The first time I heard about blockchains was at a party where a friend of mine spent the night talking my ear off about this thing called Bitcoin and why I ought to buy some. I suspect that many others have had a similar experience. Although Bitcoin can be credited with bringing blockchains—a type of distributed digital ledger—into popular discourse, it wasn’t the progenitor of the obscure technology’s key features.

In fact, the world’s oldest blockchain predates Bitcoin by 13 years and it’s been hiding in plain sight, printed weekly in the classified section of one of the world’s most widely circulated newspapers: The New York Times.

The world’s first blockchain

At its core, a blockchain is just a database that is maintained by a network of users and secured through cryptography. When new information is added to the database it is parceled in “blocks,” which can be thought of as containers for this data. Every so often a new block is created and linked to a “chain” of previously created blocks. Each block has a unique ID called a hash that is created by running the ID of the block that preceded it and the data stored in the current block through a cryptographic algorithm. This ensures the integrity of all the data stored on the blockchain because altering the data in any block would produce a different hash.

Read More: Watch This Hilarious Bitcoin Explainer Generated by an AI

Today, “blockchain” is treated as shorthand for the technology that underlies most cryptocurrencies and digital token systems, such as Bitcoin or Ethereum. Although blockchains can be used as an immutable record of financial transactions, this is far from their only use. In fact, any type of information can be added to a blockchain and in the past everything from weed strains and virtual kittens to sushi and rare art has been stored on a distributed ledger.

Blockchains, insofar as they constitute a chronological chain of hashed data, were first invented by the cryptographers Stuart Haber and Scott Stornetta in 1991 and their use cases were a lot less ambitious. Instead, Haber and Stornetta envisioned the technology as a way to timestamp digital documents to verify their authenticity. As they detailed in a paper published in The Journal of Cryptology, the ability to certify when a document was created or last modified is crucial for resolving things like intellectual property rights.

In meatspace, there a variety of mundane ways to timestamp a document, such as sending yourself the document in a sealed envelope or making chronological line entries in a notebook. In these cases, any evidence of tampering—like opening the envelope or trying to insert a page into the notebook—will be obvious. But when it comes to verifying the authenticity of a digital document, it’s much harder to determine if the document has been altered.

As Haber and Stornetta realized, timestamping a digital document would require solving two problems. First, the data itself would have to be time stamped “so that it is impossible to change even one bit of the document without the change being apparent.” Second, it would have to be impossible to change the timestamp itself.

Read More: Radioactive Material From Chernobyl Will Help Secure the Next Version of the ZCash Blockchain

An obvious solution to this problem is to send the digital document to a timestamping service that would retain the document in a “digital safety deposit box,” which meets both of the criteria mentioned above. The downsides of this approach are that it would compromise the privacy of the person submitting the document and it’s possible that the document could become corrupted when it is sent or stored by the service.

The solution that Haber and Stornetta arrived at instead was to run the document through a cryptographic hashing algorithm, which produces a unique ID for the document. If even a single bit is changed in the document and it is run through the hashing algorithm again, the ID will be totally different. This idea was coupled with the related idea of digital signatures, which can be used to uniquely identify the signatory. Thus, instead of sending the entire document to a timestamping service, users could just send the cryptographic hash value, which could be signed by the service to ensure that it had been received at a certain time and wasn’t corrupted—kind of like notarizing a document IRL.

But where does The New York Times come into play? In cryptocurrencies, hashes are posted to a public ledger known as a blockchain where anybody can see for themselves that the data’s integrity is intact. Haber and Stornetta realized that the nation’s paper of record could serve a similar purpose.

Inspiring Satoshi

What Haber and Stornetta described in their 1991 research paper is a prototypical version of the blockchains that power most cryptocurrencies today. In fact, when Satoshi Nakamoto first described Bitcoin in a 2008 whitepaper, three of the eight papers cited were written by Haber and Stornetta. When asked how he felt about being the inspiration for Bitcoin, Stornetta told the Wall Street Journal that it felt “pretty cool.”

But 14 years before Bitcoin was invented, Haber and Stornetta created their own timestamping service called Surety to put their scheme into action.

Surety’s main product is called “AbsoluteProof” that acts as a cryptographically secure seal on digital documents. Its basic mechanism is the same described in Haber and Stornetta’s original paper. Clients use Surety’s AbsoluteProof software to create a hash of a digital document, which is then sent to Surety’s servers where it is timestamped to create a seal. This seal is a cryptographically secure unique identifier that is then returned to the software program to be stored for the customer.

At the same time, a copy of that seal and every other seal created by Surety’s customers is sent to the AbsoluteProof “universal registry database,” which is a “hash-chain” composed entirely of Surety customer seals. This creates an immutable record of all the Surety seals ever produced, so that it is impossible for the company or any malicious actor to modify a seal. But it leaves out an important part of the blockchain equation: Trustlessness. How can anyone trust that Surety’s internal records are legit?

Instead of posting customer hashes to a public digital ledger, Surety creates a unique hash value of all the new seals added to the database each week and publishes this hash value in the New York Times. The hash is placed in a small ad in the Times classified section under the heading “Notices & Lost and Found” and has appeared once a week since 1995.

According to the company, “this makes it impossible for anyone—including Surety—to backdate timestamps or validate electronic records that were not exact copies of the original.” Or almost impossible, anyway.

As Ethereum’s cofounder Vitalik Buterin joked on Twitter, if someone wanted to compromise Surety’s blockchain they could “make fake newspapers with a different chain of hashes and circulate them more widely.” Given that the New York Times has an average daily print circulation of about 570,000 copies, this would probably be the stunt of the century.