What a tamper-evident decision log actually is
When someone asks why an agent sent a message, a decision log helps reconstruct the basis and approval. A hash chain checks the consistency of that history against a trusted reference.
What does a hash chain give you?
Each entry is linked to its predecessor’s hash. Editing a record breaks its agreement with the preserved chain. Detection depends on that chain or a preserved final hash remaining a trusted reference.
Someone who controls both records and hashes can truncate the history or recompute the whole chain. Verification of that rewritten history can still pass. The mechanism alone does not guarantee detection of every change.
Agree how records and an independent reference will be protected before deployment. Current chain consistency alone cannot establish the whole earlier history.
Why is a plain table not enough?
Because anyone with admin access can run an UPDATE. This assumes no bad faith; it assumes you cannot prove its absence. An audit that rests on trusting whoever holds the database credentials is not an audit.
The second issue is recorded ordering. A chain links each entry to a specific predecessor; a trusted reference allows that order to be checked. It does not independently establish the real event time.
What belongs in a single entry?
Enough to reconstruct the decision without asking anyone to remember it.
- The action and its inputs.
- The policy rule that classified it, and the tier that followed.
- Who approved: the machine, or a named person.
- The evidence: test results, verification, the content prepared for sending.
- The hash of the previous entry.
What does it not solve?
It does not protect against an untruth written at the start. If the system records a wrong basis for a decision, the chain preserves that error. Matching hashes do not establish input truth or complete history without a trusted reference.
It guarantees neither legal evidential weight nor compliance, and it is not certification. It helps reconstruct a decision; record protection and use during an inspection need assessment for the actual deployment.
Sources
- Article 12: Record-keepingEuropean Commission AI Act Service Desk, accessed 2026-10-14