Hash-chained audit trail
Also called: tamper-evident audit trail, SHA-512 hash chain
An append-only log where each entry includes the cryptographic hash of the previous one, so any later alteration is detectable.
What hash-chained audit trail means.
A hash-chained audit trail records events so that each entry contains a cryptographic digest of the entry before it. Because a digest changes when its input changes, editing or removing a historical entry invalidates later digests and can be detected by recomputing the chain. This is a general design explanation, not a claim that a particular Trunnion release implements a specific algorithm or complete state-change coverage.
Why it matters.
For an autonomous system the log is the only account of what happened. An ordinary log is a claim about the past that anyone with sufficient access can revise. A hash-chained log is a verifiable claim, which is the difference between telling a reviewer what an agent did and proving it. That distinction is what makes audit records usable in an accreditation review or a dispute.
How it works in practice.
Each entry commits to its predecessor
An entry stores the digest of the previous entry alongside its own content, so the entries form a chain rather than an unordered set.
Verification is recomputation
Walking the chain and recomputing digests either reproduces the stored values or identifies the exact entry where the record diverges.
Reasoning traces are part of the record
Capturing the inputs and the reasoning behind an action, not only the action, is what makes a run replayable rather than merely listed.
Append-only by construction
There is no legitimate edit path. Corrections are new entries that reference the earlier one, which preserves the original record.
What people get wrong.
That a hash chain makes a log immutable. It does not. Immutability requires storage that refuses writes, or distribution across parties who would have to collude. A hash chain makes tampering evident, which is a different and more modest guarantee, and it should be described that way.
Common questions.
Is this a blockchain?
No. It shares the hash-linking primitive and none of the rest. There is no distributed consensus, no network of validators, and no token. It is a local append-only log with cryptographic linkage, which is the part that provides tamper-evidence.
Why SHA-512 rather than SHA-256?
Both are approved in FIPS 180-4 and neither has a practical collision attack. SHA-512 has a larger digest and, on 64-bit hardware, often computes faster than SHA-256, so it is a reasonable default where output size is not constrained.
What can an auditor actually do with it?
Recompute the chain to confirm the record has not been altered, then replay any individual run to see the action taken, the policy decision that permitted it, the reasoning behind it, and the person who approved it.
Standards referenced.
Trunnion is engineered to align with these control families. Alignment by design is not a claim of current certification or authorization.
Read next.
- Agent governanceThe control layer that decides what an AI agent may do, who approves consequential actions, and how every step is proven afterward.
- Human-in-the-loop approval gateA checkpoint where an autonomous agent pauses and a named person approves, edits, or rejects a consequential action before it executes.
- Replayable audit recordAn audit record complete enough to reconstruct an agent run step by step, including the reasoning behind each action.
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