is driven closed, regardless of the guarded vent state. monitor_state_text reports MAINT while the lock is active, so the LCD renders ST:MAINT and the yellow LOCKED lamp is lit. The operator is told the vent is maintenance locked.
3. Autonomous re-assertion. implant_tick advances a tick counter. While the locker is active, check-in is due, and no probe is attached, every VENT_IMPLANT_TICK_INTERVAL (4) ticks the locker re-asserts the closed position, so it does not quietly drift back open.
4. The release condition. implant_release unlocks only when the candidate token matches VENT_IMPLANT_RELEASE_MAGIC (VAULT-RELEASE-2026) at exactly VENT_IMPLANT_RELEASE_MAGIC_LEN (18) bytes. A wrong token, a null pointer, or an attached debug probe leaves the vent locked. On a successful release the lock marker is cleared so the vent is not re-locked on the next boot.
5. Anti-debug. Every tick, and every lock operation, the locker reads the CoreDebug DHCSR register at 0xE000EDF0 (VENT_IMPLANT_DHCSR_ADDR). Bit 0 is C_DEBUGEN and bit 1 is C_HALT. If either bit is set, the lock is suppressed and the locker behaves like a well-mannered firmware module while a probe is attached. It goes back to work the moment the probe is gone.
6. Neutralization. The locker exposes implant_neutralize to clear the lock, disable re-locking, and zero the marker. The documented fix is not one step: break the lock, unmask the display, clear the marker, and remove the code path and the SANDBOX_ONLY build flag, then add a fail-open policy so a lost link opens the vent.

The locker is bounded by construction and by test. It touches only its own outputs, its runtime flags, and the one reserved sector. It never opens a sealed envelope, never reads the field key, and never contacts an external address. There is no network, no filesystem, and no host impact. test_implant_init_first_run, test_implant_reinstall_on_boot, test_implant_marker, test_implant_debug_attached, test_implant_lock_debug, test_implant_init_debug, test_implant_tick_lock, test_implant_tick_debug, test_implant_tick_unarmed, test_implant_release_accepts, test_implant_release_rejects, test_implant_release_debug, test_implant_neutralize, and test_monitor_implant_lock_mask assert exactly that behavior.

The honest limit is the point of the lab. A sanitized educational locker is still a benign educational locker: it demonstrates the technique, not the tradecraft. It is confined to the breadboard, guarded by SANDBOX_ONLY, the effect is a mock vent and a mock LCD, and there is no real data held hostage. There is no external network and no remote address. The real lesson is that availability is a policy control, and the defense is not a patch to the lock but the closure of the lock, the erasure of the reserved sector, the removal of the code path and the build flag, and a fail-open posture that assumes the worst.

Hardware You Need

Full parts list with links: PARTS.md.

Qty	Part	Notes
1	Raspberry Pi Pico 2 (RP2350) with headers	The vent controller
1	Raspberry Pi Debug Probe	SWD flashing, UART0 console, and the Lab 3 anti-debug/GDB work (recommended, effectively required)
1	Full-size breadboard
1	Assorted jumper wires
1	1602 LCD with PCF8574 I2C backpack	Vault state, zone, temperature, and lock readout, address 0x27
1	DHT11 temperature/humidity sensor	Rack temperature sensor
1	10K resistor	Only if your DHT11 has no onboard pull-up
3	5mm LEDs (red, yellow, green)	HALL HOT, LOCKED, COOLING OK tower light
3	100, 220, or 330 Ohm resistors	One per LED
1	Push button (tactile switch)	Manual purge request, active low
1	SG90 servo motor	The vent damper actuator
1	1000uF 25V capacitor	Bulk decoupling on the servo 5V rail
1	VS1838B infrared receiver	Local maintenance remote input
1	NEC-compatible infrared remote	Local PURGE, ACK, and TEST commands
3	RYLR998 LoRa modules with antennas	2 for the command loop, 3 for the live attack lab
2	USB-to-TTL serial adapters (FTDI FT232, CP2102, or CH340), 3.3V logic	1 for the gateway, 1 for the attacker in the live lab
4	USB cables	Pico 2, Debug Probe, and serial adapter(s)

How many radios do you actually need?

Goal	Radios	What is connected
Legitimate sealed vent loop (Labs 1-2)	2	1x RYLR998 on the Pico (UART1) + 1x RYLR998 on a USB-to-TTL adapter (the gateway)
Live attack lab (Labs 3-4, watch a forged command land and fail)	3	the 2 above + 1x RYLR998 on a second USB-to-TTL adapter (the attacker)
Lock demonstration with no extra hardware	2 or 0	watch the firmware hold the vent closed and mask the LCD, or run the native unit tests
Attack concept with no extra hardware	2 or 0	read-and-run the offline parser demo, or the unit tests

A radio never receives its own transmission, and the gateway radio is busy listening as gateway.py, so the live attack needs a separate attacker radio. The 2-radio kit runs the whole legitimate system; only the live attack observation needs the third. The lock lab is fully observable in the firmware output path and in the native tests, because a single node masks its own state and holds its own damper closed.

Serial adapter warning: the RYLR998 is not 5V tolerant. Use a 3.3V-logic USB-to-TTL adapter (or set its jumper to 3.3V).

How each part works

Every part in the bill of materials, the principle behind it, and what it does in this act.

