Files get edited.
When they existed shouldn’t
be a matter of trust.
A record can be rewritten. A file can be replaced.
The service you used can disappear.
A proof from Existence Anchor doesn’t depend on us. With your file and its .ots proof, anyone can check it against the Bitcoin blockchain.
This page in one minute, without sound.
How a stamp is made
From your file
to a Bitcoin block.
Four steps, one chain of hashes anyone can follow. Your document never leaves your device.
Your browser computes
the fingerprint.
Drop a file. Your browser computes its SHA-256 digest on your device and sends only that digest, plus a label if you type one. The file itself never reaches us.
Illustration. On Bitcoin a certificate is never instant: the timestamp servers decide when to anchor, and the block then needs 6 confirmations.
New stampExample
Filehello-world.txt
SHA-256, computed in your browser03ba204e50d126e4674c005e04d82e84c21366780af1f43bd54a37816b6ab340
Digest ready · nothing uploaded
We blind it and send it
to four timestamp servers.
We add a random 16-byte nonce and hash again, so the timestamp servers never see your digest. You get a submission receipt at once: a receipt, not yet a certificate.
Illustration. On Bitcoin a certificate is never instant: the timestamp servers decide when to anchor, and the block then needs 6 confirmations.
SubmissionExample
Sent to the timestamp serversSHA-256(digest ‖ nonce)
Nonce16 random bytes, part of your proof
ProtocolOpenTimestamps, 4 timestamp servers
Receipt issued · anchoring
Timestamp servers anchor it
in Bitcoin.
Each timestamp server gathers many submissions into one Merkle tree, whose root is committed in a Bitcoin transaction. It is not instant, and the timestamp servers decide when.
Illustration. On Bitcoin a certificate is never instant: the timestamp servers decide when to anchor, and the block then needs 6 confirmations.
AnchoringExample
Proof pathcommitment → Merkle root → block header
Bitcoin transactionone transaction, many stamps
In a block · waiting for 6 confirmations
We check the block,
then issue the certificate.
The block header must come byte for byte identical from two independent block explorers, carry enough proof of work, and sit at least 6 blocks deep. Only then is a certificate issued.
Illustration. On Bitcoin a certificate is never instant: the timestamp servers decide when to anchor, and the block then needs 6 confirmations.
CertificateExample
Block headermempool.space = blockstream.info
Proof of workabove the minimum-work floor
Depth6 blocks or more
Certificate issued · .ots ready
The privacy boundary
Your file stays
on your device.
We receive a digest, never the document. The timestamp servers receive even less.
Gathered with other submissions into one Merkle root, committed in one Bitcoin transaction.
The file stays with you. The proof can be checked by anyone.
A timestamp alone does not show who wrote a document or who owns it.
What you get
One stamp.
Four things to keep.
A receipt at once. A portable proof once the digest is anchored, and a certificate once it is anchored and your plan covers it, while we still store its files.
Your filecontract_final.pdfhashed in your browser
digest→
Existence AnchorYour stampsubmitted to 4 timestamp servers
anchored→
01Submission receiptPDF, at once
02CertificatePDF, once anchored, as your plan allows
03.ots proofcheckable without us
04Status linkpublic: state, digest, time
Receipts, certificates and proofs are private to your account. The public status link shows only a stamp’s state, its digest and when we received it.
What a certificate says
One sentence
Bitcoin proves.
“The document with SHA-256 digest X existed no later than the time of Bitcoin block N (its timestamp T, ±~2 h miner tolerance), independently verifiable against the Bitcoin blockchain without trusting us.”
That sentence is all Bitcoin proves, and anyone can check it with the .ots file and a Bitcoin node, even if this service disappears. Who submitted the stamp and when we received it come from our own records: the certificate lists them apart, marked as stated by us.
What it does not say
Who wrote the document, or what it means.
That it did not exist earlier: only that it existed no later than the block.
An exact moment: a block’s timestamp is accurate to about ±2 hours.
Sample
PROOF OF EXISTENCEProof of Existence CertificateCryptographic timestamp anchored in Bitcoin with OpenTimestamps. It does not prove authorship, ownership or content authenticity.
WHAT BITCOIN PROVES – CHECKABLE BY ANYONE, WITHOUT TRUSTING US
The document with SHA-256 digest 03ba204e…6b6ab340 existed no later than the time of Bitcoin block 358391 (its timestamp 2015-05-28 15:41:18 UTC, ±~2 h miner tolerance), independently verifiable against the Bitcoin blockchain without trusting us.
