chain-of-trust-design

Decision aid for the integrity chain that protects a system from first byte executed to first secret derived. Use this skill when designing or reviewing any system that has a "boot" step, an "unlock" step, or a "first run" step where code outside your trust boundary must be trusted to load code inside it.

The skill exists to force the question almost everyone forgets: what authenticates the code that authenticates everything else?

Triggers

Primary phrases match automatically. Alternates:

• "bootstrap signing" / "signed bootstrap"
• "evil maid" / "physical access attacker"
• "measured boot" / "TPM attestation"
• "verify the verifier"
• "unsigned scripts" / "is this script trusted"
• "first-run integrity"
• "USB boot integrity"

When NOT to use this skill

• Networked TLS chain-of-trust (different domain — use a peer-reviewed PKI design)
• Code-signing for application updates after boot (use Sigstore, signify, or platform store; this skill is about the layer below)
• Software-supply-chain trust upstream of build (handled by supply-chain-trust skill — Tier 2)

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Section 1: The core question

Every system that processes secrets has a chain of code, where each link must trust the previous one. The chain typically looks like:

firmware → bootloader → kernel → init/userland → application → secrets

Or, in a portable-secrets context:

travel-host firmware → BIOS → USB boot → unlock.sh → LUKS unlock → install-pkgs.sh → secrets pipeline

For each arrow, ask: what authenticates the next link?

If the answer is "the previous link verifies a manifest" — fine, but then ask: what authenticates the previous link? Trace it all the way back. The chain MUST terminate at:

• A hardware root of trust (TPM PCR measurement, Secure Boot pubkey in firmware), OR
• An operator-carried secret (a printed hash card, a signed binary on a piece of media you carry physically), OR
• An assumption (e.g., "the firmware is trusted") that is documented and accepted

If the chain terminates in a circular reference — "the manifest verifies the scripts, the scripts verify the manifest" — you have no chain of trust at all. This is the failure mode in review finding B3.

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Section 2: Anti-patterns (BLOCK on these)

Circular trust

unlock.sh on FAT32 partition
  ↓ (verifies)
manifest.sha256 on FAT32 partition
  ↓ (lists hash of)
unlock.sh on FAT32 partition

Both files live on unauthenticated media. An evil-maid attacker swaps unlock.sh AND manifest.sha256 consistently. Detection: zero. → BLOCK

Unsigned bootstrap on writable media

Any executable on writable storage that runs before the integrity chain is established. → BLOCK

"Override" prompt on integrity failure

[!] Manifest verification failed. Type Y to continue:

Operators learn to autoresponse-Y. The override defeats the integrity check on the exact pathway integrity matters most: when something is wrong. → BLOCK (see degraded-mode-design skill, Tier 2, for the multi-step ceremony alternative)

"We use SHA-256 of the scripts" without offline-signed pubkey

Hashing is not signing. Anyone with write access to the manifest can replace it. Hashing only authenticates if the hash itself is delivered through a separately-trusted channel (printed card, hardware-fused).

Scripts that mount LUKS before integrity is verified

The unlock script IS the high-value target. Verify its integrity before running it, or you've gained no security from LUKS.

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Section 3: Pattern catalog

Pick at least one. The patterns are not mutually exclusive — defense in depth combines two or more.

Pattern A: Signed bootstrap blob with offline key

Shape: Ship bootstrap.tar plus bootstrap.tar.sig plus a small static-linked verifier (verify) on the same media. The verifier has an embedded ed25519 public key. The private key is offline (HQ, hardware-stored).

USB layout:
  /verify              ← static, ~50 KB, embedded pubkey
  /bootstrap.tar       ← contains unlock.sh, install-pkgs.sh, .deb files, manifests
  /bootstrap.tar.sig   ← Ed25519 signature over bootstrap.tar

Operator runs:
  ./verify bootstrap.tar bootstrap.tar.sig && tar xf bootstrap.tar && ./unlock.sh

What this gives you: Evil-maid swap of unlock.sh requires also forging an Ed25519 signature → infeasible without HQ private key.

What this does NOT give you: Trust in verify itself. The verifier is on the unauthenticated media. Mitigations:

1. Make verify minimal and reproducibly built — anyone can audit it
2. Print a SHA-256 of verify on a card the operator carries; check before running
3. Better: combine with Pattern B or C

Suggested defaults:

• Signature: Ed25519 via libsodium or Rust ring
• Verifier build: Rust with #![no_std], statically linked, reproducibly built
• Key custody: offline, hardware-token-protected (FIDO2 PRF or HSM)

Vetted alternatives:

• signify (OpenBSD) — minimal verifier already exists; use if you want zero custom code
• minisign — signify with extras (prehashed mode)
• Sigstore Cosign — heavier, network-aware; pick when you want transparency log integration

Pattern B: Signed live image (Tails-style)

Shape: The "operating environment" is itself the signed thing. Boot from a signed read-only image; the USB you carry only contributes data, not code.

