Hardware Root of Trust (HRoT)

A Hardware Root of Trust is a physically isolated, tamper-resistant hardware component that serves as the foundational security anchor for an entire computing system. It provides an immutable starting point for cryptographic verification — the one element in the system that must be unconditionally trusted because its integrity is guaranteed by physical design, not software.

Why “Root”?

In cryptographic security, every verification depends on something else being verified first. This chain must start somewhere — at a “root” that cannot itself be compromised. A Hardware Root of Trust provides this root by storing initial verification keys in physically unextractable hardware (OTP fuses, ROM, or dedicated secure silicon).

Without a hardware root, any security built in software can be undermined: an attacker who controls boot firmware can defeat any encryption, authentication, or access control that loads after it.

How It Works

The Chain of Trust

┌──────────────────────────────────────────────────────────────┐
│  HARDWARE ROOT OF TRUST (Immutable — ROM/OTP/Secure Element) │
│  Contains: Root public key, boot verification code           │
└──────────────────┬───────────────────────────────────────────┘
                   │ Verifies signature of ↓
┌──────────────────▼───────────────────────────────────────────┐
│  STAGE 1 BOOTLOADER (First mutable code)                     │
│  Verified by: HRoT    Verifies: Stage 2 bootloader           │
└──────────────────┬───────────────────────────────────────────┘
                   │ Verifies signature of ↓
┌──────────────────▼───────────────────────────────────────────┐
│  STAGE 2 BOOTLOADER / OS KERNEL                              │
│  Verified by: Stage 1    Verifies: Application firmware       │
└──────────────────┬───────────────────────────────────────────┘
                   │ Verifies signature of ↓
┌──────────────────▼───────────────────────────────────────────┐
│  APPLICATION FIRMWARE                                         │
│  Verified by: Stage 2    Executes: Normal device operation    │
└──────────────────────────────────────────────────────────────┘

Every stage is cryptographically verified by the stage before it. If any stage fails verification, the boot process halts — preventing compromised firmware from executing.

Hardware Root of Trust Technologies

Technology	Implementation	Certification	Used In
Secure Elements	Dedicated tamper-resistant IC (STSAFE-A110, OPTIGA Trust M, SE050)	CC EAL5+/EAL6+	IoT devices, industrial controllers
TPM (Trusted Platform Module)	Discrete chip or firmware-based (fTPM)	TCG 2.0, FIPS 140-3	Servers, PCs, automotive
ARM TrustZone	CPU-level isolation (Secure World / Normal World)	PSA Certified L1-L3	MCUs (STM32, NXP i.MX)
OTP Fuses	One-time programmable memory in SoC	Varies by SoC vendor	All silicon — key hash storage
ROM Boot Code	Immutable code in mask ROM	Factory-verified	First-stage boot verification
DICE (Device Identifier Composition Engine)	TCG standard for layered device identity	TCG DICE	Constrained IoT, automotive

Why CRA Mandates Hardware Root of Trust

The EU Cyber Resilience Act requires products with digital elements to implement:

Secure boot — which is only secure if anchored in hardware.
Tamper-resistant key storage — software-only key storage fails this requirement.
Unforgeable device identity — requires hardware attestation, not clonable software IDs.
Authenticated firmware updates — signing keys must be stored in tamper-resistant hardware.

The regulatory bottom line: If your MCU doesn’t have a physical secure enclave or Secure Element, you cannot claim CRA compliance by adding software. The silicon must change.

Software Root of Trust vs. Hardware Root of Trust

Aspect	Software Root of Trust	Hardware Root of Trust
Key storage	Flash memory (readable with JTAG/SWD)	Tamper-resistant silicon (physically unextractable)
Boot verification	Can be bypassed by overwriting flash	Immutable ROM code — cannot be modified
Attack resistance	Vulnerable to firmware dumps, glitching	Designed to resist physical attacks (mesh shields, sensors)
Cost	Free (software only)	€0.50–€3.00 per unit (Secure Element)
CRA compliance	❌ Does not satisfy “tamper-resistant” requirement	✅ Satisfies hardware security requirements
Certification	Cannot be certified at high assurance levels	CC EAL5+, EAL6+, FIPS 140-3

Secure Boot — The process that uses the Hardware Root of Trust to verify firmware integrity at every boot stage.
Secure Element — A discrete tamper-resistant chip commonly used as the Hardware Root of Trust.
HSM — Hardware Security Modules that provide Root of Trust at server/infrastructure level.
CRA — The EU regulation that mandates Hardware Root of Trust for connected products.

Hardware Root of Trust