Understand what btc3r inscribes, and how.
You don’t need to be an engineer to entrust something to the blockchain. This page explains the four foundations of the service: OP_RETURN, the SHA-256 fingerprint, the choice of Bitcoin, and the verification you can perform yourself, without us.
OP_RETURN: the ledger’s margin
A Bitcoin transaction is first and foremost about transferring value. But the protocol has always included a scripting language. And that language has one particular instruction: OP_RETURN.
80 bytes of free data
OP_RETURN lets you attach up to 80 bytes of arbitrary data to a transaction: a short text, or a SHA-256 fingerprint (32 bytes). This format has been relayed as standard by network nodes since Bitcoin Core 0.9, released in 2014.
An unspendable output
The OP_RETURN output carries no value: it is marked as permanently unspendable. It serves one purpose only — to carry information in the block, like a note in the margin of a ledger.
Respectful of the network
Because it is unspendable, this output does not need to be kept in the nodes’ working memory (the UTXO set). It is the protocol’s intended, accepted mechanism for inscribing data. The transaction pays its fees like any other.
In short: when btc3r inscribes your message or fingerprint, it builds a Bitcoin transaction whose OP_RETURN output contains your data, signs it, then broadcasts it to the network. Once included in a block, it becomes part of the ledger just like any other transaction.
SHA-256: a file’s digital fingerprint
SHA-256 is a cryptographic hash function standardised by NIST, the US standards institute (FIPS 180-4). It is also the function at the heart of Bitcoin itself. Three properties make it valuable.
The same content always produces, everywhere, the same 64-character hexadecimal fingerprint — whether computed by your browser, a court expert or a server, today or twenty years from now.
Changing a single character upends the entire fingerprint: this is the “avalanche effect”. Comparing two fingerprints is therefore enough to know whether two files are strictly identical.
It is practically impossible to reconstruct content from its fingerprint, or to craft two different contents sharing the same fingerprint. Publishing a fingerprint reveals nothing about the document.
A single capital letter’s difference: two utterly unrelated fingerprints. It is this property that makes the fingerprint a tamper-proof signature of the exact content. You can reproduce this calculation with any SHA-256 tool: the result will be identical.
Why Bitcoin, and not another blockchain?
A permanent inscription is worth only as much as the ledger that carries it. Our criterion is neither fashion nor speed: it is the probability that the ledger will still exist fifty years from now.
Proven longevity
Bitcoin has run without interruption since its first block, mined on 3 January 2009. The text message inscribed in that genesis block (a newspaper headline of the day) is still readable today: the best demonstration of durability there is.
Massive replication
Thousands of independent nodes, on every continent, each keep a full copy of the chain. Erasing your inscription would mean making all these copies vanish at once, across parties who do not know one another.
The cost of rewriting
Altering a past block would require redoing all the proof-of-work that follows it, faster than every miner on the network combined. That economically prohibitive cost is the mathematical guarantee of immutability.
Other blockchains are faster or cheaper, but none combines seventeen years of uninterrupted operation, this replication and this computing power. For a service whose promise is permanence, choosing Bitcoin is not ideological: it is prudential.
Verify an inscription yourself, without btc3r
This is the cardinal principle of the service: you don’t have to take our word for it. Every inscription is verifiable end to end with free, public tools. Here is how.
Get the TXID
This is the transaction identifier, a 64-character string shown on your btc3r certificate. It uniquely identifies your inscription across the whole chain.
Open a public explorer
Go to mempool.space or blockstream.info and paste the TXID into the search box. The transaction appears, along with the block that contains it and its timestamp.
Read the OP_RETURN output
Among the transaction’s outputs is the OP_RETURN output. Explorers show its content in hexadecimal, and usually its decoding into text: this is your message, or your fingerprint.
For a document: compare
Recompute the SHA-256 of your original file (with the tool of your choice) and compare it to the inscribed fingerprint. If they match, the inscription stands for that exact file, as of the block’s date.
This verification involves btc3r at no point: it relies solely on the certificate you hold and the public ledger. That is what sets a blockchain proof apart from a mere provider’s attestation.
