STM32 Custom Bootloader

A comprehensive, multi-stage secure bootloader system for STM32F4 microcontrollers designed for mission-critical applications requiring reliable and secure firmware updates. This bootloader provides advanced functionality including firmware encryption, incremental updates via delta patching, and failsafe recovery mechanisms.

The system implements a robust four-stage bootloader architecture:

• Boot (16KB): Primary bootloader that validates and hands off to Loader
• Loader (48KB): Interactive menu-based bootloader for normal boot or update selection
• Updater (64KB): Handles firmware updates via XMODEM with encryption and delta patching
• Application (384KB): Main application firmware

Updates can be delivered as full firmware images or as delta patches to minimize bandwidth usage and update time, reducing transmission size for minor updates.

Project Goal: Security features include firmware encryption (AES-128-GCM) and authentication via mbedTLS. The bootloader implements a secure boot chain that verifies each stage of the boot process using component-specific magic numbers, CRC32 checksums, and version checks, ensuring that only authenticated firmware can execute on the device.

Bootloader Flow

The bootloader consists of four components with a secure chain of trust and multiple paths for normal operation and updates:

Update Process

The update process involves multiple security steps:

The bootloader uses a carefully desided memory layout for its components:

Each firmware component includes a 512-byte header with the following structure:

typedef struct __attribute__((packed)) {
    uint32_t image_magic;
    uint16_t image_hdr_version;
    uint8_t  image_type;
    uint8_t  is_patch;
    uint8_t  version_major;
    uint8_t  version_minor;
    uint8_t  version_patch;
    uint8_t  _padding;
    uint32_t vector_addr;
    uint32_t crc;
    uint32_t data_size;
    uint8_t  reserved[0x1E0];
} ImageHeader_t;

Each bootloader component is identified by a unique magic number:

• Loader: 0xDEADC0DE
• Updater: 0xFEEDFACE
• Application: 0xC0FFEE00