Audit Trail

Every event—from data access requests to model weight submissions—is recorded with a cryptographic hash and timestamp. This creates a tamper-proof chain of custody.

• Uses Merkle tree structures to verify record integrity
• Prevents retroactive alteration of logs
• Provides non-repudiation for all participant actions

Unlike traditional centralized logging, each client node (e.g., a hospital) maintains its own local, signed ledger. The aggregation server records only metadata and consensus proofs.

• Preserves data residency requirements
• Logs remain under local institutional control
• Enables cross-site validation of events without exposing raw data

All log entries are digitally signed using the originating node's private key. This provides mathematical proof of origin and prevents log forgery.

• Integrates with Trusted Execution Environments (TEEs) for hardware-backed signing
• Supports Secure Aggregation (SecAgg) audit events
• Enables verification that a specific institution performed a specific action at a specific time

The audit trail is structured to directly map to regulatory frameworks like HIPAA, GDPR, and FDA SaMD guidelines.

• Records all Data Use Agreement (DUA) enforcements
• Logs every access to Protected Health Information (PHI)
• Provides automated evidence generation for FDA clearance pathways

The audit trail tracks the consumption of the privacy budget (epsilon) across training rounds. Each query or model update that consumes budget is immutably logged.

• Monitors cumulative privacy loss over time
• Triggers alerts when the Differential Privacy budget nears exhaustion
• Provides verifiable proof of privacy guarantees to regulators

The audit trail captures anomalous behaviors from client nodes, such as submitting corrupted gradients or deviating from the agreed Federated Averaging (FedAvg) protocol.

• Logs events triggering Robust Aggregation rules like Krum
• Creates a forensic record for investigating Byzantine Fault Tolerance violations
• Documents Client Drift incidents and remediation actions

A write-once, read-many data structure where records, once appended, cannot be altered or deleted. In an audit trail, this is achieved through cryptographic hashing and Merkle tree structures, ensuring that any retrospective tampering is computationally infeasible. Each entry contains a hash of the previous block, creating a chain of verifiable integrity.

A security property guaranteeing that an entity cannot deny the authenticity of a recorded action. Audit trails enforce non-repudiation through digital signatures and public key infrastructure (PKI). When a clinician accesses a diagnostic model's output, the event is signed with their private key, providing cryptographic proof of the action's origin that is admissible in regulatory audits.

A design pattern where any modification to historical records leaves detectable, irreversible evidence. Techniques include:

• Forward integrity: Compromising a current key does not expose past logs
• Hash chaining: Each entry's hash is embedded in the subsequent entry
• Witness co-signing: An external trusted third party periodically countersigns the log state

The capability to trace the complete lineage of a data artifact or model decision back to its origin. In federated diagnostic systems, provenance tracking records the source institution, data version, preprocessing steps, and model checkpoint used for every inference. This enables root-cause analysis when a diagnostic discrepancy is identified across sites.

The process of generating a verifiable, machine-readable report that proves adherence to regulatory frameworks like HIPAA, GDPR, or FDA SaMD guidelines. An audit trail serves as the raw evidence for attestation, automatically compiling access logs, model update histories, and data use agreement (DUA) validations into a structured compliance package for external auditors.

A trusted third-party service that issues cryptographically signed timestamps, binding a precise moment in time to a specific data hash. Integrating a TSA into a federated audit trail ensures temporal ordering of events across institutions with unsynchronized clocks, preventing backdating attacks and establishing a legally recognized chronology of model updates and data accesses.