Understanding Hardware Trust Anchors in Automotive Embedded Systems: SHEs, HSMs, TPMs. (2024)

Muhammet Kalaycı

7 min read

Mar 8, 2024

Two cornerstone technologies in cybersecurity domain are Hardware Security Modules (HSMs) and Secure Hardware Extensions (SHEs), and they are commonly confused with each other. So here we clarify the difference and dive into their story.

Hardware Trust Anchors provide a foundational layer of security by protecting sensitive data, like cryptographic materials, from software-level manipulations. They offer a variety of crypto functions, including ECDSA signature generation, which offloads these intensive tasks from the host controller, thus enhancing system performance and security. Various standardized feature sets for HTAs, including SHE, HSM, and Trusted Platform Module (TPM), are provided by different hardware suppliers, such as Infineon’s Aurix HSM/SHE+ driver, Renesas’ Intelligent Cryptographic Unit (ICU), Freescale’s Crypto Service Engine (CSE), and ARM’s TrustZone.

Understanding Hardware Trust Anchors in Automotive Embedded Systems: SHEs, HSMs, TPMs. (2)

In automotive contexts, HSMs are crucial for protecting the communication, ensuring the integrity and confidentiality of messages exchanged between vehicles and infrastructure. This level of security is paramount in preventing unauthorized access and manipulation, which could lead to dire consequences in critical systems. The deployment of HSMs in automotive systems signifies a commitment to the highest security standards, safeguarding against sophisticated attacks that target the vehicle’s embedded systems.

Developed under the EU-sponsored EVITA project by a consortium including Robert Bosch, BMW, and Infineon, HSMs are categorized into full, medium, and small profiles based on their capabilities and intended applications. The primary objective of HSMs in automotive systems is to harden Electronic Control Units (ECUs) against both software and selected hardware attacks while providing hardware acceleration for cryptographic functions. This facilitates secure ECU-to-ECU communication, protecting the transport of sensitive information.

“The internal structure of our three proposed EVITA HSM types is described in more detail in the proceeding subsections.”

HSMs and SHEs serve distinct roles within cybersecurity, with HSMs offering a broader range of cryptographic functions and SHEs providing specific secure hardware extensions for microcontrollers.

Understanding Hardware Trust Anchors in Automotive Embedded Systems: SHEs, HSMs, TPMs. (3)

Understanding Hardware Trust Anchors in Automotive Embedded Systems: SHEs, HSMs, TPMs. (4)

We have identified three different classes of HSMs: 
- the “EVITA HSM Full Version” as hardware extension to the ECU specifically responsible for V2X applications, 
- the “EVITA HSM Medium Version” as hardware extension to the ECU connected to the in-vehicle domain controls (e.g., power train control) and, 
- the “EVITA HSM Light Version” for security-critical sensors and actuators.

The AUTOSAR 4.3 security architecture integrates HSMs to provide cryptographic services through components like the Crypto Service Manager (CSM), Crypto Interface (CRYIF), and Crypto Driver (CRYDRV), supporting both hardware and software crypto drivers. This architecture facilitates secure communications and data storage, essential for implementing the ISO/IEC 15118 standard in electric vehicle charging systems. This standard relies on X.509 certificates for public key infrastructures, ensuring the secure exchange of data between electric vehicles and charging stations.

Secure Hardware Extention? Is that an HSM?

The Secure Hardware Extension (SHE) is an on-chip extension to any given microcontroller. It is intended to move the control over cryptographic keys from the software domain into the hardware domain and therefore protect those keys from software attacks. However, it is not meant to replace highly secure solutions like TPM chips or smart cards, i.e. no tamper resistance is required by the specification.
The main goals for the design at hand are • Protect cryptographic keys from software attacks
• Provide an authentic software environment
• Let the security only depend on the strength of the underlying algorithm and the confidentiality of the keys
• Allow for distributed key ownerships
• Keep the flexibility high and the costs low
Basically SHE consists of three building blocks, a storage area to keep the cryptographic keys and additional corresponding information, a implementation of a block cipher (AES) and a control logic connecting the parts to the CPU of the microcontroller, see Figure 4.1 for a simplified block diagram. SHE can be implemented in several ways, e.g. a finite state machine or a small, dedicated CPU core.

This is AUTOSAR’s definition of SHE. So SHE is pretty much like the Light HSM of EVITA.

