Chain of Trust

A chain of trust is a sequence of components where each component verifies the next one before passing control to it. Trust in the final component follows from trust in the first.

The first component is the root of trust. It is trusted without verification, so it must be immutable, typically a boot ROM or a hardware module.

Boot Chain

Root of trust (boot ROM / TPM)
        │ verifies
        ▼
Firmware (UEFI)
        │ verifies
        ▼
Bootloader (GRUB)
        │ verifies
        ▼
Kernel and initramfs
        │ verifies
        ▼
Kernel modules and init
        │ verifies
        ▼
Applications and data

At each link:

1. The current component computes a hash of the next one.
2. The hash is compared against a known-good value or a digital signature.
3. Control is passed only if the check succeeds.

Properties

• Transitive
  Each link is trusted only because the previous link is trusted.
• Broken at the weakest link
  A single unverified component lets an attacker compromise everything after it.
• Ordered
  A component must be verified before it executes. Code that runs first can lie about everything after it.

A rootkit that infects a bootloader or the kernel breaks the chain, since the components below it can no longer be trusted.

Verification Modes

• Secure boot
  Each stage checks the next stage’s signature against keys held by the firmware. A stage that fails the check is not executed.
• Measured boot
  Each stage records a hash of the next stage in tamper-resistant storage before running it. Nothing is blocked. A separate party later decides whether the recorded values are acceptable. See TPM.