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Secure Bootloader

A platform-independent secure bootloader for embedded systems that delivers AES-CBC-encrypted, CRC-32-verified firmware over UART — without a full OTA stack.

flowchart LR
    PC[Host PC] -->|UART| BL[Bootloader]
    BL -->|AES-CBC decrypt\nCRC-32 verify| FL[(Flash)]
    FL --> APP[Application]

    classDef host  fill:#3f51b5,stroke:#1a237e,color:#fff
    classDef bl    fill:#455a64,stroke:#263238,color:#fff
    classDef flash fill:#009688,stroke:#004d40,color:#fff
    classDef app   fill:#e65100,stroke:#bf360c,color:#fff

    class PC host
    class BL bl
    class FL flash
    class APP app

Tool Ecosystem

This bootloader is one part of a three-tool firmware-update chain:

Tool Role
🔐 EncryptBIN Generates the AES-128-CBC encrypted firmware package on the PC
📡 SecureLoader Transfers the encrypted package to the device over serial (CLI + GUI)
🛡️ SECURE_BOOTLOADER (this project) Bootloader on the device — decrypts, verifies, and flashes the firmware 
flowchart LR
    A[Factory / Initial provisioning] --> SB[SecureBootloader flashing] --> D[(Device)]

    A2[EncryptBIN] -->|produces| B([encrypted .bin])
    B -->|input| C[SecureLoader]
    C -->|flashes| D

    classDef tool fill:#3f51b5,stroke:#1a237e,color:#fff,stroke-width:2px
    classDef file fill:#009688,stroke:#004d40,color:#fff
    classDef device fill:#455a64,stroke:#263238,color:#fff

    class A,A2,C tool
    class B file
    class D,SB device

Features

  • Encrypted transport — AES-128-CBC with a per-image IV embedded in the firmware header
  • Integrity verification — CRC-32/IEEE 802.3 computed across the complete firmware image
  • Atomic flash write — the application reset vector is written last, only after the full-image CRC passes; a partial transfer can never boot
  • Push-button stay-alive — a physical button extends the bootloader window without host involvement
  • Configurable timeouts — independent start, communication, and button-hold timeouts
  • Hardware CRC option — switchable between a software CRC implementation and the STM32 CRC peripheral at CMake configuration time
  • Portable architecture — all hardware access is behind a thin driver interface; porting to a new MCU requires only that interface
  • Full unit-test coverage — 44 tests (Unity + CMock) exercise every protocol state and edge case without hardware

Supported Targets

Target MCU Core Flash SRAM Notes
STM32G070RB STM32G070RBTx Cortex-M0+ 128 KB 36 KB NUCLEO-G070RB
STM32G071RB STM32G071RBTx Cortex-M0+ 128 KB 36 KB NUCLEO-G071RB
STM32G474RE STM32G474RETx Cortex-M4F 512 KB 128 KB NUCLEO-G474RE
ATMEGA328P ATmega328P AVR8 32 KB 2 KB Arduino Uno R3
STM32_TEMPLATE any STM32 any — — Porting starting point

The bootloader itself occupies the first 4 KB of flash on all STM32 targets. The application region begins immediately after.

Quick Navigation

  • Getting Started
    Build, flash and run in under ten minutes.

  • Architecture
    Block diagrams, boot flow and component responsibilities.

  • Wire Protocol
    Command reference, packet format and state machine.

  • Security Model
    AES-CBC, CRC-32 and the atomic flash-write guarantee.

  • Porting Guide
    Add a new MCU target in six steps.

  • Build System
    CMake options, key injection, device ID.

  • Testing
    Unit test suite, coverage, and how to run it.

  • EncryptBIN
    Produce the encrypted .bin package from your application binary.

  • SecureLoader
    Upload the encrypted package to the target over serial.

Repository Layout

SECURE_BOOTLOADER/
├── src/                    # Platform-independent bootloader core
│   ├── main_app.c          #   Main loop, protocol FSM, flash write logic
│   ├── main_app.h          #   Public API (single entry point)
│   ├── main_app_priv.h     #   Internal protocol types (tests & core only)
│   └── CRC/
│       ├── crc_api.h       #   CRC-32 interface (SW and HW share this)
│       └── crc.c           #   Software CRC-32/IEEE 802.3 implementation
├── lib/
│   └── tiny-AES-c/         # AES-128-CBC library (third-party)
├── hw/
│   ├── STM32G070RB/        # STM32G070RB target
│   ├── STM32G071RB/        # STM32G071RB target
│   ├── STM32G474RE/        # STM32G474RE target
│   ├── ATMEGA328P_ARDUINO_UNO_R3/  # ATmega328P target
│   ├── STM32_TEMPLATE/     # Porting template for new STM32 targets
│   ├── cmake/              # Shared CMake utilities (key parsing)
│   └── config/             # Toolchain files (ARM GCC, AVR GCC)
├── test/
│   ├── MAIN_APP/           # Unit tests for the bootloader core
│   ├── CRC/                # Unit tests for the CRC module
│   ├── unity/              # Unity test framework
│   ├── cmock/              # CMock framework
│   └── __embedded_target_manager/  # Python test orchestration
└── docs/                   # This documentation