Bitcoin’s Blockchain Contains “Hundreds of Links” to Child Pornography

In recent months, Bitcoin’s supporters have pointed to its falling use in illegal transactions as a sign of the cryptocurrency’s growth toward mainstream acceptance. But German researchers say that links to child pornography within technology underlying Bitcoin could stifle its development.

While the blockchain is largely known to be an immutable ledger of Bitcoin transactions corroborated by copies held by participating computers, it also allows its users to leave coded messages. Bitcoin’s creator, Satoshi Nakamoto, famously left a cryptic message on the blockchain’s original block: “The Times 03/Jan/2009 Chancellor on brink of second bailout for banks.”

Like that very first message, most of the content left on the blockchain has been relatively benign—tributes to the late Nelson Mandela, or messages to loved ones on Valentine’s Day. But the ones that could be illegal, containing links to child porn, for example, could be an outsized problem for the Bitcoin community.

“While most of this content is harmless, there is also content to be considered objectionable in many jurisdictions, e.g., the depiction of nudity of a young woman or hundreds of links to child pornography,” the paper authored by members of RWTH Aachen University and Goethe University read. “As a result, it could become illegal (or even already is today) to possess the blockchain, which is required to participate in Bitcoin.”

The researchers said they found 1,600 instances in which transactions on the blockchain included non-financial information, representing about 1.4% of transactions. Since the Bitcoin blockchain is immutable, those who download it are also unwittingly downloading links to child porn.

The Department of Justice did not respond to requests for comment from Fortune.

It’s not the first time curious onlookers have found links to child pornography in Bitcoin’s blockchain. Users first pointed out the links in 2013. Though this is perhaps the first time researchers have been able to quantify the volume of potentially illicit material hidden in the blockchain.

Additionally, since Bitcoin has buyers and traders all over the world, items in the blockchain also raise questions about legality in other nations. As the blockchain researchers note: “In China, the mere possession of state secrets can result in longtime prison sentences. Furthermore, China’s definition of state secrets is vague and covers, e.g., activities for safeguarding state security. Such vague allegations with reference to state secrets have been applied to critical news in the past.”

The researchers pointed out that the blockchain includes online news articles concerning pro-democracy demonstrations in Hong Kong in 2014, demonstrations that were a point of irritation for Beijing.

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