Understanding Hardware Trust Anchors in Automotive Embedded Systems: SHEs, HSMs, TPMs. (5)

Here are four examples of Hardware Security Module (HSM) implementations in the automotive sector, highlighting the diverse applications and benefits of HSMs in modern vehicle architectures:

1. NXP Semiconductors’ Secure Vehicle Architecture:

NXP focuses on a multilayer automotive security architecture to protect connected vehicles from cyberattacks. This architecture emphasizes the importance of secure interfaces, secure gateways, secure networks, and secure processors, incorporating HSMs within their automotive microcontrollers. These HSMs support secure software updates and data protection, ensuring that electronic control units (ECUs) are safeguarded against unauthorized manipulation.

Understanding Hardware Trust Anchors in Automotive Embedded Systems: SHEs, HSMs, TPMs. (6)

2. Infineon Technologies’ AURIX Security Solutions:

The AURIX family of microcontrollers by Infineon integrates HSMs to offer advanced security features. Designed for automotive applications, AURIX MCUs provide robust security mechanisms, including encryption and secure boot capabilities. These features are vital for protecting the vehicle’s ECUs from potential cyber threats, making AURIX a popular choice for implementing HSMs in the automotive industry.

3. ETAS & Infineon’s ESCRYPT CycurHSM:

This automotive embedded security software stack utilizes the hardware security module on automotive MCUs to establish a security trust anchor. ESCRYPT CycurHSM ensures the secure boot of the ECU, secure in-vehicle communication, and protection of ECU components, thereby addressing complex OEM security requirements effectively.

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4. Renesas Electronics’ RH850 Automotive MCUs:

The RH850 family includes high-performance 32-bit MCUs tailored for automotive applications. These MCUs offer embedded security features, including HSMs, to meet the requirements for functional safety and secure communication. Renesas’ approach allows for secure data exchange and storage within the vehicle’s network, contributing significantly to the overall cybersecurity strategy of automotive systems.

In addition to the embedded HSMs within automotive microcontrollers, external HSMs also play a significant role in enhancing vehicle cybersecurity. These devices offer a separate, dedicated environment for executing cryptographic operations, thus providing an additional layer of security. Here’s an exploration of the importance and application of external HSMs in the automotive sector.

There are a few products in the market for such hardware:

Secure-IC’s Securyzr™ for Automotive:

Tailored for the Xilinx Zynq UltraScale+ MPSoC platform, it combines hardware and software solutions for a comprehensive security approach, ideal for ADAS, autonomous driving, and infotainment systems

Rambus RT-64x Embedded HSM Family (Root of Trust):

Offers ASIL-B and ASIL-D compliance, supporting secure boot, EVITA and SHE+ HSM functionalities, AutoSAR, and protection of proprietary algorithms. It’s suited for automotive applications requiring robust security measures and supports multiple root of trust capabilities.

Thales Hardware Security Modules:

Known for securing cryptographic keys and provisioning encryption, decryption, authentication, and digital signing services for a wide range of applications. Thales HSMs provide high-level security by storing cryptographic keys in a tamper-resistant device, designed for easy integration and management through the Thales Crypto Command Center.

Integration: Internal HSMs are directly integrated within the MCU, offering security features without the need for external components. External HSMs, while separate, provide flexibility in updates and replacements and can enhance the security of systems that might not have integrated HSM capabilities.
Security Level: Both internal and external HSMs aim to provide high security for cryptographic operations. External HSMs can offer more specialized security features and greater scalability across multiple systems.
Flexibility and Scalability: External HSMs provide a higher degree of flexibility and scalability compared to internal HSMs. They can be updated or replaced independently of the automotive MCU, allowing for rapid adaptation to emerging security threats.
Performance: Internal HSMs offer the advantage of being optimized for the specific MCU architecture, potentially offering better performance for cryptographic operations directly within the automotive system. External HSMs, on the other hand, can offload these operations from the main automotive computing resources, preserving performance.

The integration of HSM technology in automotive MCUs by leading companies like NXP, Infineon, ETAS, and Renesas underscores the critical role of cybersecurity in the current automotive landscape. Each implementation, whether it be NXP’s comprehensive security architecture, Infineon’s AURIX MCUs, ETAS & Infineon’s ESCRYPT CycurHSM software, or Renesas’ RH850 MCUs, offers unique advantages in securing automotive systems against the growing threat of cyberattacks. These HSM implementations not only provide robust security features but also support the automotive industry’s goal of creating safer, more secure vehicles for the future. As the automotive sector continues to evolve, with vehicles becoming more connected and autonomous, the role of HSMs in automotive cybersecurity will undoubtedly expand, further emphasizing the importance of continuous innovation and collaboration among technology providers.