Hardware Security Module (HSM)

A Hardware Security Module (HSM) is a dedicated hardware device or chip that performs cryptographic operations and key management within a physically secure, tamper-resistant boundary. HSMs are designed so that cryptographic keys cannot be extracted - even if an attacker gains physical access to the device. All cryptographic operations using keys stored in the HSM are performed inside the module; the key material never leaves the HSM in plaintext. HSMs come in two main forms: network-attached HSMs (rack-mounted devices used in data centre environments for CA operations, code signing, and payment processing) and embedded HSMs (chips integrated into IoT devices, SoCs, or TPM modules). For CRA manufacturers, both types are relevant: network HSMs for the manufacturing and signing infrastructure, and embedded HSMs for device-level key protection.

Manufacturers subject to CRA requirements use network-attached HSMs in their product development and manufacturing infrastructure for:

• Firmware signing: The root key that signs firmware images for Secure Boot must be protected in an HSM. If this key is stolen, an attacker can create firmware that all devices will accept as legitimate. HSM storage ensures the key is never exposed even during signing operations.
• Certificate authority operations: Manufacturing-time certificate injection (provisioning device identity certificates) uses an HSM-backed CA to ensure certificate private keys are never exposed.
• Code signing for OTA updates: All OTA update packages must be signed; the signing key should be HSM-protected to prevent supply chain attacks via compromised build systems.
• Declaration of conformity: Some regulators accept HSM-backed digital signatures on electronic declarations of conformity as legally equivalent to physical signatures.

For the device itself, embedded HSM functionality is provided by several hardware technologies:

• TPM (Trusted Platform Module): A standardised chip (ISO/IEC 11889) providing secure key storage, attestation, and cryptographic operations. TPM 2.0 is widely available in x86 platforms and increasingly in ARM-based IoT hardware.
• Secure Element (SE): A tamper-resistant microcontroller used in smart cards, SIM cards, and IoT devices for secure key storage and cryptographic operations. Common in payment terminals and NFC-enabled devices.
• ARM TrustZone: A hardware security architecture in ARM processors that creates a 'secure world' isolated from the normal OS environment, where sensitive key material and cryptographic operations can be protected.
• SoC-integrated OTP fuses: One-time-programmable fuses in SoCs that store root keys immutably - once programmed during manufacturing, they cannot be changed or overwritten.

The presence or absence of HSM-equivalent hardware in a product design should be explicitly addressed in the product's threat model, as required by the CRA's Annex I risk assessment obligations. Threat models should consider:

• Physical attack scenarios: Can an attacker extract firmware signing keys by decapping the chip or probing debug interfaces? HSM/secure element protection mitigates this.
• Debug interface lockdown: JTAG/SWD debug interfaces that remain accessible in production firmware can bypass all software security controls. Secure Boot should be combined with debug port lockdown.
• Key injection security: The process by which production devices receive their unique keys during manufacturing is a critical security control. This process should be performed in a secure manufacturing environment with HSM-backed CA infrastructure.

Manufacturers should document their key management architecture in the Annex VII technical file as part of the security-relevant design documentation.