SHA-256 Cryptographic Evidence Hashing & Immutable Audit Trails

### Cryptographic Integrity in Financial Audit Trails

When a retail business records a payment fulfillment, missed installment, or contract breach, proving that the supporting evidence was not retroactively altered is critical for compliance and legal validity.

Voxton implements SHA-256 cryptographic hashing at the API ingestion layer to secure every evidence payload before persistence.

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The Evidence Hashing Pipeline

1. **Ingestion & Canonicalization**: When an evidence document (PDF contract, delivery confirmation JSON, or bank receipt) is uploaded, the API server canonicalizes the payload bytes into a deterministic byte array. 2. **SHA-256 Hash Generation**: A 256-bit hash digest is calculated: $$\text{Hash} = \text{SHA256}(\text{Payload}_{\text{canonicalized}})$$ 3. **Header Binding**: The generated hash digest is bound to the transaction header alongside the timestamp (ISO 8601 UTC) and the tenant ID. 4. **MinIO / S3 Storage**: The raw file is stored in encrypted, tenant-isolated object storage buckets, while the SHA-256 hash is written to the primary PostgreSQL audit log.

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Tamper Verification & Audit Mode

When a consumer or auditor inspects a transaction record: - The system recalculates the SHA-256 hash of the stored evidence object on demand. - The recalculated hash is matched against the original audit ledger. - If even a single byte of the file has been altered or corrupted, the integrity check fails immediately, triggering a security alert and isolating the record.

{
  "eventId": "evt_88f921a9x",
  "evidenceHash": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
  "currency": "GBP",
  "amountMinor": 12500,
  "timestamp": "2026-07-24T14:30:00Z",
  "tenantIsolationVerified": true
}

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Zero Leakage & GDPR Compliance

By storing SHA-256 hashes on audit trails, external auditors can verify that a specific document existed at a specific time without accessing unencrypted Personally Identifiable Information (PII), aligning with UK GDPR data minimization requirements.