Bitcoin timestamp proof for a file: what it shows and how to check it

What a Bitcoin timestamp proof contains

Most Bitcoin timestamp proofs, whatever service made them, have the same four parts:

1. The file’s fingerprint. A SHA-256 hash calculated from the file’s exact bytes. Change one character or pixel and the fingerprint changes completely.
2. A path of hash steps. Services usually combine many fingerprints into one value (a Merkle root) so that one transaction can cover thousands of files. The proof lists the steps that lead from your fingerprint to that value.
3. The Bitcoin transaction. The transaction that carries the combined value, often in an OP_RETURN output, which is a small data field in a transaction.
4. The block. The block that includes the transaction. Each block header holds a Merkle root of all its transactions and a time, so the transaction is tied to that block and to every block after it. The Bitcoin developer reference explains the header format.

The original file is not part of the proof. Only the fingerprint is recorded, so the proof can be checked later by anyone who has both the file and the proof.

What it proves, and what it doesn’t

A valid proof shows two things:

• Existence by a time. The fingerprint was recorded no later than the block. Because it is not practical to make a new file that matches an existing SHA-256 fingerprint, the file must have existed by then.
• Integrity. The file you have now is byte-for-byte the same file. If it had been edited, even by one byte, the fingerprints would not match.

It does not show:

• who created the file, or who owns it;
• that the file didn’t exist earlier, or that nobody else had something similar;
• that the content is true or accurate;
• whether AI was used to make it.

Those questions need other evidence, such as working files, drafts, emails and witnesses. A timestamp is one dated piece of a wider record. Our guide to how to prove a file existed before a date compares it with the other methods.

How precise is a Bitcoin block time?

The time in a block header is set by the miner. Bitcoin’s rules put limits on it: according to the Bitcoin developer reference, it must be later than the median time of the previous 11 blocks, and full nodes will not accept a block whose time is more than two hours in the future by their own clocks.

In practice, that means:

• treat a block time as accurate to within a couple of hours, not to the minute;
• describe the result as “existed no later than block [number], mined around [date]”;
• if minutes matter, for example in trading or bidding disputes, use a signed RFC 3161 timestamp as well. Our RFC 3161 vs blockchain timestamp comparison explains the difference.

For most creative, research and business records, the question is about days, months or years, and a Bitcoin block time is more than precise enough.

How to check a Bitcoin timestamp proof yourself

You don’t need to trust the service that made the proof. You can check each step:

1. Fingerprint the file. Calculate its SHA-256 hash with the free SHA-256 hash checker, which runs in your browser, or with your computer’s own tools (see how to check a SHA-256 checksum on Windows, Mac and Linux).
2. Compare it with the proof. The fingerprint must match the one in the certificate or proof file exactly.
3. Follow the path. Repeat the listed hash steps from your fingerprint and confirm you arrive at the value in the transaction. Open-source tools can do this for OpenTimestamps .ots files; see how to verify an OpenTimestamps file.
4. Look up the transaction. Find the transaction ID in a public Bitcoin block explorer, or in your own Bitcoin node, and note the block number and time.
5. Keep the file unchanged. Check against the original file, not an exported, resized or re-saved copy, because a copy usually has a different fingerprint.

If any step fails, the proof doesn’t cover that file. The most common reason is a different copy of the file, not a broken proof. See how to check a file still matches its fingerprint.

How TimeBinder’s Bitcoin timestamps work

TimeBinder^® makes the fingerprint in your browser, and the file stays on your device. The fingerprint is recorded on the Bitcoin blockchain, and you get a Proof of Time Certificate.

• An ASAP TimeBind gets its own Bitcoin transaction, broadcast straight away, and can be checked independently using the steps above.
• A Pooled TimeBind is batched with other fingerprints, is usually recorded within 24 to 48 hours, and is checked through TimeBinder.

How blockchain timestamping works at TimeBinder has the full step-by-step. If you’d rather use the free command-line route, see OpenTimestamps alternatives for an honest comparison.

When a Bitcoin timestamp is the wrong tool

• You need the time to the second. Use an RFC 3161 timestamp.
• You need the EU’s legal presumption for qualified time stamps. That comes from a qualified trust service provider under eIDAS, not from Bitcoin anchoring.
• You need to store or back up the file. A timestamp keeps no copy. Keep your originals safe yourself.

See the glossary for proof of work, Bitcoin node and block height.

Get a Bitcoin timestamp for your file

Your first Pooled TimeBind is free, with no credit card required. After that, TimeBinds start from US$5, and an ASAP TimeBind with its own Bitcoin transaction is US$15. See plans and pricing