ARM Cortex-M IP Protection

TL;DR

Protecting your ARM Cortex-M project from copying (especially against Chinese knockoffs) requires a layered approach. This guide covers hardware security features, code obfuscation, secure boot, and supply chain considerations.

1. Hardware Security Features

ARM Cortex-M microcontrollers often have built-in security features. Use them!

• TrustZone: If your MCU supports it, TrustZone creates a secure world separate from the normal operating environment. Critical code and data reside in this secure world.
• Memory Protection Unit (MPU): Configure the MPU to restrict access to sensitive memory regions. This prevents rogue code from reading or writing important data. Example configuration (pseudo-code):

  
  // Define memory region attributes
  RGN_ATTRIBUTES rgn_attributes = {
    .start = 0x20000000,
    .size = 0x10000,
    .access = MPU_ACCESS_READ_WRITE | MPU_ACCESS_EXECUTE
  };
  
  // Enable the region
  MPU->RNR = rgn_attributes;
  

• Hardware Crypto: Use hardware AES, SHA, and random number generators for encryption, hashing, and secure key generation. Avoid software implementations where possible as they are slower and more vulnerable.
• Unique ID/Device Serial Number: Many MCUs have a unique identifier burned into them during manufacturing. Use this to tie your code to specific devices (see section 4).

2. Code Obfuscation

Make it harder for attackers to reverse engineer your code.

• Control Flow Flattening: Randomize the order of instructions, making static analysis more difficult.
• String Encryption: Encrypt sensitive strings in flash memory and decrypt them at runtime.
• Opaque Predicates: Insert conditional branches that always evaluate to the same result but are hard for an attacker to determine statically.
• Virtualization: Transform your code into a virtual machine instruction set, making it harder to understand directly.

Note: Obfuscation is not foolproof; determined attackers can still reverse engineer code. It raises the bar but isn’t a complete solution.

3. Secure Boot

Ensure only authorized firmware runs on your device.

• Root of Trust: Establish a secure starting point for boot (e.g., using a hardware one-time programmable memory).
• Digital Signatures: Cryptographically sign your firmware images. The bootloader verifies the signature before loading the code.

  
  // Example: Verify digital signature (simplified)
  if (verify_signature(firmware_image, signature, public_key)) {
    load_firmware(firmware_image);
  } else {
    // Handle invalid signature - halt or enter recovery mode
  }
  

• Secure Firmware Updates: Implement a secure over-the-air (OTA) update mechanism with authentication and encryption.

4. Supply Chain Security

This is crucial for dealing with knockoffs.

• Trusted Manufacturers: Work with reputable manufacturers and distributors.
• Component Authentication: Verify the authenticity of key components (e.g., MCUs) upon receipt.
• Unique Device Binding: Tie your software license or functionality to the unique ID of each device.

  
  // Example: Check Unique ID
  if (device_id == authorized_device_id) {
    enable_features();
  } else {
    disable_features();
  }
  

• Tamper Evidence: Consider using tamper-evident packaging or hardware features to detect physical tampering.

5. Code Confidentiality & Licensing

• Strong NDAs: Use Non-Disclosure Agreements with all parties involved in development and manufacturing.
• Licensing Restrictions: Clearly define the terms of your software license, prohibiting reverse engineering or unauthorized copying.