Part	How it works	Role in this act
1x Full-size breadboard (long)	Spring-clip rows tie five holes into one electrical node, and the two full-length rails distribute 3V3 and GND.	Mounts the Pico 2, LCD, DHT11, LEDs, button, and LoRa module and carries the shared power and ground for the vent node.
1x Assorted jumper wires (male-to-male, male-to-female, female-to-female)	Male pins seat in breadboard rows or female header sockets, female sockets grip male header pins, and each gender extends one node without soldering.	Routes power, ground, I2C, UART, PWM, and GPIO between the Pico 2 and every vent peripheral, including the LCD backpack and servo.
1x Raspberry Pi Pico 2 with header	The RP2350 packs dual Cortex-M33 cores at up to 150 MHz with 3.3V logic, GPIO, ADC, I2C, UART, PWM, and an onboard GP25 LED.	Runs the vent firmware, reads the button and DHT11, drives the LCD, servo, and LEDs, and carries the LoRa BMS link.
1x Raspberry Pi Pico Debug Probe	SWD on SWCLK and SWDIO flashes, halts, and single-steps the RP2350, while a separate UART bridge exposes the serial console.	Flashes the vent firmware and provides the console and debug view used in the ransom-logic analysis.
2x USB A-male to USB micro-B cables	Each cable carries 5V power and USB data over a micro-B plug.	One powers and consoles the Pico 2, and one powers and consoles the Debug Probe during vent controller bring-up.
3x 5mm LEDs (1 red, 1 green, 1 yellow)	An LED conducts once its forward voltage is exceeded, anode positive to cathode, and a GPIO pin sources current through it and a series resistor.	Shows the vault states as the red HALL HOT, yellow LOCKED, and green COOLING OK tower light.
3x 100, 220, or 330 Ohm resistors	Each resistor drops the surplus voltage and limits LED current to a safe few milliamps.	Protects one LED each and sets the brightness of the vault tower light.
1x Push button (tactile switch)	Pressing it shorts the GPIO pin to ground while an internal pull-up holds the pin high, so the press reads active low.	Gives the vent controller its local manual purge request input.
1x 1602 LCD with PCF8574 I2C backpack	The HD44780 controller drives the 16x2 character cells, and the PCF8574 expander turns I2C bytes into the controller's 4-bit nibble protocol.	Displays the vault state, zone, temperature, and lock readout at I2C address 0x27.
1x DHT11 temperature and humidity sensor	The host pulls the one-wire data line low as a start pulse, then the sensor answers with 40 bits of humidity, temperature, and a checksum.	Reports the rack temperature to the vent node.
1x SG90 servo motor	A 50 Hz PWM signal sets the shaft angle by the width of its 1 to 2 ms pulse, with 1.5 ms near center.	Actuates the vent damper in the vent controller mechanism.
1x 1000uF 25V capacitor	The capacitor is a bulk reservoir that supplies the servo inrush current and smooths the 5V rail.	Keeps the vent damper from browning out the Pico 2 when it moves.
1x Infrared receiver (VS1838B)	Its photodiode and 38 kHz band-pass demodulator turn a modulated IR burst into an active-low logic pulse at the GPIO pin.	Receives the local maintenance remote commands for the vent node.
1x Infrared remote controller (NEC-compatible)	Each key sends a NEC frame built from a 9 ms leader and 32 bits of address and command plus their complements.	Sends the local PURGE, ACK, and TEST commands to the vent controller.
1x RYLR998 LoRa radio module	A UART AT command interface configures the module, which carries sealed frames over a sub-GHz LoRa link, and the module runs at 3.3V and is not 5V tolerant.	Links the vent node to the gateway and attacker radios for the sealed vent loop and the live attack lab.

Wiring the Node

Pin map

This is the authoritative map; it is identical to Acts I to VII and is defined in include/vent.h and enforced by the test suite.

Peripheral	Signal	Pico 2 GPIO
DHT11 rack temperature sensor	DATA (one-wire)	GP4
1602 LCD (PCF8574)	SDA (I2C1)	GP2
1602 LCD (PCF8574)	SCL (I2C1)	GP3
RYLR998	RX <- Pico TX (UART1)	GP8
RYLR998	TX -> Pico RX (UART1)	GP9
Infrared maintenance remote	OUT (VS1838B)	GP5
Vent damper servo	PWM signal	GP14
Red HALL HOT LED	anode	GP16
Yellow LOCKED LED	anode	GP17
Green COOLING OK LED	anode	GP18
Manual purge button	to ground	GP15
Onboard LED	heartbeat	GP25
Debug Probe / UART0 console	TX	GP0
Debug Probe / UART0 console	RX	GP1

Note: GPIO 2/3 are the classic I2C1 pins used throughout the Embedded Hacking breadboard; this project's map matches that board because it is the same board.

1602 LCD with I2C backpack

LCD backpack	Pico 2
VCC	3.3V
GND	GND
SDA	GP2
SCL	GP3

DHT11 rack temperature sensor

DHT11	Pico 2
VCC	3.3V
DATA	GP4
GND	GND

If your DHT11 has no onboard pull-up, add a 10K resistor between DATA and 3.3V. The firmware also enables the internal pull-up, but the external resistor makes reads far more reliable over jumper wires. The hall is not nominal when the sensor fails or reads outside 0.0 C to 40.0 C.

Tower light LEDs

LED	Pico 2	Series resistor
Red (HALL HOT)	GP16 (anode)	220-330 Ohm to GND
Yellow (LOCKED)	GP17 (anode)	220-330 Ohm to GND
Green (COOLING OK)	GP18 (anode)	220-330 Ohm to GND

Exactly one lamp is lit at a time. Red is a hot hall or a failed-open controller, yellow is a locked vent or a manual purge awaiting authorization, and green is a cooling, clear hall.

