FIBER Agent

Embedded Rust application for medical-grade temperature monitoring on Raspberry Pi Compute Module 4.

Overview

FIBER monitors up to 8 DS18B20 temperature sensors with a 4-level alarm system, stores readings in an encrypted database with HMAC integrity, and communicates via MQTT with EU MDR 2017/745 compliance. It also bridges LoRaWAN wireless sensors (HARDWARIO STICKER) into the same monitoring pipeline.

Hardware

Component	Detail
Target	Raspberry Pi CM4
Co-processor	STM32 via UART — LED control, ADC (Vbat, VIN)
Sensors	8x DS18B20 (1-Wire via DS2482 I2C bridge)
Display	ST7920 128x64 LCD (SPI)
Buttons	UP (GPIO23), DOWN (GPIO25), ENTER (GPIO24)
Accelerometer	LIS2DH12 (I2C, 0x19) — motion/tamper detection
Buzzer	GPIO17, PWM volume control (0-100%)
Connectivity	MQTT (TLS), BLE, LoRaWAN (ChirpStack)

Implemented Features

Temperature Monitoring

• 8 independent 1-Wire sensor lines with per-sensor configurable thresholds
• 4-level alarm states: Normal, Warning, Critical, Disconnected
• Failure debouncing and warmup filtering to prevent false alarms
• Periodic aggregation (min/max/mean) with crash recovery via JSON persistence

Alarm System

• Per-sensor LED indication: green (normal), yellow (warning), red (critical), blinking red (disconnected)
• Buzzer patterns with priority management (critical > disconnected > power alerts)
• Configurable alarm thresholds and LED/buzzer patterns per alarm level
• MQTT alarm event publishing (QoS 2)

Power Management

• Battery monitoring via STM32 ADC (Vbat: 3100-3400mV range)
• AC power detection via VIN threshold (>12V = AC, <12V = battery)
• Power LED: green (AC), yellow (battery), blinking yellow (low battery)
• Buzzer alerts on AC disconnect, battery mode reminder, critical battery

Display & UI

• Sensor Overview — 4 sensors per page, 2 pages, with alarm indicators
• Sensor Detail — thresholds, current temp, alarm state, location
• LoRaWAN Sensors — remote sensor temp, humidity, battery, signal
• QR Code — BLE pairing info (ble://{hostname}/{mac}/{pin})
• System Info — version, hostname, network, battery, storage, uptime (paginated)
• Pairing Mode — 6-character code display
• Button navigation: UP/DOWN scroll, ENTER select, UP+DOWN 3s hold enters pairing

MQTT Communication

• TLS with CA certificate validation (insecure_skip_verify blocked in production)
• Per-topic QoS (0 for telemetry, 1 for power, 2 for alarms)
• Last Will and Testament for offline detection
• Exponential backoff reconnection (1s to 60s)
• Rate-limited command subscription with audit logging
• Publishes: sensors, power, network status, system info, alarms, aggregations

Secure Command Protocol (EU MDR)

• Ed25519 signature verification on all incoming commands
• Challenge-response protocol: request → challenge → confirm → apply
• Nonce-based replay attack prevention
• Authorized signers registry with per-signer permissions and expiration
• Audit trail of every command attempt (success and failure)

Device Pairing

• Initiated by 3-second button hold (UP+DOWN)
• 6-character code shown on display, entered in viewer app
• AES-256-GCM encrypted key exchange using pairing code as seed
• Device CA signs admin Ed25519 certificate for the paired user

Storage

• SQLCipher encrypted SQLite database (WAL mode)
• HMAC-SHA256 integrity on every sensor reading
• Immutable audit trail of all operations
• Auto-generated HMAC key on first boot
• Configurable max size (default 5GB) with FIFO auto-purge
• Non-blocking writes via background thread with 100ms flush interval

LoRaWAN Gateway Bridge

• Auto-detects ChirpStack gateway hardware
• Subscribes to ChirpStack uplink events on local Mosquitto
• Parses HARDWARIO STICKER payloads: temp, humidity, voltage, illuminance, motion
• 4-level alarm system per wireless sensor (same as wired)
• Sensor timeout tracking (default 1 hour)
• Bridges data into FIBER MQTT topic hierarchy

Additional

• Accelerometer: motion/tamper detection with configurable threshold and debounce
• BLE: advertising control via system service, PIN-based pairing
• Network: WiFi/Ethernet status detection, signal strength, IP reporting
• Buzzer volume: software PWM, MQTT-adjustable
• Screen brightness: PWM backlight control, MQTT-adjustable
• Config applier: atomic file writes with backup, validation, and rollback

Architecture

The application spawns dedicated threads for each subsystem:

Thread	Interval	Role
Sensor Monitor	1s	Read DS18B20 sensors, evaluate alarms
Power Monitor	1s	Read Vbat/VIN via STM32 ADC
LED Monitor	50ms	Drive per-sensor and power LEDs
Display Monitor	250ms	Render LCD screens
Button Monitor	event	Handle physical button input
Buzzer Controller	event	Priority-managed audio patterns
Storage Thread	100ms flush	Non-blocking SQLCipher writes
MQTT Monitor	event	Publish telemetry, process commands
Pairing Monitor	event	Handle secure pairing protocol
LoRaWAN Monitor	30s	Bridge ChirpStack to FIBER MQTT
Accelerometer	100ms	Motion state machine

All threads communicate via Arc<Mutex<T>> shared state and crossbeam channels.

