Logs can explain what a service thought happened. They do not prove what happened. Klevar Docs needed an audit trail for rendered documents, invoice events, credit note applications, signatures, voids, and attachments. The usual answer is an events table. Insert a row whenever something happens. Add timestamps. Keep it forever. That is useful, but it is still just a table unless the table can detect tampering. The hash chain is the difference. Each entity gets its own ordered chain. A row stores the event payload, a SHA-256 hash of the canonical payload, the previous row hash, the chain index, and a link hash that binds those values together. If someone changes a payload, deletes a prior row, swaps entity rows, or reorders entries, verification fails. The append path is transactional with the document change. That detail matters more than the hashing. If the document row commits and the chain row rolls back, the proof is incomplete.…