Embedded System Security: Certificates

TL;DR

This guide explains how to use authentication certificates in an embedded system (microcontroller) to secure communication and device identity. We’ll cover generating a certificate, storing it securely on the microcontroller, and using it for verification.

1. Understanding Certificates & Why Use Them?

Certificates are digital documents that prove the identity of your embedded system or another communicating party. They’re essential for:

• Secure Communication: Ensuring data exchanged with other devices is genuine and hasn’t been tampered with (e.g., using TLS/SSL).
• Device Authentication: Verifying that the device connecting to your network or cloud service is actually who it claims to be.
• Preventing Man-in-the-Middle Attacks: Protecting against attackers intercepting and modifying communication.

Certificates rely on a Certificate Authority (CA), which issues the certificates after verifying identity.

2. Generating Your Certificate

You’ll need to create a certificate for your device. This can be done using tools like OpenSSL or cloud-based services.

• Create a Private Key: This is kept secret on the microcontroller.
• Generate a Certificate Signing Request (CSR): Contains information about your device.
• Get the Certificate Signed by a CA: Submit the CSR to a trusted CA (or use a self-signed certificate for testing – not recommended for production).

Example using OpenSSL:

openssl req -x509 -newkey rsa:2048 -keyout device.key -out device.crt -days 365

This creates a private key (device.key) and a self-signed certificate (device.crt) valid for 365 days.

3. Securely Storing the Certificate on the Microcontroller

Storing the certificate securely is crucial. Here are some options:

• Hardware Security Module (HSM): The most secure option; dedicated hardware for key storage and cryptographic operations.
• Secure Element: Similar to an HSM, often used in IoT devices.
• Protected Flash Memory: Use the microcontroller’s built-in flash memory with read/write protection. This is a common approach.
• Encryption at Rest: Encrypt the certificate data before storing it in flash memory using a key derived from a secure source (e.g., a unique device ID).

Important Considerations:

• Never store the private key in plain text!
• Protect access to the flash memory containing the certificate.
• Consider using tamper detection mechanisms.

4. Implementing Certificate Verification

Your microcontroller code needs to verify certificates received from other devices (or use its own certificate for authentication).

• Load the Trusted CA Certificates: Store a list of trusted CAs on your device.
• Verify the Certificate Chain: Check if the received certificate is signed by a trusted CA.
• Check Expiration Date: Ensure the certificate hasn’t expired.
• Validate Hostname/Device ID: Confirm that the certificate matches the expected identity of the communicating party.

Example (simplified) using a hypothetical library:

bool verify_certificate(const char *received_cert, const char *trusted_ca_list) {
  if (check_chain_of_trust(received_cert, trusted_ca_list)) {
    if (is_expired(received_cert)) {
      return false;
    }
    if (validate_hostname(received_cert, expected_hostname)) {
      return true;
    } else {
      return false;
    }
  } else {
    return false;
  }
}

5. Using the Certificate for Communication

Once verified, use the certificate to establish a secure connection (e.g., TLS/SSL).

• Configure your communication stack: Enable TLS/SSL and specify the trusted CA certificates.
• Present your device’s certificate: During handshake, provide your device’s certificate for authentication.
• Handle errors gracefully: Implement robust error handling to deal with invalid or expired certificates.

Most embedded systems use a TLS/SSL library (e.g., wolfSSL, mbedTLS) which provides functions for managing certificates and establishing secure connections.

6. Regular Updates

Certificates expire! Implement a mechanism to update the certificate on your device periodically:

• Over-the-Air (OTA) Updates: Download new certificates securely from a trusted server.
• Secure Bootloader: Verify the integrity of the updated certificate during boot.