Building

Prerequisites

• Rust toolchain (see rust-toolchain.toml)
• Cross-compilation linker: aarch64-linux-gnu-gcc
• arm64 dev headers:
  • libssl-dev (required by SQLCipher)
  • libdbus-1-dev (required by bluer for the in-app BLE GATT server)

sudo apt install gcc-aarch64-linux-gnu pkg-config

On Ubuntu, arm64 packages require the ports.ubuntu.com mirror. Add arm64 architecture and configure apt sources:

sudo dpkg --add-architecture arm64

Edit /etc/apt/sources.list.d/ubuntu.sources — add Architectures: amd64 i386 to existing entries and append an arm64 block:

Types: deb
URIs: http://ports.ubuntu.com/ubuntu-ports/
Suites: noble noble-updates noble-backports noble-security
Components: main restricted universe multiverse
Signed-By: /usr/share/keyrings/ubuntu-archive-keyring.gpg
Architectures: arm64

Then install the cross-compilation dependencies:

sudo apt update
sudo apt install libssl-dev:arm64 libdbus-1-dev:arm64

Build for target

pkg-config needs to be told it's allowed to cross-compile and where to find the arm64 .pc files:

PKG_CONFIG_ALLOW_CROSS=1 \
PKG_CONFIG_PATH=/usr/lib/aarch64-linux-gnu/pkgconfig \
cargo build --release --target aarch64-unknown-linux-gnu

Dev-platform build

Disables cryptographic verification. Requires /data/fiber/config/DEV_MODE_ENABLED on the device.

PKG_CONFIG_ALLOW_CROSS=1 \
PKG_CONFIG_PATH=/usr/lib/aarch64-linux-gnu/pkgconfig \
cargo build --release --target aarch64-unknown-linux-gnu --features dev-platform

Native build (for tests + lint only — does not run on hardware)

If you only want to compile the code and run unit tests without producing a target binary:

sudo apt install libssl-dev libdbus-1-dev pkg-config
cargo build --release      # native x86_64
cargo test --lib

The resulting binary will not work on the device (Pi GPIO, STM32 UART, BLE adapter not present), but this is the fastest way to validate code changes.

Yocto build

When building via the meta-fiber recipe (bitbake fiber), bitbake provides the cross-sysroot automatically. The recipe declares DEPENDS += "openssl dbus", so no manual setup is needed.

Configuration

Runtime configuration is loaded from /data/fiber/config/fiber.config.yaml. See fiber.config.yaml for defaults. Sensor thresholds are in fiber.sensors.config.yaml.

File	Purpose
fiber.config.yaml	Main config: power, MQTT, serial, storage, display, buzzer
fiber.sensors.config.yaml	Per-sensor thresholds, alarm patterns, names, locations
authorized_signers.yaml	EU MDR authorized public keys for remote commands

EU MDR 2017/745 Compliance

Classification: Class IIa | Software safety: IEC 62304 Class B

Area	Status	What's implemented
Data integrity	Complete	HMAC-SHA256 on every sensor reading, SHA-256 hash-chain on audit logs
Encryption at rest	Complete	SQLCipher (AES-256) on all databases
Encryption in transit	Complete	TLS on MQTT (8883) and HTTPS (443)
Command authorization	Complete	Ed25519 signatures, challenge-response protocol, nonce replay prevention
Access control	Complete	JWT auth, 8 RBAC roles, bcrypt passwords, rate limiting, session IP binding
Audit trail	Complete	Tamper-evident hash-chain on firmware and viewer, API access logs, frontend action audit
Configuration tracking	Complete	All signed changes stored with signer ID, signature, nonce, verification status
Privilege separation	Complete	Dedicated fiber user, systemd hardening (NoNewPrivileges, ProtectSystem=strict)
Firewall	Complete	iptables default-deny, only SSH/MQTT-TLS/HTTPS/mDNS/DHCP allowed
OTA updates	Complete	RAUC A/B partitions, signed bundles, auto-rollback on failed boot
Data retention	Complete	3-year retention with FIFO auto-purge at 90% capacity
GDPR (code-level)	95%	Consent, right to erasure, processing records. Missing: DPIA document

Cryptographic algorithms

Algorithm	Usage
Ed25519	Command signing, certificate chain
AES-256-GCM	Pairing key encryption
HMAC-SHA256	Sensor reading integrity
SHA-256	Audit hash-chain
SQLCipher (AES-256)	Database encryption at rest
PBKDF2 (480k iterations)	Key derivation for pairing
bcrypt	Password hashing

Remaining gaps (documentation, not code)

ISO 14971 risk management, IEC 62304 software development plan, ISO 13485 QMS, clinical evaluation report, technical file, instructions for use (IFU), IEC 62366 usability evaluation, GDPR DPIA.

License

See LICENSE for details.