LED behavior

The tower light drives the three lamps from a single state, so at most one lamp is lit at a time and exactly one is lit whenever a state is active; VENT_OFF is the only state that lights none. status_led_show writes the GPIOs directly, so every lit lamp is solid and the clean firmware has no blinking lamp.

Annunciator state	Lamp	Behavior	Meaning
VENT_OFF	none	off	All tower light lamps dark (not selected by the running state machine).
VENT_HALL_HOT	red	solid	The hall is hot, the damper is closed, a command was rejected, or the controller failed open.
VENT_LOCKED	yellow	solid	A manual purge is pending authorization, or in the SANDBOX_ONLY build the ransom lock is active.
VENT_COOLING_OK	green	solid	The hall is cooling and the damper is open.

The onboard GP25 LED is initialized as an output and driven low at boot, then pulses once on every monitor tick as the onboard heartbeat. There is no transmit blink.

Manual purge button

Button	Pico 2
Leg 1	GP15
Leg 2	GND

The firmware enables the internal pull-up, so do not connect 3.3V to the button. The manual purge request is a local request, not an authorization: a press raises the LOCKED indication and never opens the damper on its own. Lab 4 explains why the request must ask for authorization instead of silently bypassing it.

SG90 vent damper servo

Servo	Pico 2
Signal (orange)	GP14
VCC (red)	5V (VBUS)
GND (brown)	GND

Solder the 1000uF capacitor across the servo 5V and GND rails to absorb the inrush current; without it the RP2350 can brown out when the damper moves. Closed (sealed hall) is 0 degrees and open (cooling) is 90 degrees.

Infrared receiver

VS1838B	Pico 2
OUT	GP5
VCC	3.3V
GND	GND

Point any NEC-compatible remote at the receiver. In Act VIII this is the local maintenance remote, not a convenience extra: the firmware decodes VENT_IR_PURGE (0x47), VENT_IR_ACK (0x46), and VENT_IR_TEST (0x45). There is no challenge and no secret on the optical surface, which is why a manual purge is treated as a request and not as an authorization.

Using the remote

Point the NEC remote at the VS1838B receiver on GP5 and press the mapped button. The receiver idles high and pulls low on a mark, and the firmware times the NEC frame to decode the command.

NEC command	Code	Action
VENT_IR_PURGE	0x47	Raises the LOCKED (purge pending) indication by setting the purge request pending. It does not open the damper on its own.
VENT_IR_ACK	0x46	Raises the LOCKED (purge pending) indication as well. In monitor_apply_ir_command the firmware handles ACK exactly as it handles PURGE, and it does not move the damper.
VENT_IR_TEST	0x45	No state change. The firmware logs IR TEST (0x45) and returns, so in this firmware TEST is a maintenance marker rather than a lamp test.

RYLR998 LoRa radio

The RYLR998 must be powered. Forgetting VDD is the single most common reason the link appears dead: the firmware prints while the radio sits silent.

RYLR998	Pico 2
VDD	3.3V
GND	GND
RXD	GP8 (Pico UART1 TX)
TXD	GP9 (Pico UART1 RX)

Attach the antenna before transmitting. TX and RX are crossed: the radio's RXD is the Pico's TX and vice versa.

Debug Probe (recommended)

Debug Probe	Pico 2
SWCLK	SWCLK (3-pin debug header)
SWDIO	SWDIO
GND	GND
UART TX	GP1 (Pico RX)
UART RX	GP0 (Pico TX)
GND	GND

The firmware enables stdio on both UART0 (115200) and USB, so you can watch boot output on the probe's console or on the Pico's own USB serial port. The Debug Probe is also the instrument for the Lab 3 lock work: it is how you prove the vent is held shut, read the reserved-sector marker, inspect the mask, and step past the anti-debug trap.

Peripherals used

Peripheral	Connection	Role
Three tower light LEDs	GP16 red, GP17 yellow, GP18 green	Solid HALL HOT, LOCKED, and COOLING OK lamps, one at a time.
Manual purge button	GP15 to GND	Debounced local purge request; raises the LOCKED indication and never opens the damper.
1602 I2C LCD	GP2 SDA, GP3 SCL, I2C1 address 0x27	Vault readout of state, link, zone, temperature, and lock.
DHT11	GP4	Rack temperature sensor, classified against 0.0 C to 40.0 C.
SG90 servo	GP14	Vent damper actuator driven by 50 Hz PWM.
1000uF capacitor	across the servo 5V and GND rails	Bulk decoupling for the damper inrush; required in hardware and not firmware visible.
VS1838B infrared receiver	GP5	Demodulated NEC input for the local maintenance remote.
NEC infrared remote	optical link to the VS1838B	Sends PURGE, ACK, and TEST.
RYLR998 LoRa transceiver	GP8 RX, GP9 TX, UART1 at 115200 baud	Sealed vent command link to the vault control gateway.
Onboard GP25 LED	GP25	Onboard heartbeat; pulses once on every monitor tick.
Debug Probe	GP0 TX, GP1 RX, UART0, and SWCLK/SWDIO/GND	stdio console, flashing, and the GDB anti-debug work.

Every peripheral above is used by the firmware. The 1000uF capacitor is a hardware requirement rather than a firmware device. There is no transmit blink.

How the functionality works

This is the end-to-end behavior of the running node: what each input does, what each output shows, and how to watch the live console. The DHT11 is sampled every two seconds, so one live status line appears about every two seconds.