Merkle root, from the proof and in the block header8a1b66ecb7cbd07d8139a7e7d7f2c41a ab1f5009b8364aaf61d03ad245e47e00
What we assert – stated by us, from our records. Bitcoin does not prove these.
Account[Your account]Issued by[Operator]
A sample in our layout, filled in with the hello-world proof that ships with the OpenTimestamps client, so every value on it can be recomputed. Not issued by us.
Where it helps
When a file date
isn’t enough.
Anyone can change a file’s date. Nobody can move a stamp to an earlier Bitcoin block.
Ideas
Show when your idea already existed.
Stamp a design, a manuscript or lab notes before you share them.
concept_v1.pdfContracts
The version everyone agreed to.
Stamp the signed file. Later, anyone can check whether a copy put in front of them is that exact file.
agreement_signed.pdfResearch
Results before publication.
Record that a dataset or a result existed before the paper went out.
results_dataset_v7.csvCreative work
Your work, before it goes out.
Photos, audio, source files: stamp them before you send them to a client or a platform.
shoot_2026-09-12.zipRecords
An export that can’t be backdated.
Stamp a ledger or a log export at the close of each period.
ledger_2026-09.csvSoftware
The release you shipped.
Stamp release artifacts straight from CI with an API key.
release-4.2.0.tar.gz
For developers
Stamps from
your own code.
Hash locally, send the 64-character digest with an API key, and fetch the proof once the stamp is anchored and the certificate once it is issued. Your files never leave your systems.
That is a stamp’s status. Its certificate is issued once the stamp is anchored and your plan covers it, while we still store the stamp’s files. API access comes with the Pro plan; each member creates their own keys and can revoke them at any time.
$ pip install opentimestamps-client
$ sha256sum hello-world.txt
03ba204e50d126e4674c005e04d82e84c21366780af1f43bd54a37816b6ab340 hello-world.txt$ ots verify hello-world.txt.ots
Assuming target filename is 'hello-world.txt'Success! Bitcoin block 358391 attests existence as of 2015-05-28 UTC
The stock OpenTimestamps client on its own hello-world example, run next to a Bitcoin Core node on a machine set to UTC. Nothing in it talks to us.
For the long term
A proof that stays
in your hands.
Keep the file and its proof together. They don’t need your account, or ours.
Download everything
The receipt, the certificate PDF and the .ots proof. The certificate and the proof also come together as one .zip.
Works if we disappear
The .ots file is a standard OpenTimestamps proof. The stock client and a Bitcoin node check it, with or without this service.
How long we store it
We store a stamp’s .ots proof and its certificate for 30 days from the stamp and, with a subscription, while it is current and 30 days after. A .ots you downloaded once the stamp was anchored keeps working after that.
Plans
Start free. Grow into a team.
Every plan anchors in Bitcoin and includes certificates. Each plan says how many stamps a month it covers, how long we store a stamp’s files, how many members it has and whether it includes API access.
Free
$0
no card needed
Up to 25 stamps a month (fair use)
Your .ots proof and certificate stored 30 days from each stamp
Your .ots proof and certificate stored while subscribed, and 30 days after
Certificates included
Up to 3 members
No API access
Available soon
Pro
$24a month
billed through Stripe
Up to 10,000 stamps a month (fair use)
Your .ots proof and certificate stored while subscribed, and 30 days after
Certificates included
Up to 25 members
API access
Available soon
A stamp beyond the monthly allowance is still anchored, and its .ots proof still verifies; only its certificate waits, and it is issued if the account’s plan comes to cover it while we still store the stamp’s files. A month is a calendar month in UTC.
The files we store for a stamp are its .ots proof and its certificate; we delete them after that time, together with the stamp’s label, and from then on no longer provide the stamp’s receipt. The rest of our record of the stamp stays, its digest included, and so does its public status page. Once a stamp is anchored the proof itself is permanent: a .ots downloaded then verifies without us. Subscribed means the subscription is active, in a trial, or retrying a failed payment.
Prices in US dollars (USD). Paid plans renew automatically each month or year until cancelled; we email a reminder before each renewal. The account’s admin can cancel at any time in Plan and billing, and the plan runs to the end of the period already paid for; a failed renewal payment is still retried, and if no retry succeeds, the plan ends after the last one. At checkout you ask for the service to start at once and so give up the 14-day right of withdrawal; payments are not refunded. Refunds and cancellation →
TeamsTurn your account into an organisation and invite members by email. One admin; everyone shares the plan.
API keysOn Pro, every member can create keys and revoke them. A removed member’s keys stop working.
BillingPaid through Stripe. Change plan, card or cancel in the customer portal.
