Lurking within Bitcoin's ever-growing blockchain is a disturbing surprise. A team of researchers say they've found links to online child pornography recorded into the technology.

The researchers at RWTH Aachen University in Germany claim to have uncovered the 274 links while investigating the non-financial data that's been stashed into Bitcoin's blockchain.

Unfortunately, that's not all. A pornographic image, possibly of a minor, was also scribbled into the blockchain. This all could theoretically make the blockchain illegal to possess in 112 countries, the researchers warned in a new paper that was presented at a conference earlier this month.

How did the sexual content get there? Although the Bitcoin blockchain is designed to record every transaction with the cryptocurrency, you can also adds snippets of non-financial data too. This has benefits. For instance, a notary service can tie a Bitcoin transaction to a digital document.

The blockchain network is decentralized, which helps make the data recorded practically immutable. But this also introduces a problem: potentially anyone can anonymously insert controversial data on the blockchain.

The researchers at the German university decided to download Bitcoin's blockchain —which at the time was 122 GB in size— and scan for what they called "objectionable content." They found that only 1.4 percent of all transactions on the blockchain carried non-financial data, which amounted to 118.5 MB in size. An even smaller amount, at 22.6 MB, contained readable files when viewed over applicable software.

Among the data was the child pornographic content found in three files; the 274 links were lists that directed to sites found on the Dark Web. The researchers also uncovered six wedding-related images, 609 transactions containing public chat logs, emails and forum posts discussing Bitcoin and money laundering, in addition to backups of WikiLeaks data.

Others have also warned about blockchain's dangers. Back in 2015, Interpol worried about bad actors embedding malware and other illegal data, including child abuse images, into the technology.

Fortunately, the researchers at Aachen University didn't find any malware on Bitcoin's blockchain. Nevertheless, the researchers warned: "Illegal content has the potential to jeopardize a whole cryptocurrency."

To be clear, bitcoin holders don't need to download the entire blockchain to make transactions in the virtual currency. Many investors buy and send transfers through third-party cryptocurrency services that directly interact with the Bitcoin blockchain.

A US law also states it's a crime to possess child pornography when the offender "knowingly" retains or transfers it.

In their paper, the researchers noted a number of ways you can insert content on the cryptocurrency's blockchain, which include services such as CryptoGraffiti, Satoshi Uploader and Apertus. But on the flip side, the added data cannot be reversed. The capability to do so would harm Bitcoin's security and diminish trust in the cryptocurrency, said Roman Matzutt, one of the paper's authors, in an email.

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He added that it wouldn't be easy for an average Bitcoin user to stumble on the illegal content. The researchers only found it by actively scanning the blockchain for evidence of inserted data.

To the human eye, the illegal content will also appear as a string of scrambled numbers and alphabet characters. However, the data can be decoded to produce text or an image. "Once you know where to look, it is rather easy to extract [the] data," Matzutt said.

Although the pornographic content can't be removed from the blockchain, Matzutt and his colleagues are researching a possible solution. It involves Bitcoin operators needing to only run the essential parts of virtual currency's blockchain, rather than the full copy. The irrelevant portions of the blockchain could then be ignored and stripped out.

"We hope that, as some sort of by-product, we can also destroy the structure of non financial content," Matzutt said.

Members of the Bitcoin community have also talked about this "pruning" as a way to reduce the blockchain's ever-growing size.

Editor's note: This article has been updated with a comment from one of the researchers and information about US child pornography law.

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