Understand UEFI Secure Boot and Chain of Trust

Topic Overview

UEFI Secure Boot and trust

Secure Boot uses firmware-stored keys to verify each boot stage before execution. The platform key (PK), key-exchange keys (KEK), and signature databases (db/dbx) form a chain of trust from power-on to the OS loader. For example, a shim bootloader signed by a CA in db can load a Linux kernel only if the kernel's signing certificate is also trusted.

Chain of trust stages

1. Firmware: Verifies its own integrity (often via hardware roots like TPM)
2. EFI application: Boot loader .efi must carry a valid Authenticode signature
3. Kernel / initrd: Optional enforcement via kernel lockdown and signed modules
4. Revocation: dbx blocks known-bad hashes and certificates

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• Microsoft KEK is preinstalled on most retail PCs; Linux uses shim + MOK
• Disabling Secure Boot is a firmware setup option, not a runtime API
• Measured boot (TPM PCRs) extends trust into the running OS

Why for this exercise

You will document how Secure Boot keys and signatures gate each loader stage. This exercise requires explaining PK, KEK, db, and dbx and how a signed .efi binary is accepted or rejected at boot.

Your Task

UEFI Secure Boot checks every .efi image against two databases before running it: db, the trusted signers, each with an expiry date, and dbx, revoked hashes. Apply its rules in this order:

1. The hash is in dbx: rejected as revoked, even if the same hash is also in db. Revocation always wins.
2. The hash is not in db: rejected as untrusted.
3. The image date is after that db entry's expiry: rejected as expired. A date equal to the expiry is still valid.
4. Otherwise: accepted.

Input

1. A count, then that many db lines HASH EXPIRY, with the expiry as YYYYMMDD.
2. A count, then that many dbx lines HASH.
3. A count, then that many image lines NAME HASH DATE.

Each database has at most 16 entries.

Output

One line per image:

• shim: ACCEPTED
• evil: REJECTED (hash in dbx, revocation overrides db)
• malware: REJECTED (no signer in db)
• oldloader: REJECTED (db entry expired 20260101)

Then verified 3 image(s), db=2, dbx=1.