#KEYMASTER: Understanding Secure Boot: How Devices Establish Trust from Power-On

In today’s connected world, devices—from laptops and cars to medical equipment and IoT sensors—must defend themselves against increasingly sophisticated cyber threats. One of the most critical protections enabling this security is Secure Boot.

In this Keymaster episode, host Sven Rajala speaks with Jérôme Ducros, Solution Engineer at Keyfactor, to explain what Secure Boot is, why it matters, and how trust is established every time a device powers on.

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What Is Secure Boot?

Secure Boot is a security mechanism that ensures a device only runs trusted firmware during startup. Before executing any code, the device verifies that the software hasn’t been modified or replaced with malicious content.

Rather than assuming trust, Secure Boot builds trust step by step.

A helpful analogy is entering a secure office building:

• You pass through an outer gate,
• Then a building entrance,
• Then a floor access checkpoint,
• And finally restricted rooms.

Each checkpoint validates access before allowing you to proceed. Secure Boot works the same way—every stage verifies the next before execution continues.

This process protects devices across industries, including:

• IoT systems
• Automotive platforms
• Industrial equipment
• Medical devices
• Consumer electronics

Any system that executes firmware during startup benefits from Secure Boot.

The Root of Trust: Where Security Begins

At the heart of Secure Boot lies the Root of Trust—the foundational element that the device inherently trusts.

Because software can be modified, the root of trust is typically hardware-based, making it far harder to tamper with.

It usually consists of two main components:

1. Boot ROM Code

A small piece of immutable code embedded directly into the chip’s memory during manufacturing. This code cannot be altered after production.

2. Hardware Fuses

Fuses act as permanent, one-time programmable memory locations used to store secrets such as cryptographic keys. Once programmed, they cannot be changed—similar to a one-way switch.

Together, these elements form the device’s trusted starting point.

What Happens When a Device Powers On?

When a device starts, Secure Boot follows a precise sequence:

1. Reset occurs — the device begins execution from the first trusted instruction in Boot ROM.
2. Boot ROM runs — trusted hardware code initiates verification.
3. Next stage verification — the Boot ROM checks the digital signature of the bootloader stored in flash memory.
4. Chain of trust continues — each verified component validates the next:
   a. Bootloader → Operating system
   b. OS → Drivers and applications
   c. Applications → Additional components

Each step performs cryptographic verification before execution. If any verification fails, trust is broken. The device typically stops booting and enters a recovery or safe mode to prevent compromise.

The Role of Keys, Signatures, and Certificates

Secure Boot relies heavily on cryptography, specifically public-key infrastructure (PKI).

Use of Key Pairs

• Private key: Used to sign firmware.
• Public key: Used by the device to verify signatures.

But managing many individual keys can become complex. This is where certificates play a crucial role.

Why Certificates Matter

Certificates allow devices to trust authorized signers rather than individual keys. A device stores a trusted root certificate and validates a certificate chain back to that root.

Benefits include:

• Scalability across many devices
• Easier key management
• Flexibility in software signing
• Long-term maintainability

Certificates transform Secure Boot from a rigid mechanism into a manageable security system.

Silicon Vendors vs. Device Manufacturers

Secure Boot is a shared responsibility between hardware vendors and device manufacturers.

Semiconductor Vendors Provide:

• Boot ROM implementation
• Secure hardware features
• Fuse-based key storage
• Cryptographic acceleration engines

Device Manufacturers (OEMs) Define:

• Which keys are trusted
• The certificate hierarchy
• Firmware signing policies
• Overall trust model

Hardware enables security, but OEMs decide what is trusted.

Why Secure Boot Matters Today

As devices become increasingly connected, attackers often target the earliest stages of system startup. If malicious firmware loads before security controls activate, the entire system can be compromised.

Secure Boot prevents this by ensuring:

• Only authenticated code runs
• Firmware integrity is preserved
• Unauthorized modifications are detected immediately

In short, it protects devices before they even fully turn on.

Key Takeaways

• Secure Boot verifies firmware before execution, preventing malicious code from running at startup.
• Trust is built progressively through a chain of cryptographic verification.
• The Root of Trust is hardware-based and immutable, forming the security foundation.
• Boot ROM and hardware fuses anchor device identity and trust.
• Certificates make Secure Boot scalable and manageable across large device fleets.
• Semiconductor vendors provide secure hardware capabilities, while OEMs define trust policies.
• If any verification step fails, the device halts or enters recovery mode to maintain security.

Learn more

• #KEYMASTER: Trusted Device Bootstrapping: The Foundation of Product Security
• #KEYMASTER: Understanding the Cascading Impact of the EU Cyber Resilience Act (CRA)
• Use Case: Secure boot and OTA updates