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Documents and contracts

How to prove what the contract said, and since when

When the other party produces a different version of the annex, your copy is worth the same as theirs. The exact version, sealed the day it was closed, is not.

Seal the document

Private. Only the file fingerprint is registered.

Word against word “That’s not the annex we signed.” “The clause was different.” “You changed that filing afterwards.” Two PDFs, two versions and six emails with attachments called “final_v3_OK”.
The dated proof wins The exact version of the document, sealed the day it was closed. What it said, down to the comma, and since when. No longer your copy against theirs: a date and a fingerprint.
Cents against a clause One disputed penalty, deposit or invoice is worth more than sealing every contract you close all year. And today, sealing costs nothing.

The scene

“That’s not the annex we signed.”

You closed a service contract in January: the contract and an annex with the scope and the penalties for delay, signed as a PDF and sent by email. In September there is a delay and the client claims the penalty “as per the annex”. Their annex says 2% per week. Yours says 0.5%. Both are signed, both arrived by email, and the thread holds six attachments called “annex_final_v3_OK.pdf”. You know which one was final. But you cannot prove that your copy is the one that existed in January and not one edited later. Neither can the client.

The ritual: before you send it, and when it comes back signed

  • Seal the final version before you send it: the contract, the quote, the report or the filing, exactly as it leaves your computer. That is the date you will be able to prove.
  • When it comes back signed, seal the signed PDF or the scan too. That is the document that will be argued over, not the draft.
  • Every annex, addendum or amendment gets its own seal. What changes later is sealed later: that is how you show which version came first and which came after.
  • If you file it with a public body, seal the exact file you submit and the registration receipt you get back. You keep together what you filed and when it was recorded.
  • With the free account, Sellat Local computes the fingerprint on your computer and the contract is not uploaded. Only its fingerprint leaves.
  • Keep the exact file and the certificate in the client’s or the case’s folder. If someone produces another version, check theirs against your seal: if it matches, it is the one that existed that day; if not, it is not.

Just want to certify a document, without the story? Certify a document →

What Sellat proves in this dispute

Sellat does not say whether the contract is valid, who signed it or who is right. The contract itself, an agreement or a judge decides that. Sealing does not sign anything either: that is something else. Sellat proves one concrete thing: that this exact PDF, with that clause and that figure, existed on the date you sealed it and that nobody has modified it since. If both parties seal the same final version, the proof is the same for both: the fingerprint matches.

What you keep after sealing

When you seal, you receive a PDF certificate and a verification ZIP by email. That ZIP is what you forward when the other version appears: it holds the fingerprint of the exact document, the date and the instructions for the other party, their lawyer or whoever needs to check it on their own, without trusting you or Sellat. Sellat does not keep your contract: with Sellat Local, it never even receives it. Only its fingerprint.

An email proves you sent something. Sellat proves what it said.

An email attachment gets forwarded, renamed and saved again; versions multiply, and the mail server belongs to someone else. And a signed PDF proves who signed, not that the copy you show today is the one that existed back then. Sealing the final version does for your document what a registry stamp does for a filing: it fixes exact content and a date that depend on neither your word nor the other party’s. It works for a two-page contract or a case file with forty annexes: you seal the file, or one ZIP with all of them.

Inside the package

This is exactly what you open two years later.

Not a promise and not a screenshot: the real list of files Sellat puts in your ZIP. What you see here is generated by the same module that builds the package.

Open the package Close the package
  • original/<archivo> If the original is kept

    Your file, byte for byte

    The exact photo you sealed, in a folder of its own so no other file in the package can be mistaken for it. It travels inside when the original is available: the upload that created the proof, or custody.

  • certificado-<id>.pdf Always

    The readable certificate

    The PDF you show a person or attach to a filing. It is an informative document: what actually gets checked is the file, not the PDF. If the file carried Content Credentials (C2PA), the certificate records them too — who signed, with which tool and whether they declare AI use — exactly as your browser read them when you protected it.

  • proof.json Once the batch is anchored

    The portable proof

    Your file fingerprint, its Merkle leaf and inclusion path, the batch root and the public transaction that records it. This is the piece that lets anyone check everything without Sellat.

  • sello-cualificado-<id>.tsr With a qualified seal

    The qualified seal

    The RFC 3161 response from the qualified timestamping authority, exactly as it was issued. This is the file that fixes the date under eIDAS.

