What is LTV in a PDF signature?

Why a good signature stops verifying

Verification asks three questions: do the bytes match, is the certificate trustworthy, and was it valid when used. The first is self contained. The other two normally reach outside the file, to the issuer's certificate and to a revocation service that says whether the certificate had been withdrawn. Certificates expire, issuers reorganise, and revocation responders stop answering for old certificates. None of that means the signature was bad, but a reader with no way to check simply reports that it cannot determine validity, which in a dispute is uncomfortably close to the same thing.

Freezing the evidence at signing

The remedy is to capture the supporting material at the moment of signing and store it inside the document. The certificate chain goes in, along with the revocation responses that were current, so a reader in ten years can evaluate the signature against what was true then rather than what is reachable now. This has to happen while the data is still available, which is why long term validation is a decision made when the document is produced. It cannot be added retrospectively to an old file with any real meaning.

Why a timestamp is the anchor

Embedded revocation data is only useful if the date it relates to can be established independently. A trusted timestamp from a third party fixes that date, so the whole package can be read as this signature existed at this moment, and at that moment the certificate was valid and unrevoked. Without a timestamp the file is asserting a date supplied by the signer's own machine, and every other piece of the evidence hangs from that unreliable point. This is why the levels that add validation data assume a timestamp is present.

Archive timestamps and the long view

Beyond ten or twenty years the concern shifts from expired certificates to ageing cryptography. Hash functions and key sizes considered strong today may be weak later, so the highest level adds a timestamp over the signature, the certificates and the revocation data together, and that timestamp can be renewed with stronger algorithms before the old ones fail. It is genuine engineering for archives, and it is more than a commercial contract with a seven year life needs.

Who actually needs it

Documents with long tails: leases, guarantees, facility agreements, land and construction records, employment files in jurisdictions with long retention rules, and anything a regulator may ask about a decade later. For a purchase order or a short services agreement it is over engineering. The practical rule is to look at how long the document could matter and whether anybody would need to prove its integrity to a stranger after that period, rather than at the value of the transaction. Where a signing platform does not embed validation data, the gap can be narrowed outside it. Keep the completed file unchanged, keep the completion certificate and the event record with it, and consider timestamping the file with an independent authority after download, which at least fixes a date that does not depend on anybody's word. None of that matches a properly constructed long term signature, but it is far better than a file whose only evidence of age is a modification date, which changes whenever a copy is made carelessly.

What the signatures produced here contain

The completed document is signed and certified with a PAdES structure, which covers the bytes and detects later change. A timestamp is only attached when a timestamp authority address is configured, and in the deployed configuration that setting is empty, so signed files currently carry no RFC 3161 token and no embedded validation data. The recorded algorithm string reflects this: PAdES-SHA256-DocMDP without a timestamp, and PAdES-SHA256-DocMDP-RFC3161 when one is attached. For long lived documents, the practical mitigation is to keep the certificate of completion and the event record alongside the file. Storing those three together, named so they are obviously one set, is what makes them useful to somebody retrieving the matter years later, when nobody involved in the original transaction is still available to explain what happened.

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