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Five ways to prove a file existed on a date. Which should you use?

2026-10-08 · OpenStela

If you just need to show that a file existed by a certain date, a hash timestamp is the cheapest thing that holds up: OpenTimestamps on your own, or a registry that does it for you. If you're in the US and this is a work you'd actually go to court over, registering it with the Copyright Office does things no private method can. Emailing the file to yourself is the one to drop. The US Copyright Office says outright that it isn't a substitute for registration.

All five methods answer the same narrow question: did this exact file exist no later than this date? None of them decides who made it or who owns it. They differ in cost, speed, how many files you can realistically cover, and who can check the result without asking you. The table below sums it up. The sections after it explain each row, with links to each method's own documentation.

The five methods side by side

Fees and times checked 2026-10-08. Sources are linked in each section below.

1. Emailing the file to yourself

This is the habit people call "poor man's copyright": mail yourself the file, or post it to yourself in a sealed envelope, and keep the dated copy. The US Copyright Office's general FAQ answers it directly: "There is no provision in the copyright law regarding any such type of protection, and it is not a substitute for registration."

The technical problem is simple. The evidence sits in an account you control. You'd be asking someone to trust both the mail provider's records and you. It costs nothing and it's better than having nothing at all, but don't build on it. If it's already your habit, keep doing it as a backup and add one of the methods below.

2. A notary

A notary is a public officer, and their record is strong evidence of whatever they actually witnessed. What they witness is the catch. In Taiwan, Article 101 of the Notary Act (last amended 2019-04-03) says that when a notary certifies a private document, the signer signs it in front of the notary or confirms the signature as theirs. That's an act about the signature. A notary doesn't judge whether your drawing is original. What a notarised file-and-hash statement gives you is a public officer's dated record that this document, naming this hash, was presented by you on that day.

Cost is set by statute in Taiwan. A deed whose value can't be calculated is NT$1,000 (Art. 112), certifying a document costs half of the corresponding deed fee (Art. 120), and work outside office hours adds half (Art. 122). Notary rules and fees differ by country. If you're elsewhere, ask a local notary what their certificate actually covers.

The real cost is the visit. You do it once per document, so people notarise a finished album or a signed contract, not every draft.

3. Government copyright registration

Where it exists, this is the only option on the list with legal effects built into the statute. In the US the Copyright Office FAQ says registration is voluntary, because copyright exists from creation, but "you will have to register, however, if you wish to bring a lawsuit for infringement of a U.S. work." Registered works "may be eligible for statutory damages and attorney's fees," and registration within five years of publication is "prima facie evidence in a court of law."

What it costs, from the Office's fee schedule: $45 for a single author filing one work online, $65 for the Standard Application, $125 on paper. How long it takes, from the Registration Processing Times sheet covering cases closed 2026-04-01 to 2026-09-30: online claims with a digital upload and no back-and-forth averaged 3.4 months. Claims that needed correspondence averaged 5.3 months.

Two things to know if you work with AI. The Office's registration guidance of 2023-03-16 (88 Fed. Reg. 16,190) asks you to disclose AI-generated material in the application. That means the registration covers your human contribution, not the generated parts. Can AI art be copyrighted? goes through that in detail. And the fee is per filing, so it doesn't scale to fifty drafts a week.

Not every country has this. Taiwan doesn't. The Intellectual Property Office's copyright Q&A collection (October 2021) explains that under Article 10 of the Copyright Act, protection starts when the work is finished, with no registration at any agency. It adds that because copyright is a private right, you carry the burden of proving it, so keep drafts, backups, edit histories and discussions. That's exactly the gap the other four methods fill.

4. OpenTimestamps

OpenTimestamps is a free, open standard for blockchain timestamping. You hash the file (in the browser, so the file isn't uploaded) and public calendar servers batch the hash into Bitcoin. You keep a small .ots proof file next to the original. The servers are donation-funded and need no account or API key.

Per the client's README, it "takes a few hours" for the timestamp to be confirmed in Bitcoin. Until then the proof is incomplete, and you "upgrade" it to add the path into the chain. Verifying with the command-line client needs a local Bitcoin Core node (a pruned one is fine). The website also has an in-browser verifier.

It's honest about its scope: "A timestamp proves that some data existed prior to some point in time." That's all it says. The proof doesn't record what the file is, what you called it, or what you claim about it. If you lose either the original file or the .ots file, you can't verify anything. For a bare date on a hash, it's a very good answer, and you don't need anybody's service for it.

5. A registry like OpenStela

A registry does what OpenTimestamps does, plus it writes down what the hash is about. When you register a work, OpenStela builds an authorship record first: the file's SHA-256 fingerprint, the name you publish under, your stated creation date, the AI tools you used, your own description of what you contributed (required when AI was involved), and a rights declaration. That record is hashed and goes into a Merkle tree with that night's other records. The tree's 32-byte root is written to Arbitrum One. The file itself never goes on chain.

What you get on top of a bare timestamp: a public page others can read, a fingerprint anyone can paste into the verify page, and a Proof Pack you can download. The Proof Pack holds the record, its Merkle proof and the chain transaction, so the check can be redone offline without trusting us. How a registered work is verified walks through that recomputation. The record itself is frozen once written. You can only change the title, a note and whether it's public.

Cost: registering works is free, within 300 MB of uploaded files per account (links take almost none) and a daily cap on how many works one account can register. Characters are free up to 50 per account. See pricing. The limits are real. The record is only as good as the file you keep, and the contribution text is your own statement, which we don't check.

Which one to use, and when

Most creators end up combining them. Here's a reasonable default:

1. Every draft and every release: a hash timestamp. Use OpenTimestamps if you only want the date, or a registry if you also want your attribution and contribution on the record and a page you can link to.
2. Finished work you'd sue over, in the US: register it with the Copyright Office as well. Nothing private gets you the legal effects quoted above.
3. A contract, or a one-off document someone else must accept: a notary, if the other side or a court in your country expects one.
4. Emailing yourself: keep it as a habit if you like, but don't count it as one of your methods.

Whatever you choose, keep the original file byte-for-byte unchanged. Every hash method vouches for a fingerprint, and none of them can vouch for a file you edited or lost. Write down how you made it at the time, too. The older post How do I prove a file existed on a certain date? explains why that note is worth as much as the date. What does an ID actually prove? sets out the line between evidence and ownership.

This is general information, not legal advice.