Chain of Trust

The immutable foundation of the entire chain, physically embedded in silicon during manufacturing. This root is inherently trusted and cannot be altered by software. It stores the first cryptographic keys and initiates the secure boot process by verifying the firmware signature before execution. Without a genuine hardware root, all subsequent trust assumptions collapse.

Each component in the boot sequence computes a cryptographic hash of the next stage before passing control. These measurements are stored in Platform Configuration Registers (PCRs) within a TPM or vTPM. The process creates an unforgeable audit trail of the exact software state. Key aspects:

• Uses SHA-256 or stronger hashing algorithms
• Measurements are cumulative and irreversible
• Forms the basis for remote attestation claims

Trust flows in a single direction: from root to application. The ROM verifies the bootloader, which verifies the OS kernel, which verifies the application. Each link extends the chain only after successful validation. If any verification fails, the chain breaks and the system halts or enters a remediation state. This transitive property ensures that compromising one layer does not grant trust to subsequent layers.

The chain is structured in distinct, sequential layers:

• Layer 0: Hardware Root of Trust (ROM, eFuses)
• Layer 1: Platform Firmware (UEFI, BIOS)
• Layer 2: Bootloader (GRUB, shim)
• Layer 3: Operating System Kernel
• Layer 4: Application and Enclave Code Each layer authenticates the next using digital signatures verified against a certificate chain anchored to the hardware root.

A properly implemented chain of trust provides tamper evidence, not just tamper resistance. Any unauthorized modification to a measured component changes its hash, which is detectable during attestation. The system can respond by:

• Refusing to boot
• Revoking cryptographic keys
• Triggering alerts in a security operations center This ensures that even if an attacker gains physical access, the breach is detectable and contained.

A security standard that ensures a device boots using only software that is trusted by the Original Equipment Manufacturer (OEM). Each component of the boot sequence—from the firmware to the bootloader—verifies the digital signature of the next component against a database of authorized keys before execution. This establishes an unbroken chain of trust from the hardware root to the operating system kernel.

A process that cryptographically measures and logs each component of the boot chain into a Platform Configuration Register (PCR) within a Trusted Platform Module (TPM). Unlike secure boot, which halts on a mismatch, measured boot records the exact state of the system, allowing a remote party to later verify the boot log against the PCR values to detect any unauthorized modifications.

The set of all hardware, firmware, and software components that are critical to a system's security. A single vulnerability or misconfiguration in the TCB can compromise the entire chain of trust. The goal of confidential computing is to minimize the TCB by excluding the hypervisor and host OS, reducing the attack surface to only the CPU and the application itself.

A security property that allows a relying party to verify that the code running inside a TEE is exactly the code they expect. This is achieved by publishing a cryptographic hash of the trusted codebase and including this measurement in the attestation report. It extends the chain of trust to the application layer, ensuring that not only is the hardware genuine, but the software is unmodified.