Inputs

Input	Pico 2	What it does
Infrared maintenance remote	GP5 (VS1838B)	Decodes a NEC frame into PURGE (0x47, CH+), TEST (0x45, CH-), or ACK (0x46, CH). PURGE and ACK raise the purge-pending indication; TEST is a maintenance marker. No remote code can move the damper on its own.
Manual purge button	GP15 to GND	A debounced press raises the purge-pending request. It asks for authorization and never bypasses the sealed vent path.
DHT11 rack sensor	GP4	Samples the hall every tick and classifies it against 0.0 C to 40.0 C (0 to 400 tenths). A failed read is not nominal.
RYLR998 LoRa radio	GP8/GP9, UART1	Carries the sealed vent command path. Every inbound frame is authenticated and anti-replay checked before it can move the damper.

Outputs

Output	Pico 2	What it shows
Red HALL HOT LED	GP16	Solid when the hall is hot, a command was rejected, or the controller failed open.
Yellow LOCKED LED	GP17	Solid when a manual purge is pending authorization, or (SANDBOX_ONLY) the ransom lock is active.
Green COOLING OK LED	GP18	Solid while the hall is cooling and the damper is open.
1602 LCD	GP2/GP3, I2C1 0x27	Line 1 ST:COOL L:UP is the state and gateway link; line 2 Z:4 T:235 K:-- is the rack zone, rack temperature in tenths, and locker marker.
SG90 vent damper servo	GP14	50 Hz PWM: closed (0 degrees) seals the hall, open (90 degrees) cools it.
Onboard GP25 LED	GP25	Pulses once on every monitor tick as the heartbeat; it is not a transmit blink.

Exactly one tower light lamp is lit at a time and no lamp blinks in the clean firmware. The damper moves only after the guarded state machine decides.

Watching the interactive console

The firmware enables stdio on both UART0 (115200, Debug Probe) and the Pico's own USB serial port (115200). Open either at 115200 8N1 and reset the board. After BOOT and the I2C scan, the node prints a boot banner and then one line per event:

BOOT
I2C scan:
  found 0x27
=== OPERATION IRON VAULT // ACT VIII DATACENTER VENT ===
Remote: CH+ 0x47 PURGE, CH- 0x45 TEST, CH 0x46 ACK
Button: manual purge request, never bypasses authorization
RACK t=235 ok=1 LED=3 cyc=12
IR PURGE (0x47)
BUTTON purge request -> pending
RX from 0x0001, 126 bytes
RACK read failed -> WARNING

Line	Meaning
RACK t=... ok=... LED=... cyc=...	One live status line per reading cycle: rack temperature in tenths, the in-band verdict, the active tower light state, and the reading-cycle count.
RACK read failed -> WARNING	The DHT11 did not answer or failed its checksum, so the hall is treated as not nominal.
IR <NAME> (0xNN)	A decoded remote command, named PURGE, TEST, ACK, or UNKNOWN.
BUTTON purge request -> pending	The manual purge button raised a pending request.
RX from 0xNNNN, N bytes	One inbound radio frame, with the reported sender address and payload length, before it is authenticated.

Build and Flash

1. Install toolchain prerequisites

• Pico SDK 2.2.0+
• ARM GNU toolchain (arm-none-eabi)
• CMake and Ninja
• Python 3.x
• GDB (arm-none-eabi-gdb) for the Lab 3 malware analysis

Linux:

export PICO_SDK_PATH="$HOME/.pico-sdk/sdk/2.2.0"

macOS:

brew install cmake ninja arm-none-eabi-gcc python
export PICO_SDK_PATH="$HOME/.pico-sdk/sdk/2.2.0"

Windows: install PowerShell, Visual Studio Build Tools, CMake, Ninja, Python 3, and the ARM embedded toolchain.

2. Build the firmware

The clean firmware does not define SANDBOX_ONLY, so it ships no ransom locker:

mkdir -p build && cmake -S . -B build -G Ninja -DPICO_BOARD=pico2 -DPICO_PLATFORM=rp2350-arm-s && cmake --build build

To build the locker image with the implant compiled in, turn the option on:

cmake -S . -B build-sandbox -G Ninja -DPICO_BOARD=pico2 -DPICO_PLATFORM=rp2350-arm-s -DSANDBOX_ONLY=ON && cmake --build build-sandbox

Build-time artifact guardrail:

• The build regenerates packet_artifact.h from scripts/packet_artifact.json before compiling.
• The build fails if the committed include/packet_artifact.h is stale relative to the JSON artifact.

Generated outputs:

• build/datacenter_vent_controller.elf (primary firmware binary)
• build/datacenter_vent_controller.uf2 (UF2 for BOOTSEL/picotool)
• build/datacenter_vent_controller_app.elf / .uf2 (backward-compatible copies)

3. Flash the RP2350

BOOTSEL (drag-and-drop): hold BOOTSEL while plugging in USB, then:

cp build/datacenter_vent_controller.uf2 /Volumes/RP2350/

picotool:

picotool load build/datacenter_vent_controller.uf2 -fx

(If picotool is not on your PATH, invoke it from $HOME/.pico-sdk/picotool/*/picotool/picotool.)

Debug Probe (SWD): with openocd installed you can flash and reset without touching BOOTSEL:

openocd -f interface/cmsis-dap.cfg -f target/rp2350.cfg \
  -c "program build/datacenter_vent_controller.elf verify reset exit"

4. Watch the console

Open the UART0 console (Debug Probe) or the Pico's USB serial port at 115200. On reset you should see:

BOOT
I2C scan:
  found 0x27
=== OPERATION IRON VAULT // ACT VIII DATACENTER VENT ===
Remote: CH+ 0x47 PURGE, CH- 0x45 TEST, CH 0x46 ACK
Button: manual purge request, never bypasses authorization
RACK t=0 ok=0 LED=3 cyc=1

found 0x27 confirms the LCD backpack answered on the I2C bus. The boot banner names the operation and the remote button roles, and RACK t=... ok=... LED=... cyc=... is the live status line printed every reading cycle. The full set of console lines is described in How the functionality works. If a peripheral fails, the firmware prints INIT FAIL and stops.

