Secure Boot

Every boot component—from UEFI firmware to OS bootloader—must be signed with a private key. The firmware verifies each signature against a database of authorized public keys before execution.

• Uses RSA-2048 or ECDSA asymmetric cryptography
• Signature database stored in db (authorized) and dbx (revoked) variables
• Prevents bootkits and rootkits from injecting malicious code before the OS loads

The Platform Key sits at the root of the Secure Boot trust hierarchy. The platform owner uses it to sign the Key Exchange Key (KEK), which in turn signs the authorized signature databases.

• PK is typically enrolled during manufacturing or by the device owner
• KEK separates OS vendor keys from platform owner control
• Enables multi-stakeholder trust without sharing root secrets

Secure Boot anchors its trust chain in an immutable hardware root of trust, typically a Trusted Platform Module (TPM) or on-die ROM. This ensures the initial firmware cannot be tampered with.

• First-stage bootloader stored in write-protected ROM
• TPM Platform Configuration Registers (PCRs) record boot measurements
• Enables remote attestation of the boot state to external verifiers

The dbx (forbidden signatures database) allows immediate revocation of compromised bootloaders or drivers. When a critical vulnerability like BlackLotus or BootHole is discovered, updated dbx entries prevent exploitation.

• Revoked hashes and certificates block known-malicious binaries
• Timestamp-based revocation prevents rollback to vulnerable versions
• UEFI Capsule Updates deliver revocation lists securely

While Secure Boot enforces execution policy, Measured Boot records cryptographic hashes of every loaded component into TPM PCRs. Together they provide both enforcement and auditability.

• Each boot stage extends a PCR with its hash before executing the next
• Logs stored in the Event Log for post-boot verification
• Enables trusted computing attestation workflows for enterprise device compliance

A physically immutable, inherently trusted source within a computing platform that serves as the foundation for all subsequent security operations. It is the absolute anchor for Secure Boot, providing the first unalterable code executed at power-on.

• Typically embedded in ROM or one-time programmable fuses
• Stores the root key used to verify the initial bootloader's digital signature
• Any compromise of the root of trust invalidates the entire chain of trust

A process that cryptographically measures and logs each component of the boot chain into a Platform Configuration Register (PCR) within a TPM. Unlike Secure Boot, which enforces policy by halting on failure, Measured Boot records the exact state for later remote verification.

• Stores SHA-256 hashes of firmware, bootloader, and OS kernel
• Enables Remote Attestation to prove boot integrity to a third party
• Does not stop an untrusted component from loading; it records the fact that it did

The process by which a Trusted Execution Environment generates a cryptographically signed report proving its identity, integrity, and that it is running specific code on genuine hardware. Secure Boot establishes the clean state that attestation then verifies to a remote relying party.

• Binds the platform's identity to a specific software hash
• Prevents replay attacks with fresh nonces from the verifier
• Forms the basis for Secure Provisioning of secrets into verified enclaves

A dedicated microcontroller designed to secure hardware through integrated cryptographic keys. The TPM stores the authorized signature database that Secure Boot consults during verification and provides the PCRs used by Measured Boot.

• Compliant with ISO/IEC 11889 standard
• Securely stores platform measurements in shielded locations
• Performs key generation, encryption, and digital signing operations

A hierarchical sequence of validation where each component in a system is authenticated by the preceding component, beginning with the immutable Hardware Root of Trust and extending through firmware, bootloader, OS kernel, and ultimately to the application layer.

• Secure Boot enforces the first critical links in this chain
• A break at any single link compromises all subsequent layers
• Extends into user space with technologies like Code Transparency

The set of all hardware, firmware, and software components that are critical to a system's security. A vulnerability in any TCB element can compromise the entire system. Secure Boot's role is to minimize and verify the early boot TCB.

• Includes the CPU, firmware, bootloader, and hypervisor
• A smaller TCB reduces the attack surface for Side-Channel Attacks
• Confidential computing aims to shrink the TCB by removing the host OS