Conceptual Health Trust Root Protocol v1.

External attacker. Outside the corporate perimeter. May achieve XSS against an officer's browser, phish credentials, run malware on an officer's laptop, intercept TLS via a state-level CA compromise. Defeated by: (a) hardware-bound WebAuthn — credential never leaves the secure element; (b) per-ceremony challenges with origin binding — phishing-proof; (c) cross-channel notification — malware on one device is visible on the other two.

Insider (compromised officer). A single custodian's session, key, and device all under adversary control. Defeated by the threshold — 1-of-5 is insufficient; the adversary must compromise three independent officers simultaneously. Each compromise is a chain-stamped event with cross-channel notification.

Coerced custodian. A custodian under physical or legal duress, forced to release their share. Defeated by the duress-release path — looks identical to a normal release but covertly alerts the security team and chain-stamps a separate alarm row. The ceremony's chain output is marked suspect; no key material flows.

Out of scope: a compromise of three or more custodians simultaneously by an attacker who also defeats the cross-channel notification on all three. At that threshold the protocol commits the breach to the chain — recovery happens through governance, not cryptography.

Threshold · Shamir Secret Sharing, 3-of-5, GF(2²⁵⁶) field, polynomial degree 2.

Share wrap (Phase 2B work) · AES-256-GCM, key derived per-custodian from the WebAuthn public key via HKDF-SHA-512. DB read alone returns nothing without a matching device signature.

Audit chain · HMAC chain with per-row hash_algo dispatch — SHA3-512 forward (since sequence #24971, 2026-05-26), SHA-256 history preserved exactly as written. Crypto agility is a corporate principle, not a one-time fix — see the governance note at NOTES/2026-05-26-crypto-agility-governance.md.

Signature algorithms accepted on credentials · ES256 (P-256), EdDSA (Ed25519), RS256 (RSA-2048 SHA-256). Whitelisted in the policy file; CHECK constraint at the database level rejects anything else.

Hash for challenge bytes · 32 bytes from os.urandom (Linux kernel CSPRNG / RDRAND seeded). Stored as bytea; UNIQUE constraint at DB level.

The chain of tables — each FK-linked, each with CHECK constraints that enforce policy at the row level:

• chtr_custodian_shares — Shamir share ciphertext, share index 1–5, KEK generation, revocation. Existing.
• chtr_custodian_credentials — WebAuthn credentials per custodian (multi-credential supported). Hardware-only DB-level CHECK. New (Phase 1A).
• chtr_ceremony_sessions — initiation reason, purpose, cooling-off, expiry, KEK generation, status, chain-stamp columns. Existing.
• chtr_webauthn_challenges — per-ceremony challenges, 5-min TTL (max 15 by CHECK), single-use. New (Phase 1A).
• chtr_ceremony_share_grants — grant record per custodian per ceremony, mfa_factor, mfa_proof_id (challenge ID), duress flag, anomaly_flags. Existing.

Full DDL is at backend/migrations/chtr_p1a.sql (when committed in the same PR as this page).

• NIST SP 800-63B AAL3 — Authenticator Assurance Level 3. CHTR-P1 exceeds AAL3 on every requirement (hardware-bound, attestation-verified, user-verification-required, replay-protected).
• FIPS 140-3 — for FIPS-certified authenticators (Yubikey 5 FIPS). The AAGUID whitelist marks these explicitly.
• FIDO L2 — preferred certification level. The whitelist marks L1 vs L2 per AAGUID.
• HIPAA Security Rule §164.308(a)(1)(ii) — administrative safeguards. The CHTR protocol satisfies the requirement for "appropriate access controls" on the system-of-record encryption root.
• HIPAA Security Rule §164.312(a)(2)(iv) — encryption / decryption. The master KEK never exists outside the threshold-reconstruction window of an active ceremony.
• 21 CFR Part 11 §11.10(g) — electronic signatures. The grant assertion (signed by the custodian's hardware key) is an electronic signature meeting the §11.50 requirements.

Key escrow for legal compulsion. CHTR-P1 does not provide a back door for law enforcement, regulatory production orders, or any other lawful-access pathway. The master key is reconstructed only by the corporation's own threshold custodians via the live ceremony protocol. A subpoena to Conceptual Healthcare Corporation can only compel the corporation to execute the ceremony itself if there is a lawful basis to do so — and any such ceremony is chain-stamped publicly.

Quantum-resistant signatures. The accepted public-key algorithms (ES256, EdDSA, RS256) are classical. A post-quantum migration path follows the same crypto-agility pattern as the SHA-3 migration: per-row public_key_alg column allows mixed-era credentials; CRYSTALS-Dilithium / SLH-DSA addition is a future policy_id version bump.

Side-channel resistance of host hardware. CHTR-P1 trusts the authenticator's secure element; we do not solve laptops with cold-boot vulnerabilities, dirty RAM, or compromised microcode. Custodian devices are subject to a separate physical-security policy.