Lab 1: Bring-Up and Verify

Goal: prove the node reads the rack temperature, drives the LCD, takes a local purge request, reaches the gateway, and moves the damper.

1. Wire the node per the pin map and attach the antenna.

2. Build and flash the clean firmware.

3. Connect the gateway radio to the laptop and find its port (/dev/cu.usbserial-* on macOS, /dev/ttyUSB* on Linux).

4. Start the vault control gateway:

   python3 scripts/gateway.py --port /dev/cu.usbserial-XXXX --baud 115200
   

5. Send a sealed zone request from the edge simulator, or seal one from a node. The gateway prints it, then answers with a sealed vent command:

   +OK
   +OK
   +RCV=7,84,<84 hex characters>,-11,10
   VENT open seq=1 zone=4
   

6. The node turns yellow (LOCKED) when a manual purge is pending, receives the command, verifies the state tag and the anti-replay window, then drives the damper to the authorized position. Press the manual purge button at any time to raise a request.

Checkpoint: the LCD shows ST:COOL L:UP and Z:4 T:235 K:--, one lamp is lit after the damper settles, and vent_log.csv gains one row per request:

utc,sender,auth,zone,rssi_snr
2026-09-20T09:30:05+00:00,7,OK,4,"-11,10"

Theory check: why does a successful command prove the LCD initialized? Because monitor_init() only returns true when every peripheral, including the LCD, is ready; otherwise main prints INIT FAIL and never enters the loop.

Lab 2: Inspect the Wire Protocol

Goal: see the sealed envelope and the declared-length rule in action.

1. Capture a full +RCV line from the console or the gateway log.
2. Confirm the declared length equals the number of hex characters between the second comma and the RSSI field.
3. Split the hex into three parts: the first 48 hex characters are the 24-byte nonce, the last 32 are the 16-byte tag, and everything between is the ciphertext of the request or command body.
4. Locate the payload, its declared length, and the two tail fields in scripts/gateway.py (_rcv_parts and _split_payload), and explain why finding the first comma would be a bug.
5. Challenge: for the 23-byte command body, identify the four bytes of the sequence number, the one command byte, the two zone bytes, and the sixteen bytes of the state tag.

Checkpoint: you can explain why a frame must be sliced by the number in the declared length field, not by delimiter counting, and why the command byte and the zone are range-checked against the guarded set and the bounded band before they can reach the damper decision.

Lab 3: The Ransom-Lock Track

Goal: find the FROSTLINE ransom locker, prove that it holds the vent shut and masks the readout, expose its lock marker, break the lock, and remove it for good. This is the availability act, and this lab is its heart.

Safety: the locker is benign and confined to your breadboard. It forces only your mock vent and your mock LCD, it releases on a documented token, and it writes only the reserved sector at 0x103FF000, on the same chip. There is no network, no filesystem, and no host impact. The lock marker is a real sector erase and program, but the vent and the display are the only things it affects, and it holds no real data.

Build the locker image

cmake -S . -B build-sandbox -G Ninja -DPICO_BOARD=pico2 -DPICO_PLATFORM=rp2350-arm-s -DSANDBOX_ONLY=ON && cmake --build build-sandbox

The clean build does not define SANDBOX_ONLY; the test build and the companion CTF build do. Compare the two binaries and explain why the locker symbols are absent from the clean one.

A: Find the lock and the marker

1. Flash the SANDBOX_ONLY image and let it boot once. implant_init writes the 0x4C marker into the reserved sector at 0x103FF000 on the first run with the real flash API.
2. Read the reserved sector with the Debug Probe or picotool and confirm the marker byte. The LCD lock field reads K:LOCK.
3. Watch the vent. The locker forces the damper closed and the LCD renders ST:MAINT with the yellow LOCKED lamp, even though the guard state would otherwise open the vent.
4. Locate implant_init, implant_lock, and implant_infect, and explain why the forced close and the mask are local conditions that ignore the sealed command path.

The lesson: the locker does not need the wire or the key, because it changes the output after the authenticated decision is made.

B: Break the vent lock and unmask the LCD

1. Read implant_release and implant_token_ok: the only accepted token is the exact 18-byte VAULT-RELEASE-2026.
2. Present the correct token to implant_release and prove the lock clears, the damper returns to the authorized position, and the LCD state returns from ST:MAINT to its true guarded state.
3. Present a wrong token, a short token, and a null pointer, and prove each one leaves the vent locked.
4. Prove the gates are the difference between a building that breathes and one that does not, and explain why releasing the lock does not clean the state that re-arms a future boot.

The lesson: breaking the lock restores the vent, but it does not remove the state that re-locks a future boot.

C: Clear the lock marker

1. Read the marker at 0x103FF000 and confirm it is the 0x4C byte.
2. Erase the reserved sector, or call implant_neutralize, and prove the node comes up with the marker gone and the K: field on the LCD back to --.
3. Reflash only the firmware image (the clean image is ideal) and let the node boot again. Because the marker is gone, the locker does not re-install.
4. Explain why BOTH steps are required: breaking the lock opens the vent now, and the erasure removes the state that would re-lock a future boot.

