PUF Challenge-Response Health Verification

Mechanism

Each unit incorporates a PUF instance — typically an SRAM-startup PUF, an arbiter PUF, a ring-oscillator PUF, or a butterfly PUF — whose physical structure produces a deterministic but unit-unique mapping from challenge bitstrings to response bitstrings. During unit provisioning, the manufacturing authority enrolls a population of challenge-response pairs (CRPs) under a credentialed enrollment ceremony; the enrolled CRPs are stored in the governance ledger bound to the unit's identity record, while the PUF itself remains within the unit and never leaves. The enrollment record carries the enrollment authority, the enrollment timestamp, the PUF type, the operating-condition envelope under which CRPs were captured, and a salted hash of each enrolled response.

Health monitoring issues credentialed challenges drawn from the enrolled CRP set. Each challenge is wrapped in a freshness nonce, signed by the monitoring authority, and delivered to the unit over the operational channel. The unit applies the challenge to its PUF, captures the raw response, applies a fuzzy-extractor or error-correcting helper-data step to convert the noisy physical response into a stable bitstring, and returns a signed response containing the stabilized bitstring, the freshness nonce, and the unit's signing credential. The architecture verifies the signature, compares the stabilized bitstring against the enrolled response, and records the verification result — pass, fail, or marginal — as a credentialed observation in the unit's lineage.

Repeated challenges traverse the enrolled CRP set without replay. Each issued challenge is consumed: the monitoring authority marks the CRP as exercised and excludes it from future challenges, preventing a recorded response from authenticating a subsequent challenge. When the exercised CRP set approaches exhaustion, a credentialed re-enrollment ceremony refreshes the population, again under the unit's operating-condition envelope and again recorded in lineage. The cryptographic protocol surrounding challenge issuance and response verification is bound to the unit's signing credential, ensuring that an adversary observing the channel cannot synthesize valid responses without access to both the PUF and the signing key.

Operating Parameters

CRP population size at enrollment is governance-configurable; reference deployments enroll between 2^14 and 2^20 pairs per unit, balancing enrollment-ceremony duration against expected operational lifetime and challenge cadence. Challenge cadence ranges from one challenge per operational hour for low-criticality units to multiple challenges per minute for high-criticality units, with the cadence itself recorded as a credentialed parameter subject to safety-class governance.

Response stability is parametrized by the fuzzy-extractor error-correction strength. Typical helper-data schemes tolerate intra-PUF Hamming distances of 5 to 15 percent across the operating-condition envelope while rejecting inter-PUF distances above 40 percent, yielding false-accept rates below 2^-80 and false-reject rates below 10^-6 under nominal conditions. The operating-condition envelope itself encompasses temperature, supply voltage, and aging windows; PUF responses captured outside the enrolled envelope are flagged as marginal rather than fail, prompting re-challenge under controlled conditions.

Verification latency from challenge issuance to recorded result typically falls between tens of milliseconds and single-digit seconds, dominated by the unit's PUF evaluation and helper-data processing rather than the network or signing path. Aging and burn-in effects on PUF stability are tracked across the unit's lineage; a measurable drift in the marginal-response rate triggers a credentialed re-enrollment under safety-class authority before the drift compromises false-reject rates.

Alternative Embodiments

In one embodiment, the PUF is an SRAM-startup PUF leveraging the power-on bias of uninitialized memory cells; this embodiment requires no dedicated PUF circuitry beyond on-die SRAM and is appropriate for cost-constrained units. In a second embodiment, the PUF is an arbiter PUF or ring-oscillator PUF synthesized as dedicated logic, supporting much larger CRP populations at the cost of additional die area. In a third embodiment, the PUF is implemented optically via coherent-scattering speckle patterns from an embedded scatterer; this embodiment resists semi-invasive attacks that target electrical PUFs.

A fourth embodiment substitutes a strong-PUF protocol — such as a controlled PUF wrapped in a secure key-derivation function — for direct response disclosure. The unit returns a key-derived authenticator rather than the raw stabilized bitstring, frustrating modeling attacks that accumulate disclosed CRPs to train a software model of the PUF mapping. A fifth embodiment composes the PUF verification with secure-boot attestation: the PUF response keys a measured-boot quote, binding the hardware identity to the firmware-and-configuration measurement and producing a single composite attestation rather than two separable verifications.

A sixth embodiment introduces cross-unit PUF federation. A monitoring authority issuing challenges to multiple peer units in a cohort can correlate response timing, aging-drift signatures, and operating-condition reports across the cohort, detecting coordinated supply-chain compromise that any single-unit verification might miss. A seventh embodiment binds the PUF response into the unit's signing-credential generation: the signing key is derived from the PUF, eliminating the separate-storage attack surface for the signing key and ensuring that any unit failing PUF verification also fails signature verification on subsequent operational messages.

An eighth embodiment introduces challenge-stratification policies that interleave low-cost frequent challenges drawn from a small reservoir with high-cost infrequent challenges drawn from the full enrolled CRP set. The frequent reservoir supports rapid liveness verification at every operational hand-off, while the infrequent draws provide stronger statistical evidence against modeling attacks that might gradually learn the small reservoir. A ninth embodiment supports field-extensible re-enrollment at distributed depots: a mobile re-enrollment authority, itself credentialed under safety-class governance, performs CRP refresh in the field under controlled environmental conditions, enabling extended-lifetime deployments without requiring units to return to a central enrollment facility.