  • autoridad-sellado.pem With a qualified seal

    Who signed the seal

    That authority signing certificate, with its SHA-256 written into the README. It travels inside so the check works offline and nobody has to go looking for it.

  • anclaje-bitcoin.ots Once the batch is anchored

    The Bitcoin anchor

    The OpenTimestamps proof of the batch root. It may still be waiting for a block: it is a valid file and whoever receives it can upgrade it themselves.

  • LEEME.txt / README.txt Always

    The instructions, in two languages

    What each file is, what it proves and what it does not, and the exact commands to check it. In Spanish and English, so an expert or a lawyer never has to ask you anything.

The seal is signed by FNMT-RCM, not by Sellat.

Sellat is not the party attesting the time. The qualified seal is issued by a qualified timestamping authority of the FNMT-RCM, and its signing certificate travels inside your package. We are the intermediary that joins your fingerprint to that seal and hands it to you in a tidy folder.

It checks out without us and without the internet.

With the seal and the authority certificate inside the ZIP, the date is verified with standard tools on the machine of whoever receives it. If Sellat disappeared tomorrow, your package would still check out.

The package proves that this exact file existed on that date and has not been modified. It does not prove who created it, who took the photo, or what happened in front of the camera.

Seal the final version before you send it. And the signed one, when it comes back.

One disputed clause is worth more than sealing every contract you close all year. Today, sealing costs nothing. Upload the exact PDF and keep the certificate with the case file.

Seal the document

Frequently asked questions

Before you ask.

Does it work for a PDF that is already signed?

Yes, and that is the one worth sealing: the signature travels inside the file, and the seal fixes that this signed PDF existed on that date. Sealing does not sign anything and does not replace a signature: it adds the date and the integrity of the version you both already signed.

What if the other party has a different version?

Check their copy against your seal. If it matches, it is the version that existed on the day you sealed; if it does not, it is another file and nobody can say it existed then. Cleanest of all is for both parties to seal the same final version: the fingerprint is the same for both.

Does it work for filings with a public body?

Yes. The registry gives you a receipt with the filing date; sealing the exact file you submitted, and the receipt, also leaves you with what that filing said, down to the comma. If the content of an application, an objection or an appeal is disputed later, you have both: when it went in and what went in.

What about the qualified eIDAS timestamp?

If you ask for it when sealing, the fingerprint of the document also receives the qualified timestamp, with the seals in your account. It enjoys the legal presumption of accuracy of date and time and of integrity of the data across the European Union (art. 41.2 of Regulation (EU) 910/2014).

Will this hold up in court?

Sellat does not promise that a proof will hold up in court, and it cannot: the judge or arbitrator in each case is the one who weighs the evidence. What you can bring is technical evidence, verifiable by anyone, that that contract and its annexes existed on that date and have not been modified since. On the legal page we explain precisely what Sellat proves and what it does not.

How much does it cost?

Sealing is free with the free account, with the fingerprint computed on your computer. The limits of each account are on the pricing page.

Can anyone see my file?

It depends on the mode. In the normal flow the file is uploaded to Sellat only to calculate its SHA-256 fingerprint and prepare the proof; afterwards it is not retained. With a free account, Sellat Local calculates the fingerprint on your own device and the original is never uploaded. And if you turn on custody — optional, and only with an account — Sellat does keep the original, checked against its fingerprint, until you remove it. In all three cases the only thing in the public record is the hash, a code that cannot be used to read or rebuild the content.

Certify a contract with a date: proving the exact version of a document

Proving what a contract, a quote or a filing said has two parts: the exact content and the date since which it existed that way. An email or a signature proves that something was sent or who signed; a sealed version proves what was inside and since when. For freelancers, firms and small businesses, that is the difference between arguing over a clause and showing it. Technical detail, in case you need it: Sellat computes the SHA-256 fingerprint of the exact file (on your computer with Sellat Local, if you have an account), records it in a public reference on Polygon and on Bitcoin through OpenTimestamps, and sends you a PDF certificate and a verification ZIP. In the normal flow the document is not kept after the proof is created: only its fingerprint. With an account you can enable custody so that Sellat keeps it too, encrypted. Whoever receives the proof can check it with their exact copy, without an account and without trusting Sellat. For firms that close documents every day, there is an API.