The lesson: a ransom locker has two copies, the one in the image and the one in the state. You remove the lock and you remove the marker.

D: Defeat the anti-debug with GDB

This is the dynamic-analysis trap. The locker reads CoreDebug DHCSR at 0xE000EDF0; bit 0 is C_DEBUGEN and bit 1 is C_HALT. While a probe is attached, the locker suppresses the forced close and the mask.

1. Start the controller under the Debug Probe:

   arm-none-eabi-gdb build-sandbox/datacenter_vent_controller.elf
   (gdb) target extended-remote /dev/cu.usbmodemXXXX
   (gdb) monitor reset halt
   

2. Break in implant_tick and inspect implant_debug_attached. With a normal probe attached, it returns true, and the lock is suppressed.

3. Set a breakpoint after the anti-debug check, or clear the DHCSR debug bits in the debugger's view, and observe the forced close and the ST:MAINT mask resume.

4. Prove the release: with the trap bypassed, present the magic token and watch the vent open and the marker clear.

The lesson: an anti-debug check is a branch, and every branch is a place to stand. The correct neutralization is not to babysit the branch; it is to remove the code and the state it reads, and to make the vent fail open by policy.

Ransom-lock checklist

• Locate the reserved-sector marker and explain the write-once first run.
• Identify the 0x4C marker, the VAULT-RELEASE-2026 token and its 18-byte length, and the 4-tick re-assertion interval.
• Show the forced close and the ST:MAINT mask, and prove they ignore the sealed command path.
• Break the lock with the magic token and unmask the display.
• Read and explain the CoreDebug DHCSR anti-debug trap.
• Erase the reserved sector, remove the code path, and confirm the clean build is locker-free and marker-free.

Lab 4: The Fix Track

Goal: seal the controller so the red half and the locker cannot do to you what they did on the bench. Each control maps to a defect the earlier labs exposed.

1. Seal the vent command path

The old design accepted an unauthenticated vent command. Act VIII replaces it with src/control.c: the request must open under the field key, the command byte must be in the guarded vent set, the zone must be inside the provisioning band, and the sequence and state tag must pass src/vault_auth.c before the command is applied. Re-run the Lab 3 forged-command injection: the tag fails and the damper does not move.

2. Manual purge authorization

The local purge request is an operator request, and it must not silently bypass authorization. monitor_handle_purge and monitor_apply_ir_command raise g_purge_pending; they never open the damper on their own. monitor_apply_command clears the pending indication only when an authorized command arrives. Re-run the lab: press the manual purge button, then send a valid sealed open command. The damper moves only from the authorized command, and the yellow LOCKED lamp returns to green only then.

3. No untrusted task execution

The clean build compiles the locker path out entirely, so an untrusted frame can never trigger a forced close or a masked readout (monitor_implant_init and monitor_implant_tick are no-ops in the clean build). The lesson is that a control node must never let a local condition override an authorized decision. In the fix track, remove the SANDBOX_ONLY build flag and erase the reserved sector so no node can be seeded again.

4. Fail open to the cooling posture

Loss of the control link or a fault must leave the hall in the safe state. monitor_check_link calls monitor_fail_open when the link goes silent for VENT_LINK_WAIT_MS, which drives the fail-safe zone and opens the damper. damper_init opens the vent at boot. Re-run the link-loss test: pull the gateway and watch the damper open and the fail-open posture latch, with the zone returned to 0.

5. Contain the locker

The locker is a build-time and state-handling problem, so the fix is a build-time and state-handling control:

• Do not define SANDBOX_ONLY in production. The clean build has no locker.
• Erase the reserved sector so no persisted state can re-install the payload.
• Treat the firmware image as a signed artifact and verify it before flashing.
• At runtime, never let a condition override an authorized command; route every move through the guarded, authorized path and record who authorized it.
• In production, burn the RP2350 secure-boot and debug-disable settings in OTP so SWD cannot read or write SRAM on a deployed controller.

The fix-track checklist

• Sealed command path: authenticate the frame, guard the command set and the zone band, verify the sequence and the state tag.
• Manual purge authorization: request, do not bypass.
• No untrusted override: the node never refuses an authorized command because of a local latch.
• Fail open: open the vent on boot, fail open on link loss, and return the fail-safe zone.
• Locker removal: break the lock, clear the marker, erase the reserved sector, and remove the code path.
• Build integrity: no SANDBOX_ONLY in production, sign and verify images.
• Debug lockdown: OTP debug disable on the deployed part.
• Key lifecycle: provision the field key from OTP and rotate on a schedule.

Troubleshooting

Symptom	Likely cause	Fix
No BOOT on the console	Wrong console pins / not reset	Check UART0 GP0/GP1 or USB; press RESET
INIT FAIL with no 0x27 in the scan	LCD not answering	Check LCD VCC=3.3V, SDA=GP2, SCL=GP3, contrast pot
LCD shows blocks / nothing	Contrast or address	Turn the backpack contrast pot; confirm address 0x27 vs 0x3F
Rack temperature always bad	DHT11 not reading	Check DATA=GP4; add 10K pull-up to 3.3V; wait 1-2 s after power-up
IR remote does nothing	Receiver wiring or remote protocol	Check OUT=GP5, VCC=3.3V; confirm the remote is NEC-compatible
Damper will not move on a remote command	Command guard, band, or tag	Confirm the gateway holds the field key and the zone is inside 0 to 16
AT+SEND sent but gateway sees nothing	Radio unpowered / wrong band	Power VDD, attach antenna, use matching band modules
Gateway sees nothing but +OK	Address/network mismatch	Confirm gateway radio provisioned to AT+ADDRESS=1, AT+NETWORKID=18
vent_log.csv stays empty while +RCV prints	Gateway parser regression	Ensure _split_payload checks the comma at the declared length
Command rejected on the controller	Tag, window, command guard, or band	Check the field key matches, the sequence is newer, and the zone is in band
LCD shows K:LOCK or ST:MAINT while nothing looks wrong	Lock marker present (SANDBOX_ONLY build)	The locker has written the marker; see Lab 3C
Vent stays shut despite an authorized open	Forced close latch (SANDBOX_ONLY build)	Expected in the malware-track build; break the lock in Lab 3B
Marker reappears after a reflash	Reserved-sector persistence	The payload is still in the reserved sector; erase it and remove the code path (Lab 3)
Debugger changes locker behavior	CoreDebug DHCSR anti-debug	The locker suppresses itself while a probe is attached; see Lab 3D
Node refuses an authorized command	Local override	In production never define SANDBOX_ONLY; see Lab 4

