RFC 3161 timestamp

An RFC 3161 timestamp is a signed token from an outside time stamping authority that proves a given hash existed at a given moment.

In plain words

RFC 3161 is the internet standard for trusted timestamps. The flow is simple. Your software takes a hash of a file and sends only that hash to a time stamping authority. The authority adds the current time, signs the pair with its own key and sends the token back. The authority never sees the file.

Later, anyone can check the token against the file. If the hash matches and the authority's signature is good, the file existed in that exact form at that time.

Why it matters

Without an outside clock, the only time on a signed file is the one the signing computer reported. A person can set their own clock. A timestamp from a third party removes that doubt.

It also helps with keys. If a signing key is later revoked, a timestamp made before the revocation can show the signature came first. That is why the PAdES B-T level is defined by adding one.

An example

A sales agreement is sealed on 3 March at 14:02. Two years later, someone suggests the PDF was swapped in afterwards. The file's timestamp token, signed by an outside authority, says this exact file existed at 14:02 on 3 March. The question moves from "when" to "who", which the audit trail answers.

How it shows up in docuinked

Every sealed docuinked PDF carries an RFC 3161 timestamp from an outside authority inside its PAdES seal. docuinked also timestamps checkpoints over its event log every hour, so a rewrite of the log after a checkpoint would show. The public PDF checker validates the timestamp chain as well as the seal.

Common confusions

A timestamp is not a date field. The date a signer sees on the page is text. An RFC 3161 token is a signed proof. Both can appear on one document and mean different things.

Expired timestamp messages. Some readers say a timestamp "has expired" once the authority's certificate expires. That refers to the certificate's age, not to a change in the file.

Sources

Checked 2026-11-07.

Common questions

Does the time stamping authority see my document?

No. Only the hash leaves the system. A hash can't be turned back into the file, so the authority learns nothing about the contents.

Why does a PDF reader say the timestamp has expired?

The authority's own certificate has passed its end date. The file can still be unchanged. Some readers let you accept expired timestamps in their settings.

Is the timestamp the signing time or the completion time?

In docuinked it is the time the finished file was sealed, after the last signer. Each signer's own signing time is in the evidence log.

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