How to Prove a Document Existed at a Certain Time
Whether it's a contract dispute, a copyright claim, an invention idea, or a receipt, "prove it existed before this date" comes up more often than you'd think. Here's how to do it with Bitcoin.
The 3-step method
1. Stamp it now
Go to satohash.io/stamp, drop your file in, and click stamp. Satohash hashes the file in your browser (the file never leaves your device) and anchors the fingerprint to Bitcoin via OpenTimestamps. It's free, no account needed.
2. Keep the proof package
You get two things to keep together:
- Your original file — unchanged, on your device.
- The
.otsproof file — the timestamp receipt.
Store them together (same folder, a USB stick, encrypted cloud, or print the certificate). The proof is useless without its matching file, and the file is "undated" without the proof.
3. Verify anytime, anywhere
Anyone — you, a judge, an opposing lawyer — can verify the proof:
- Easily: open your proof card at
satohash.io/p/<your-hash>or use the interactive verify page. - Independently: upload the
.otsfile to opentimestamps.org, or verify locally with the freeotscommand-line tool against a Bitcoin node. Satohash doesn't need to exist for this to work — the proof is self-contained.
What the proof shows
When verified, the proof reveals the Bitcoin block your fingerprint was anchored in — a specific block number with a specific timestamp. Because Bitcoin blocks are chained cryptographically and globally agreed upon, that block's time is effectively undeniable proof that your document existed before it was mined.
Real-world uses
Common mistakes to avoid
- ❌ Losing the
.otsfile — without it there's no proof - ❌ Editing the file after stamping — the hash won't match anymore (stamp the new version too)
- ❌ Keeping only a screenshot — keep the original bytes and the
.ots - ❌ Not timestamping early enough — stamp at the moment of creation, not after a dispute begins
Stamp your first document free: satohash.io/stamp