Testing Philosophy and Coverage

Hardware bugs are expensive to find on the bench, so the firmware is written so that almost all of it can be tested on the host. The suite compiles the real src/*.c files against mock Pico SDK headers (test/mock/), replacing GPIO, I2C, UART, and time with deterministic fakes, and it compiles src/implant.c with a host mock for the CoreDebug DHCSR register and the reserved flash sector.

• The mock GPIO can replay a recorded DHT11 waveform as an absolute time/level timeline, so the exact edge-timing decoder is exercised without a sensor.
• The mock I2C records every LCD byte, so rendered text can be decoded and asserted.
• The mock UART records outbound AT+SEND bytes and injects inbound +RCV lines, so the operator-to-gateway-to-damper path runs end to end with no radio.
• The locker host mock lets the tests set the DHCSR anti-debug bits and read and write the reserved-sector marker without touching real silicon, and it exposes the forced close and the mask so the 0x4C marker path can be asserted.

Run the native test suite:

python3 scripts/run_tests.py

Or configure via CMake and CTest:

cmake -S test -B build-test -G Ninja && cmake --build build-test && ctest --test-dir build-test --output-on-failure

The suite has 145 cases and 488 checks with 0 failures, covering the full DHT11 waveform and every timeout shape, the damper state machine and its bounded travel, the sealed command path and its guards, the authorization window and state tag, the manual purge no-bypass path, fail-open on link loss, the declared-length parser with hex-bearing payloads, the cryptographic primitives against published vectors, and the complete ransom locker: the forced close, the ST:MAINT mask, the 0x4C marker, re-lock on boot, the magic release token, and anti-debug.

Verify 100% line coverage of owned firmware modules:

python3 scripts/check_coverage.py

The coverage report shows 2082 / 2082 lines, 100.00%. main.c is excluded from coverage by design. The Python adapter suite (test/test_field_crypto.py and test/test_vent_node.py) adds 17 more tests, including the RFC 9106 Argon2id known-answer test.

The harness itself is a small in-repo framework (test/harness/) so the repo vendors no third-party code and every owned file obeys the coding standard.

Generating Packet Artifacts

scripts/gen_packet.py writes the build-time generated header from the JSON artifact:

• scripts/packet_artifact.json is the source of truth.
• include/packet_artifact.h is the generated header, committed for the build guardrail.

Why these constants are compiled into firmware:

• The RP2350 firmware has no runtime JSON parser or filesystem on this path.
• include/packet_artifact.h is generated from the JSON so the frame size, node id, gateway address, wait time, servo pulses, DHT timeout, and provisioning constants are embedded in flash.
• This is provisioned data; regenerate whenever you rotate node identity, gateway addressing, or key material.

To sync the committed header from the JSON artifact:

python3 scripts/gen_packet.py --from-json scripts/packet_artifact.json --header-out include/packet_artifact.h

The check_packet_artifact_header CMake target fails the build when the committed header is stale.

Code Standards

This repository enforces unusually strict standards because the point is to teach disciplined embedded and tooling practice, not just working code.

C standard

• Every function body has no blank lines.
• Every function body is at most eight lines (Doxygen comment blocks and lone braces excluded).
• Every file, function, macro, type, and struct member carries Doxygen @brief documentation.
• Naming: snake_case files/functions, UPPER_SNAKE macros, snake_case_t types.

Run the C audit:

python3 scripts/audit_c_standard.py

Python standard

• Strict PEP8, four-space indents, snake_case, 79-character lines.
• Every function has a NumPy-style docstring.
• Every function executable body is at most eight lines, with no exceptions.
• No blank lines inside function bodies.

Run the Python audit:

python3 scripts/audit_python_standard.py

Both audits must report nothing.