Composition with Other Properties

PUF challenge-response composes with tamper-evident-seal monitoring to produce a hardware-integrity attestation that resists both substitution and intrusion. Tamper-evident-seal monitoring detects physical opening, casing breach, or chip-level decapping events; PUF verification detects cloning, counterfeiting, or wholesale substitution. A unit that has been opened and reassembled fails the seal monitor; a unit that has been replaced wholesale with a counterfeit fails the PUF challenge. Composition of both checks produces a structural attestation that defeats either attack class in isolation and raises the cost of any combined attack to the level of physically extracting and re-instantiating the PUF, which the unclonability property defines as infeasible.

The PUF verification result enters composite admissibility evaluation as a safety-class observation with high weight. A failed PUF verification produces a refuse-weighted observation that propagates into every composite evaluation referencing the unit, halting the unit's contribution to mutation acceptance until a credentialed remediation event clears the refusal. Marginal PUF results enter as defer-eligible observations, prompting re-challenge under controlled conditions before the unit's contribution is admitted to downstream composition.

The PUF primitive composes with cross-mesh reconciliation: a unit failing PUF verification during a partition interval produces a refuse-weighted observation that, on reunion, propagates into reconciliation and identifies the mutations admitted under the unit's contribution during the partition. Such mutations are flagged for post-reunion review under the dispute primitive rather than silently retained or silently rolled back, preserving operator agency over remediation. The PUF primitive further composes with audit reproducibility: re-execution of the recorded challenge against the recorded response yields the recorded verification outcome, allowing post-hoc auditors to confirm both that a challenge was issued and that the response matched without re-issuing live challenges to operational hardware.

Prior-Art Distinction

Conventional hardware identity rests on stored cryptographic keys — fused into one-time-programmable memory, retained in tamper-resistant secure elements, or provisioned into TPM-style co-processors. These approaches share an attack surface: a sufficiently capable adversary with physical access can extract or reconstruct the stored key, after which a counterfeit unit indistinguishable from the original can be fielded. PUF challenge-response differs structurally in that no key is stored; the identity is reconstituted on demand from physical structure that cannot be copied because the structure was determined by uncontrollable manufacturing variations rather than by a programmable input.

Software-only attestation schemes — including remote-attestation protocols built on TPM quotes or measured-boot logs — verify firmware-and-configuration state but do not bind that state to a physically-unique hardware identity. A counterfeit unit running identical firmware produces identical attestation quotes. PUF challenge-response binds the identity to the silicon, and composition with measured-boot attestation produces the binding the software-only schemes lack.

Prior PUF deployments in the academic and industrial literature have generally treated PUF verification as a one-time provisioning step or as an isolated authentication primitive. The disclosure differs in incorporating PUF challenge-response as a recurring, lineage-bound, cryptographic-protocol-wrapped health-monitoring observation composed with tamper-evident-seal monitoring and admitted into composite admissibility evaluation. The recurring credentialed-observation structure, the CRP-exhaustion and re-enrollment protocols, and the composition with both seal monitoring and admissibility evaluation are not anticipated by isolated-authentication PUF deployments.

Disclosure Scope

The disclosure encompasses PUF challenge-response as a recurring health-monitoring primitive, the credentialed enrollment ceremony, the lineage-bound CRP record, the freshness-nonced challenge protocol, the fuzzy-extractor stabilization step, the signed-response verification, the CRP-exhaustion and re-enrollment cycle, and the composition with tamper-evident-seal monitoring and composite admissibility evaluation. The disclosure spans SRAM-startup, arbiter, ring-oscillator, butterfly, optical-scattering, and controlled-PUF embodiments, and includes embodiments in which the PUF response keys a measured-boot quote or generates the unit's signing credential.

Defense hardware-integrity assurance — protecting against supply-chain substitution of fielded units, contested-environment counterfeiting, and depot-level cloning — gains a structural attestation primitive that conventional stored-key approaches cannot supply. Civilian critical-infrastructure deployments — grid relays, financial-settlement HSMs, healthcare device fleets — gain the same structure. The architecture supports PUF technology evolution: as new PUF constructions and helper-data schemes mature, verification protocols update through the same governance-credentialed procedures that admit ordinary mutations, and the lineage record preserves the verification provenance across protocol generations.

The disclosure expressly contemplates equivalents and variations within its scope. CRP population sizes below the reference 2^14 and above the reference 2^20 are within scope, with the corresponding adjustments to challenge cadence and re-enrollment intervals. Helper-data schemes other than reference fuzzy extractors — including syndrome-coding constructions, code-offset constructions, and lattice-based stabilization — are within scope where the resulting verification observation enters lineage as disclosed. PUF constructions not enumerated above — magnetic-tunnel-junction PUFs, memristive PUFs, and emerging post-CMOS device PUFs — are within scope where the construction admits the disclosed enrollment, challenge, and re-enrollment structure. Cryptographic protocols wrapping challenge issuance and response verification beyond the reference signed-nonce protocol — including zero-knowledge response disclosure and threshold-distributed verification — are within scope where the protocol preserves the freshness, signing-credential binding, and lineage-recording properties.