Proof of existence is a way to show that a specific file existed no later than a certain time, without revealing what the file says. You calculate the file’s cryptographic fingerprint (a hash) and record that fingerprint somewhere public and hard to change, usually the Bitcoin blockchain. Later, anyone with the same file can recalculate the fingerprint and find the dated record. It proves existence and integrity. It does not prove who made the file, who owns it, or that its content is true.
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Where the idea came from
The idea of dating a document by publishing its hash is older than Bitcoin. Bitcoin made it easy to do without a trusted middleman. In 2013 a service called Proof of Existence began embedding the SHA-256 digest of a document in a Bitcoin transaction, using an OP_RETURN output, a small data field in a transaction. Its documentation explains that the documents themselves are not stored, only the digest, and that once the transaction is confirmed the document is “proven to exist at least as early as the time the transaction was confirmed”.
That original service is still online, and later tools do a similar job. See our comparison of Proof of Existence alternatives, including OpenTimestamps and a TimeBind.
Later projects such as OpenTimestamps combined many hashes into one transaction, so one Bitcoin transaction can cover a very large number of files at no extra cost per file. Most blockchain timestamping services, including TimeBinder, follow the same basic approach.
How proof of existence works, in three steps
- Fingerprint. A hash function such as SHA-256 turns the file’s exact bytes into a short code. The same file always gives the same code. Change one byte and the code changes completely.
- Record. The fingerprint, or a combined value that includes it, is written into a Bitcoin transaction. Once the transaction is in a block, changing it would mean rewriting every later block.
- Check. Later, you fingerprint the file again and follow the proof to the transaction and block. If everything matches, the file existed in that exact form no later than the block’s time.
For the details of each step, including how precise block times are, see Bitcoin timestamp proof for a file: what it shows and how to check it.
What proof of existence proves, and what it doesn’t
| It shows | It does not show |
|---|---|
| The exact file existed no later than the recorded time | Who created the file |
| The file has not changed since, if the fingerprint still matches | Who owns it, or any copyright |
| That nobody had the same idea or a similar file earlier | |
| That the content is true or accurate |
Some early services described proof of existence as a way of “demonstrating data ownership”. Be careful with that idea. A fingerprint shows that someone had the file by a certain time. It doesn’t show they made it, or that they own the rights to it. Ownership and authorship are shown with other records, such as working files, drafts, contracts and witnesses. Our guide to what evidence helps with copyright without registration covers those. For how courts treat these records, see is a blockchain timestamp admissible in court?
What people use it for
- Creative drafts: manuscripts, lyrics, demos, design files and artwork, before they are shared or submitted.
- Business records: contracts, proposals, bids and specifications, so a later version can be told apart from the one that was sent.
- Research and code: datasets, notebooks and source code at key points in a project.
- Inventions: dated invention notes before a pitch or a patent filing.
Privacy: what a public fingerprint can reveal
A fingerprint doesn’t reveal the content of a long or unique file. Two things are still worth knowing:
- Anyone who already has the same file can match it. If you share the proof or the fingerprint, a person holding an identical copy can confirm it is the file you recorded.
- Short, guessable content can be guessed. If the “file” is a single short phrase or a number from a small range, someone could try guesses until one produces the same fingerprint. Record whole files, not short snippets.
Proof of existence with TimeBinder
TimeBinder® is a proof of existence service built for people who don’t want to use a command line. The fingerprint is made in your browser and the file stays on your device. The fingerprint is anchored to the Bitcoin blockchain, and you get a Proof of Time Certificate that can be checked later. See how blockchain timestamping works at TimeBinder, or compare it with free blockchain timestamp options and with emailing yourself, Git dates or a notary.
Frequently Asked Questions
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Is proof of existence the same as a blockchain timestamp?
In practice, yes. “Proof of existence” describes what you get, and “blockchain timestamp” describes how it is made. Both record a file’s fingerprint in a blockchain to show the file existed by a certain time.
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Does proof of existence store my document?
No. Only the fingerprint is recorded. You need to keep the original file safe yourself, because the proof can only be checked against the exact same file.
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Can proof of existence prove I created something first?
Not on its own. It shows your file existed by a certain time. It doesn’t show that nobody created something similar earlier, or who the author was.
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Is proof of existence legally valid?
Courts decide how much weight to give any evidence. Some laws say electronic or blockchain records can’t be rejected just because they are electronic. This is general information, not legal advice.
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What happens if the service I used shuts down?
If the proof leads to a public Bitcoin transaction and you keep the proof and the original file, you can still check it yourself. Keep both safe. See our note on proof services that have closed.
Record proof of existence for your files
Your first Pooled TimeBind is free, with no credit card required. After that, TimeBinds start from US$5, and an ASAP TimeBind is US$15. See plans and pricing