What an inscription is worth
Under French law, proof of facts is free (article 1358 of the Civil Code): a blockchain timestamp is admissible as evidence, whose weight the judge assesses. The European eIDAS Regulation (No 910/2014, article 41) further prohibits denying an electronic timestamp legal effect solely because it is not “qualified”. An inscription replaces neither an authentic instrument nor a legal deposit, but it provides a certain date and integrity verifiable by anyone, indefinitely. The subject is developed, provision by provision, on our proof of anteriority page.
The words of inscription
| Term | Definition |
|---|---|
| TXID | Unique identifier of a Bitcoin transaction (64 hexadecimal characters). It is the “reference number” of your inscription: it appears on your certificate and lets you find it on any explorer. |
| Block | A page of the ledger grouping transactions, added to the chain roughly every ten minutes. Each block is numbered and timestamped, and references the previous block — hence the word “chain”. |
| Confirmation | A transaction is “confirmed” once it is included in a block. Each block added afterwards is a further confirmation, making rewriting ever more improbable. |
| Node | A computer running the Bitcoin software, which verifies every transaction and keeps a full copy of the chain. Thousands of independent nodes operate worldwide. |
| Mempool | The queue of transactions broadcast to the network but not yet included in a block. Your inscription waits there between broadcast and confirmation. |
| Fingerprint (hash) | The result of a hash function such as SHA-256: a compact, unique identifier of an exact content. See the SHA-256 section above. |
| OP_RETURN | A Bitcoin protocol instruction allowing up to 80 bytes of data to be attached to a transaction, in an unspendable output. See the OP_RETURN section above. |
| Mainnet | The main Bitcoin network, the one that counts. As opposed to the test networks used by developers. All btc3r inscriptions are made on mainnet. |
Learning: what people ask us
What is OP_RETURN in a Bitcoin transaction?
OP_RETURN is a standard instruction in Bitcoin’s scripting language that lets you attach up to 80 bytes of arbitrary data to a transaction. The output it creates is marked unspendable: it carries no value, it carries information. It is the protocol-recognised mechanism for inscribing data, relayed as standard by nodes since Bitcoin Core 0.9 (2014).
What is a SHA-256 fingerprint?
SHA-256 is a cryptographic hash function standardised by NIST (FIPS 180-4). It turns any digital content, whatever its size, into a fixed 256-bit fingerprint, i.e. 64 hexadecimal characters. The same content always produces the same fingerprint; the slightest change produces a completely different one; and it is practically impossible to reconstruct the content from the fingerprint.
Why does btc3r use Bitcoin rather than another blockchain?
Because the value of a permanent inscription depends on the durability of the ledger that carries it. Bitcoin is the oldest blockchain (running without interruption since January 2009), the most replicated (thousands of independent nodes) and the most expensive to rewrite (protected by the largest cumulative computing power). To inscribe something meant to last decades, it is the most prudent choice.
How can I verify a btc3r inscription without going through btc3r?
Take the TXID shown on your certificate, open a public block explorer such as mempool.space or blockstream.info, paste the TXID into the search box, then locate the transaction’s OP_RETURN output: it contains your message (decodable from hexadecimal) or your SHA-256 fingerprint. For a document, recompute its SHA-256 and compare with the displayed fingerprint. The procedure is detailed above.
Does an OP_RETURN inscription clutter the Bitcoin network?
No: that is precisely why OP_RETURN exists. Because the output is marked unspendable, nodes don’t need to keep it in their working memory (the set of spendable outputs, called the UTXO set). It is the network-respectful way to inscribe data, and the transaction pays its network fees like any other.
What these foundations make possible
Proof of anteriority
Date a creation (manuscript, invention, code) before showing it to the world.
Learn more →Document timestamping
Contracts, minutes, reports: freeze the exact state of a professional document.
Learn more →Proof of existence
Show that a file existed on a given date, verifiable by any third party.
Learn more →Permanent messages
A declaration, a birth, a tribute, a time capsule: words inscribed forever.
Learn more →Now that you know how, inscribe.
Plain-text message or SHA-256 fingerprint: every inscription costs 9.60 €, network fees included, and verifiable without us.