How to verify an OpenTimestamps file (and what the result means)

What’s inside an .ots file

OpenTimestamps is a free, open-source way to timestamp data on the Bitcoin blockchain. When you stamp a file, the tool works out a hash of it. A hash is a fingerprint of the file: a short code calculated from its exact contents. Change one character and you get a completely different fingerprint. Your file itself isn’t sent anywhere; on opentimestamps.org, the page notes that the hash is calculated in your browser.

The .ots file is a small proof file that sits next to your original. It holds:

• the fingerprint of your file
• a list of steps that link that fingerprint, through other fingerprints, to a single value
• an “attestation”: a note saying where that value was recorded, such as a particular Bitcoin block, or, while it’s still waiting, which calendar server is holding it

The calendar servers are free public servers that collect many timestamps and record them together in one Bitcoin transaction. The OpenTimestamps client README explains that “we’re not doing one transaction per timestamp”, which is why a new proof is pending for a while.

The README also notes that file contents are protected with nonces (random extra data), so a calendar server “learns nothing about the contents of anything you timestamp”. One side effect: you can’t rebuild a lost .ots file from the original alone. Keep them together.

How to verify an OpenTimestamps file without the command line

This is the simplest method, and it’s the one most people need.

1. Go to opentimestamps.org.
2. In the Stamp & Verify area, drop in the .ots proof file.
3. When asked, drop in the original file as well. The page needs both: it says that to verify, you need to provide the file and its .ots receipt.
4. The page works out the file’s fingerprint in your browser and compares it with the one in the proof.
5. If the proof was still pending when it was saved, the page tries to upgrade it (fetch the finished Bitcoin path from the calendar) before checking.

A successful check shows a message in this form: “Bitcoin block [number] attests existence as of [date]”.

Good to know: the web verifier looks up Bitcoin block details through a public block explorer service rather than your own copy of the blockchain. For most people that’s fine. If you need to check without relying on any outside service, use the command-line method with your own Bitcoin node.

The opentimestamps.org layout may change. Check the page’s current labels on publishing day.

Method 2: the ots verify command

The official Python client suits developers and anyone who wants to run the check on their own machine. According to its README on GitHub:

1. Install it with pip3 install opentimestamps-client (it needs Python 3).
2. Put the original file and its .ots file in the same folder.
3. Run ots verify yourfile.ots. The client assumes the original is the same name without “.ots”. If the original has a different name, point to it with -f.

The README is clear about one requirement: while you can create timestamps without your own Bitcoin node, “to verify timestamps you need a local Bitcoin Core node (a pruned node is fine)”. You can point the client at your node with the --bitcoin-node option.

Two other commands are handy:

• ots upgrade yourfile.ots adds the finished path to the Bitcoin blockchain into the proof file itself, so it no longer needs the calendar server.
• ots info yourfile.ots shows what’s inside the proof, including the file’s SHA-256 fingerprint.

Reading the result

A successful check looks like the README’s example:

Success! Bitcoin block 358391 attests existence as of 2015-05-28 CEST

In plain English:

• The fingerprint matched. The file you supplied is byte-for-byte the same file that was stamped.
• The proof leads to a real Bitcoin block. That block was mined at the date shown.
• So the exact file existed no later than that block’s time. It may have existed long before. The timestamp only sets the latest possible date.

What it doesn’t tell you: who made the file, when they started it, whether the contents are true, or whether AI was used. It also can’t tell you how a court, platform or employer will treat it.

Common problems and how to fix them

”Pending confirmation in Bitcoin blockchain.” The calendar hasn’t had its transaction confirmed yet. The README says it “takes a few hours” for a timestamp to be confirmed. Wait, then run ots upgrade (or re-drop the files on the website) and check again.

A “FAILURE!” message or a failed check. The file isn’t the exact one that was stamped. Opening and re-saving a document, exporting a photo again, or even changing metadata can change the fingerprint. Find the original copy, not a later save. You can compare fingerprints with our free SHA-256 file hash checker.

Lost .ots file. Without it, there’s no practical way to connect the file to the Bitcoin record. Store the .ots file with the original, and back both up.

Proof was never upgraded and the calendar is unavailable. An un-upgraded proof depends on the calendar server to finish the path. Upgrading as soon as a proof is confirmed makes it self-contained. The README notes that past timestamps stay verifiable “provided that the relevant calendar data is available”.

Error messages from the client. The README lists known issues, including Python package conflicts. Check there first.

How TimeBinder verification compares

TimeBinder also records a file’s SHA-256 fingerprint on the Bitcoin blockchain, and also never uploads your file: it’s fingerprinted on your device. The difference is mostly in what you keep and how checking works. For a fuller side-by-side, see TimeBinder vs OpenTimestamps.

• ASAP TimeBinds (3 Tokens) get their own Bitcoin transaction. They can be checked independently with the certificate and the Bitcoin blockchain, without TimeBinder.
• Pooled TimeBinds (1 Token) are batched with others and anchored, usually recorded within 24 to 48 hours, and often sooner, depending on Bitcoin network activity. They’re checked through TimeBinder.

Each TimeBind comes with a readable certificate: a PDF with a QR code and the Bitcoin transaction ID. A TimeBinder certificate isn’t an .ots file, so use the steps in how blockchain file verification works rather than the OpenTimestamps tools. To choose between the two types, see ASAP vs Pooled verification. For the step-by-step check of a TimeBind, see how to check a file still matches its fingerprint.

OpenTimestamps and TimeBinder compared

If you’re comfortable keeping .ots files safe and you don’t need a certificate for someone else to read, OpenTimestamps may be all you need. It’s free and open. TimeBinder suits people who want a guided app and a certificate they can share.