Travel host:
  → BIOS verifies bootloader signature (Secure Boot)
  → Bootloader verifies live-image signature
  → Live image kernel + userland is read-only
  → User USB plugs in for data only

Threat collapses to: trusted firmware + your USB.

What this gives you: Massive blast-radius reduction (review finding H4). The travel host stops being a trust dependency.

What this does NOT give you: Protection against firmware-level adversary or hardware implant. Combine with Pattern C.

Suggested defaults:

• Build: NixOS image (reproducible by construction) OR Tails (community-vetted, anonymity-focused)
• Signing: integrate with the distro's existing signing (NixOS: nix-store --verify; Debian-based: signed Release file)
• Boot: Secure Boot with operator-controlled keys (sbctl), not Microsoft's defaults

Pattern C: Measured boot + TPM remote/local attestation

Shape: TPM measures every stage of boot into PCRs. Refuse to derive secrets if PCRs don't match a known-good set.

PCR measurements taken at:
  PCR 0    — firmware
  PCR 4    — bootloader
  PCR 7    — Secure Boot policy
  PCR 8    — kernel + initrd
  PCR 9    — kernel command line

Unlock requires: TPM2_Unseal succeeds against the known-good PCR set

What this gives you: Binds secret access to a non-tampered boot chain. Evil-maid swap → PCRs change → unseal fails → no secrets.

What this does NOT give you: Anything if the TPM is compromised, the firmware is malicious, or the operator is coerced into running with sealed values that match a malicious chain ("seal your secret to my malicious kernel").

Suggested defaults:

• Tooling: systemd-cryptenroll --tpm2-pcrs=..., tpm2-tools, clevis for higher-level sealing
• PCR set: 0, 2, 4, 7, 8 (firmware, option ROMs, bootloader, Secure Boot policy, kernel)
• Backup unseal: secondary path with hardware token (review finding H5: backup story)

Pattern D: Printed hash card (operator-carried)

Shape: The operator carries a piece of paper (or laminated card) with a SHA-256 of the bootstrap blob. Manually compares before running.

Card:
  bootstrap.tar.sig SHA-256:
  a1b2 c3d4 e5f6 0708 1a2b 3c4d 5e6f 7081
  9a8b 7c6d 5e4f 3021 1a2b 3c4d 5e6f 7080

Operator runs:
  sha256sum bootstrap.tar.sig
  # compares first/last 4 groups against card

What this gives you: A truly out-of-band trust anchor, immune to USB tampering.

What this does NOT give you: Operational ergonomics; it's tedious and skipped under time pressure. Use as a SECONDARY check, not the only one.

Suggested defaults:

• Compare a prefix and suffix (first 4 + last 4 hex groups), not the full hash — operators won't read 64 hex digits accurately
• Card laminated, signed by HQ, dated, with revocation procedure documented

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Section 4: Decision matrix

                              Pattern A      Pattern B      Pattern C      Pattern D
                              (signed       (signed live   (measured      (printed
                              blob)         image)         boot/TPM)      hash card)

Defeats evil-maid swap        YES           YES            YES            partial*
Defeats compromised host      no            YES            YES            no
Defeats firmware compromise   no            no             partial**      YES
Recovery if hardware fails    moderate      easy           hard           moderate
Operator burden               low           lowest         low            HIGH
Hardware dependency           none          UEFI/SB        TPM 2.0        none
Greenfield-friendly           YES           YES            YES            yes
Retrofit-friendly             yes           moderate       moderate       yes

*  Pattern D detects evil-maid only if the operator actually checks
** Pattern C with operator-controlled Secure Boot keys; otherwise firmware compromise unseals secrets

Recommended starting point: combine Pattern B (signed live image) + Pattern A (signed user-data blob). The live image is the trusted environment; the USB user-data has a signed manifest validated against an HQ Ed25519 pubkey embedded in the live image. This is the answer for review finding B3 + H4.

