How to prove a file existed on a certain date.

The principle

You do not need to reveal a file to prove it existed. You only need to publish its fingerprint somewhere permanent and independently timestamped. A blockchain is exactly that. Because the fingerprint can only come from that exact file, and the blockchain’s timestamp is set by the network, the result is tamper-evident proof of existence you can hand to anyone.

Step 1 — Fingerprint the file

Compute the file’s SHA-256 hash. On the command line:

shasum -a 256 mydocument.pdf
# or on Linux:
sha256sum mydocument.pdf

You will get a 64-character hex value. Prefix it with 0x. This is your fingerprint — it reveals nothing about the file and cannot be reversed. (Prefer not to touch a terminal? Drop the file on the notaros home page to see its fingerprint computed in your browser.)

Step 2 — Anchor the fingerprint to a blockchain

Record the fingerprint on a public chain so a block timestamp binds it in time. You can submit it to OpenTimestamps yourself for free, or let notaros do the full job: notarize the document and notaros anchors the fingerprint to Base and Bitcoin, fronts the on-chain cost and gas, and returns a certificate. Either way, the date comes from the blockchain — not from notaros.

Step 3 — Keep the certificate

Save the proof certificate: {hash, algorithm, chainId, contract, txHash, blockNumber, logIndex, timestamp} and, for the Bitcoin tier, an .ots proof. It is self-contained — no account or service is needed to read it later.

Step 4 — Verify it (and let others verify it)

To confirm the proof, re-hash the file and read the on-chain anchor. The free verifier does this in your browser without contacting notaros, or you can do it by hand with cast and ots verify. This is the part that matters: the person you are proving something to can check it themselves.

Free vs. full-service — which should you use?

A caveat worth stating

This proves existence and integrity in time — a genuinely useful fact — but not authorship, ownership, or truth of contents, and its legal weight varies by jurisdiction. See what proof of existence does and does not prove.