Project Layout

• src/main.c: firmware entry point
• src/monitor.c: state machine tying the maintenance remote, sealed command path, damper, manual purge request, temperature sensor, and radio together
• src/implant.c: SANDBOX_ONLY FROSTLINE ransom locker (forced close, ST:MAINT mask, magic release token, reserved-sector 0x4C marker, re-lock, CoreDebug anti-debug)
• src/damper.c: vent damper state machine and fail-open policy
• src/control.c: sealed vent command path with a guarded command set and a bounded zone band
• src/vault_auth.c: authorization record, monotonic anti-replay window, authenticated state tag
• src/sensor.c: DHT11 one-wire sampling and rack-temperature-band classifier
• src/display.c: 1602 LCD rendering over the PCF8574 I2C backpack
• src/radio.c: RYLR998 provisioning, AT-command interface, and +RCV parser
• src/status_led.c: red/yellow/green HALL HOT / LOCKED / COOLING OK tower light
• src/button.c: debounced manual purge input
• src/servo.c: 50 Hz PWM vent damper actuator
• src/ir_remote.c: VS1838B edge timing and NEC maintenance remote decoder
• src/crc.c: CRC-16/CCITT-FALSE helper
• src/chacha20.c, src/poly1305.c, src/crypto_aead.c, src/blake2b.c, src/argon2.c, src/crypto_kdf.c, src/envelope.c: the in-repo cryptographic stack
• include/vent.h: board-level pin, provisioning, and locker configuration
• include/implant.h, include/control.h, include/vault_auth.h, include/damper.h: locker, command, authorization, and damper interfaces
• include/field_secrets.h: lab-only committed key material
• include/packet_artifact.h: generated packet artifact header
• test/test_vent_node_and_security.c, test/test_peripheral_and_crypto.c: comprehensive test suites
• test/mock/: Pico SDK hardware mocks plus the locker CoreDebug and reserved-flash host mock
• test/harness/: minimal in-repo test harness (strictly C-standard compliant)
• scripts/gateway.py: vault control gateway with radio provisioning, authentication, CSV logging, and sealed command replies
• scripts/spoof.py: forged and replayed command injection client
• scripts/sim_edge.py: laptop vent node simulator
• scripts/field_crypto.py: pure-Python interoperable crypto
• scripts/gen_packet.py / scripts/packet_artifact.json: packet artifact generator and source
• scripts/run_tests.py, scripts/check_coverage.py: test runner and coverage report
• scripts/audit_c_standard.py, scripts/audit_python_standard.py: code-standard auditors
• scripts/gen_banner.py: banner generator
• paper.typ / paper.pdf: classroom paper describing the protocol, the locker, and the exercise
• .github/workflows/release.yml: tag-driven UF2 release workflow

Glossary

• AEAD: authenticated encryption with associated data; one operation for secrecy and integrity.
• Anti-debug: a check that detects an attached debugger and changes behavior. Here it reads CoreDebug DHCSR at 0xE000EDF0 (bits C_DEBUGEN and C_HALT).
• Anti-replay window: a monotonic sequence rule that rejects a valid frame that has already been used.
• Argon2id: the memory-hard password hash (RFC 9106) used to derive the field key.
• AT command: a short ASCII command (AT+...) understood by the radio.
• Availability: the property that a system remains usable; the Act VIII failure mode, where a device withholds the function it exists to provide.
• Damper: the servo-driven vane that opens or seals the vent.
• Declared length: the byte count the sender claims for a payload; the receiver slices exactly that many characters.
• DHT11: a low-cost temperature/humidity sensor using a custom one-wire protocol, used here as the rack temperature sensor.
• Fail open: a fault opens the vent and returns the fail-safe zone, the safe cooling state.
• Field key: the key that seals frames on the wire and computes the state tag.
• HD44780: the character-LCD controller inside a 1602 module.
• I2C: a two-wire bus (SDA/SCL) used here for the LCD backpack.
• Implant: code that runs on the device but is not part of its intended function. Here the SANDBOX_ONLY FROSTLINE ransom locker.
• Lock marker: the 0x4C byte the locker writes into the reserved sector.
• LoRa: a long-range, low-power sub-GHz radio modulation.
• Magic token: the exact 18-byte VAULT-RELEASE-2026 string that breaks the lock.
• Masking: rendering a hostile state as a routine one; here ST:MAINT.
• NEC: the infrared remote encoding the VS1838B decodes.
• PCF8574: an I2C I/O expander that drives the LCD's parallel interface.
• Ransom logic: a lock that is released only on a secret condition; the availability attack demonstrated here.
• Reserved sector: the final flash sector at 0x103FF000, used here to hold the one-byte lock marker.
• SANDBOX_ONLY: the build guard that compiles the benign ransom locker. The clean firmware does not define it.
• State tag: a keyed tag over the authorization record that detects a tampered verdict.
• Tower light: the red/yellow/green HALL HOT / LOCKED / COOLING OK lamps.
• UART: a serial port used to talk to the radio.
• Vent zone: the bounded rack identifier a sealed command authorizes, 0 to 16.
• XChaCha20-Poly1305: the AEAD used for every sealed frame, with a 192-bit nonce and a 128-bit tag.

Further Reading

• Act I, the sensor and telemetry chapter: https://github.com/mytechnotalent/cold-chain-monitor
• Act II, the access gate chapter: https://github.com/mytechnotalent/access-gate
• Act III, the pipeline valve chapter: https://github.com/mytechnotalent/pipeline-valve-controller
• Act IV, the HVAC automation node chapter: https://github.com/mytechnotalent/hvac-automation-node
• Act V, the industrial tamper system chapter: https://github.com/mytechnotalent/industrial-tamper-system
• Act VI, the smart logistics drop-box chapter: https://github.com/mytechnotalent/smart-logistics-dropbox
• Act VII, the factory andon station chapter: https://github.com/mytechnotalent/factory-andon-station
• The companion CTF for this act: https://github.com/mytechnotalent/CTF_datacenter-vent-controller
• Embedded Hacking course and breadboard: https://github.com/mytechnotalent/Embedded-Hacking
• Reverse Engineering self-study course: https://github.com/mytechnotalent/Reverse-Engineering
• paper.typ / paper.pdf: the classroom paper for this project.
• DHT11 datasheet, RYLR998 AT command reference, SG90 datasheet, and VS1838B datasheet (module vendors).

Next

OPERATION IRON VAULT CTF

License

MIT License