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Section 5: Trust anchor inventory (mandatory exercise)

Before declaring any chain-of-trust design complete, fill out:

Anchor	What is it?	Who controls it?	Compromise impact
HQ signing key	Ed25519 private key for bootstrap signing	HQ ops, hardware token	Total: attacker can sign malicious bootstrap
Operator USB	The physical USB device	Operator	Loss alone: nothing (LUKS at rest); loss + travel host compromise: partial
Travel host firmware	UEFI + Secure Boot keys	…	…
TPM PCR baseline	Known-good PCR set sealed at provisioning	HQ + per-device	…
Operator-carried card	Printed hash	Operator	Detection of out-of-band tampering

Every anchor MUST be:

1. Identified: who or what holds it
2. Protected: how is it kept from being lost or copied
3. Rotatable: what is the procedure when it's lost or compromised
4. Tested: have you actually run the rotation procedure?

If any row has "TBD" in any column, the design is not complete.

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Section 6: The "verify the verifier" recursion

For every signed component, you have to answer "what verifies the signer?". The recursion terminates at one of:

1. Hardware — a key fused into firmware (Secure Boot, TPM endorsement key)
2. Physical — something the operator carries (a printed key fingerprint, a signed binary on physically separate media)
3. Documented assumption — explicitly stated and accepted

A common failure mode is incomplete recursion: stating "the bootstrap is signed by HQ" without saying how the operator validates HQ's pubkey. The pubkey must:

• Be embedded in a verifier whose integrity is established by some other means (Pattern D card, Secure Boot embedded firmware, etc.), AND
• Be rotatable when compromise is suspected

If you can't draw this recursion ending at hardware or physical, the chain is incomplete.

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Section 7: Recovery and rotation

A chain of trust must include:

Procedure	Documented?	Tested?
Operator USB lost	…	…
Travel host fails / unavailable	…	…
HQ signing key compromise	…	…
TPM hardware failure	…	…
Firmware update breaks PCR seal	…	…
Bootstrap pubkey rotation	…	…

Untested procedures don't exist. Every project should run a tabletop exercise (review recommendation #10) walking through "USB stolen but keys retained" and "USB plus one key stolen" before declaring the design production-ready.

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Section 8: Worked example — review finding B3

Original design:

unlock.sh + install-pkgs.sh on FAT32 (unencrypted)
checksum.manifest on FAT32 (unencrypted)
unlock.sh "verifies" checksum.manifest — but checksum.manifest names unlock.sh

What this skill flags:

• Section 2 anti-pattern: circular trust → BLOCK
• Section 5: no trust anchor inventory exists
• Section 6: recursion does not terminate at hardware or physical anchor

Remediation options (review's "pick at least one"):

1. Pattern A only: ship unlock.sh.sig + a static verify binary + an embedded Ed25519 pubkey. Operator runs ./verify first.
2. Pattern B: boot the travel host from a signed live image. The image embeds the HQ pubkey. The USB only carries unlock.sh.sig + encrypted data.
3. Pattern C: enroll the LUKS unlock against TPM PCR measurements; if unlock.sh is swapped, PCRs change, unseal fails.
4. Pattern D as backup: print SHA-256 of verify (or of unlock.sh.sig) on a card the operator carries.

The strongest combination: B + A + D. Live image gives you trusted environment; signed user-data gives you tamper-evidence on the USB; printed card gives you an out-of-band sanity check.

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Section 9: Output format

When this skill is invoked as part of a review, produce findings in this format:

### Finding: <SHORT-NAME>

**Severity**: BLOCK | HIGH | MEDIUM | LOW
**Location**: <file or component>
**Section**: Chain-of-trust design, <section reference>

**Issue**: <what's broken in the chain>

**Trust anchor inventory** (if relevant):
- Anchor 1: …
- Anchor 2: …
- Missing: <what isn't anchored>

**Remediation**:
<concrete pattern recommendation, A/B/C/D combination>

**References**:
- <NIST SP 800-193, NIST SP 800-147, RFC, etc.>

When a chain-of-trust-design.md (template — see templates/chain-of-trust-design.md) is being created, this skill drives the Trust anchors, Boot chain diagram, Verification matrix, Recovery and rotation, and Open assumptions sections.

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Related

• Companion skill: crypto-primitive-selection (you'll use it for choosing the signing primitive)
• Tier 2 skill: physical-threat-modeling (enumerates evil-maid, DMA, hostile peripheral threats this skill mitigates)
• Tier 2 skill: degraded-mode-design (override-prompt anti-pattern remediation)
• Agent: secure-bootstrap-reviewer — narrow-scope reviewer that loads this skill on activation
• Template: chain-of-trust-design.md

Standards referenced

• NIST SP 800-147 (BIOS Protection Guidelines)
• NIST SP 800-155 (BIOS Integrity Measurement Guidelines)
• NIST SP 800-193 (Platform Firmware Resiliency Guidelines)
• TCG TPM 2.0 Library Specification
• UEFI Secure Boot specification
• Reproducible Builds (reproducible-builds.org)