What a signature cannot tell you.
Hand someone a VIRP bundle and they can check every signature and every hash link. Then they ask two questions the bundle cannot answer.
How do I know this is when it happened? The timestamps were written by your daemon, on your box, using your clock. And how do I know this is all of it? You could have stopped the export before the inconvenient part, and every entry they hold would still verify.
Neither question is answerable by more signing, because the party signing is the party being asked.
What it does
Sends a fingerprint, not the record.
When a session head is signed, the node sends the witness a hash of that head, its key id, and a signature. No commands, no device names, no observation bodies. The witness cannot read the chain and is never asked to hold it.
Records it where you cannot reach.
The witness appends the submission to an append-only Merkle log, RFC 9162, the same construction used by Certificate Transparency, and returns a receipt: the entry's position, a proof of inclusion, and a signed tree head with a timestamp the operator did not choose.
Publishes so it can be held to it.
The witness signs and publishes its tree head daily, to a public repository and an external timestamp. A witness that later rewrote its log would contradict something already public.
What it stops
- Backdating. A witnessed head cannot be claimed to have happened earlier than the witness recorded it.
- Truncation. A chain presented as complete that ends before a witnessed entry is contradicted by the receipt.
Limits
One witness is one party. A witness can refuse to log, and can lie about time; publishing its tree heads makes both detectable, and two witnesses that do not share an operator is the real answer. Today Third Level IT operates this witness and also operates one of the chains it witnesses. That is a genuine limit, stated here rather than left to be discovered, and the invitation is open to anyone who wants to run a second one.
The log, the client, and the reproducible build are done and the test suite passes. Third Level IT installs the witness alongside the gate, and fits it to the chains you already run.