Today’s automotive industry is completely dependent on microprocessors for controlling every operation in cars. The microprocessor controls the main user interface which then sends commands to small, connected microcontrollers to control different peripherals known as ECU. Therefore, the integrity and security of car firmware hold great importance for automotive manufacturers.
According to Motor Trend, a typical new car has over 150 million lines of code and 25GB an hour of data flowing through its systems. If someone notoriously makes small changes in this car firmware, it can cause abnormal car behaviors as well as put users’ critical data on risk. Therefore, it is essential to ensure that only trusted firmware built by automobile manufacturers can run on automobiles.
Secure Boot is a technique that allows only authenticated code to be executed on automotive microprocessors during the bootup. Secure Boot starts its action when someone powers on the automobile and ensures that only trusted codes execute on car’s hardware till the complete screen is loaded. In this way, it prevents critical cyberattacks like malware injection, boot kits, and rootkits in automotive systems.
Secure Boot – Implementation in Automotive ECUs
Modern vehicles incorporate dozens of electronic control units (ECUs) to provide essential functionality for safety, comfort, and efficiency. These ECUs rely on software to control critical vehicle systems. However, the growing number of adversarial attacks against automotive systems makes secure boot mechanisms essential to preserving the reliability and integrity of these embedded systems.
One of the most fundamental security measures for embedded systems is secure boot, which ensures that only authenticated and untampered software is executed during the boot process. By utilizing cryptographic algorithms and hardware mechanisms, secure boot safeguards the integrity of the software loaded onto an ECU. Despite its importance, secure boot implementations often face challenges in achieving their intended security goals, particularly in automotive systems.
This article aims to explore the critical components of secure boots, common weaknesses in their implementations within automotive ECUs, and recommendations for improving their security in future vehicle generations.
Secure Boot: Definition and Root of Trust
Secure boot is designed to verify the authenticity and integrity of software components during the boot process. When an unauthorized modification is detected, secure boot triggers countermeasures such as halting the boot process or restricting access to cryptographic secrets. Unlike “authentic boot,” which primarily logs the system’s state for third-party verification, secure boot takes immediate action to prevent unverified software from executing.
A secure boot process relies heavily on a Root of Trust (RoT)—a combination of hardware and software components that provide the foundation for trust in the boot sequence. In automotive systems, the RoT is often referred to as a trust anchor. It typically includes:
- Root of Trust for Measurement (RTM): This performs software integrity checks by measuring and verifying the authenticity of the software components.
- Root of Trust for Storage (RTS): Provides a secure, tamper-resistant storage mechanism to safeguard cryptographic keys or reference values for verifying software integrity.
- Root of Trust for Reporting (RTR): Reports the state of the system, allowing the execution of active countermeasures if unauthorized modifications are detected.
In some cases, the RoT is initiated by an immutable portion of code known as the Core Root of Trust for Measurement (CRTM), which provides the first step in establishing a chain of trust. Each subsequent boot stage verifies the integrity and authenticity of the next stage, creating a “chain of trust” from the initial hardware boot to the operating system or application layer.
Software Integrity Schemes
Several cryptographic schemes are used in secure boot to ensure software integrity:
- Signature-Based Authentication: Utilizes asymmetric encryption (e.g., RSA, DSA, ECC) with a private-public key pair to generate and verify software signatures.
- Hash-Based Authentication: Relies on one-way hash algorithms (e.g., SHA2, SHA3) to generate a hash value of the software. This value is stored on the ECU, and during the boot process, the hash of the software is compared against this reference.
- Message Authentication Code (MAC)-Based Authentication: A symmetric-key approach where both the authentication and verification steps use the same secret key. The MAC value of the software is generated and verified using algorithms like HMAC or CMAC.
For each of these schemes, the secure boot process depends on securely storing cryptographic keys and reference values in a tamper-resistant memory, such as One-Time Programmable Memory (OTP) or hardware modules.
Execution Modes and Hardware Platforms
Secure boot can be executed in various modes based on the ECU’s performance requirements:
- Sequential Mode: Authenticates each boot stage sequentially before handing over control to the next.
- Concurrent Mode: Authenticates the next boot stage while the current one is executing, requiring at least two independent processing units.
- Parallel Mode: Allows the next boot stage to execute before the authentication process completes, which is typically used for authentic boot rather than secure boot.
To ensure the secure boot process remains tamper-resistant, it requires specific hardware support:
- One-Time Programmable Memory (OTP): Immutable memory that stores cryptographic keys or reference values, preventing unauthorized modifications.
- Hardware Security Module (HSM): A dedicated, shielded execution environment that includes secure storage for cryptographic secrets. An HSM can implement all three software integrity schemes (signature-based, hash-based, and MAC-based) and protect sensitive operations like key management.
- Secure Hardware Extension (SHE): A standardized module used in automotive systems, often providing CMAC-based authentication. SHE ensures the confidentiality and integrity of symmetric keys and can disable access to keys if authentication fails.
Common Weaknesses in Secure Boot Implementations
Despite the security guarantees provided by secure boot, vulnerabilities in its implementation can still be exploited. Penetration testing and research have revealed several common weaknesses in automotive ECUs:
- Insufficient Cryptographic Strength: Using deprecated or weak cryptographic algorithms, short key lengths, or reusing symmetric keys across multiple ECUs can compromise secure boot.
- Failure to Prevent Code Execution: Some implementations allow code to execute even if it has failed authentication or has not been checked. This opens the door for attacks that bypass secure boot entirely.
- Manipulation of the Secure Boot Mechanism: Attackers can manipulate the secure boot process by exploiting diagnostic procedures, modifying unauthenticated configuration parameters, or executing code before secure boot is fully initialized.
- Roll-Back Attacks: Attackers can revert the system to an earlier, less secure version of the software, bypassing modern security protections.
- Error Handling Abuse: Exploiting error handling mechanisms, such as loading an unsecured boot image in case of failure, can allow attackers to bypass secure boot.
While secure boot is a critical security mechanism for automotive ECUs, its effectiveness depends on rigorous implementation and hardware support. By ensuring the correct use of cryptographic algorithms, properly handling key management, and preventing code execution for unauthenticated software, automakers can significantly improve the security of future vehicle generations.
Additionally, addressing challenges like hardware vulnerabilities, secure storage, and robust error handling will further strengthen secure boot implementations against evolving threats.
Why Choose EPTeck for Automotive Secure Boot Development?
Expert Team: Our engineers specialize in secure boot implementation for automotive ECUs, ensuring your systems are protected against modern cyber threats.
Tailored Solutions: We provide customized secure boot solutions to meet the specific needs of your vehicle’s hardware and software architecture.
Comprehensive Hardware Support: Expertise in implementing secure boot across multiple automotive platforms, including PowerPC, ARM, and custom ECUs.
Strengthen Your Vehicle’s Security: Contact us today to enhance your vehicle’s firmware integrity with a secure boot solution tailored to your needs.