Evidence & Trust Capsule

Evidence management and trust capsule services providing tamper-evident evidence collection, structured bundling with provenance tracking, Merkle-tree-anchored trust capsule generation for regulatory audit, and financial guarantee validation. Every AI-driven decision is backed by a traceable evidence chain.

Components

Module	Purpose
trust_capsule_service.py	Merkle-tree-anchored compliance evidence capsules sealed as a ZIP (ADR-0017)
evidence_bundle_service.py	PydanticAI chain-of-thought evidence bundling with source provenance (ADR-0021)
evidence_service.py	SHA-256 evidence hashing + MinIO bundle storage (PEPPOL verifications)
guarantee_validator.py	LLM extraction of customs/VAT financial guarantees + VoP cross-reference

Trust Capsule (ADR-0017)

TrustCapsuleService.assemble(case_id, tenant_id) collects every compliance artefact for a case (case data, audit events, signal/domain events, decision) and seals them into a cryptographically anchored ZIP bundle. The pipeline:

1. Collect artefacts from the database via _fetch_case_data and _build_artifacts
2. Build a binary Merkle tree (build_merkle_tree) over the per-artefact SHA-256 leaf hashes (compute_leaf_hash → sha256:<hex> over canonical JSON); the merkle_root anchors the whole capsule
3. Render a PDF via WeasyPrint + Jinja2 templates (_render_capsule_pdf)
4. Sign the PDF (optional) with a PAdES-B-T signature via pyhanko (_sign_pdf_pades)
5. Timestamp (optional) with an RFC 3161 token via rfc3161ng against capsule_tsa_url (default https://freetsa.org/tsr)
6. Create the ZIP containing the PDF and a machine-readable manifest.json
7. Persist to MinIO ({case_id}/trust-capsule/{capsule_id}.zip) and the database

verify(capsule_zip) re-extracts manifest.json, rebuilds the Merkle tree from the artefact hashes, and compares the computed root against the manifest's merkle_root. download(capsule_id) retrieves the sealed ZIP from MinIO.

:::note Signing and timestamping degrade gracefully PAdES signing and RFC 3161 timestamping are best-effort: if pyhanko / rfc3161ng are not installed, or the signing cert/key (capsule_signing_cert / capsule_signing_key) are absent, the capsule is still produced without a digital signature or timestamp (logged as a warning, non-fatal). The Merkle root provides per-artefact tamper evidence regardless, but a capsule is not guaranteed to carry a cryptographic signature unless the certs are configured. verify() checks the Merkle root only; it does not currently perform full PAdES signature validation (signature_valid is returned as None). :::

Evidence Bundle (ADR-0021)

EvidenceBundleService.build_bundle(...) captures the full PydanticAI agent chain-of-thought (all_messages()) for an AI operation — request/response metadata, model identification, and the reasoning chain — into an EvidenceBundle. Bundles are stored in and loaded from MinIO (store_bundle / load_bundle / list_bundles). This is the EU AI Act Art. 12 logging artefact for AI-driven findings.

Guarantee Validator

GuaranteeValidator.validate(markdown_content, case_context) uses a PydanticAI LLM agent (prompt template customs_guarantee_extraction from the PromptRegistry) to extract structured FinancialGuarantee data from a customs/VAT guarantee document, then cross-references the guarantor bank against Verification-of-Payee (VoP) results in the case context using rapidfuzz (match threshold 80). This is a fiscal-representation feature, not a generic "guarantee validation against collected evidence" engine.

ADRs

• ADR-0017: Trust Capsule Cryptographic Architecture (docs/adr/ADR-0017-trust-capsule-cryptographic-architecture.md)
• ADR-0021: Evidence Bundle System for EU AI Act Chain-of-Thought Capture (docs/adr/ADR-0021-evidence-bundle-system-eu-ai-act-audit-trail.md)