The discovery of illegal child abuse links on the bitcoin blockchain is raising questions.

Researchers from the RWTH Aachen University and Goethe University in Germany have found approximately 1,600 different files currently stored on bitcoin’s blockchain, the open source ledger that records each bitcoin transaction.

As well as noting the transactions themselves, the blockchain can also store small quantities of non-financial data, including files and links.

Several objectionable files located

Of the 1,600 files uncovered by the researchers, at least eight of the files could be classed as sexual content and one is thought to be an image of child abuse. Two other files contained almost 300 links to child abuse services, with 142 of them linked to dark web services such as Tor.

The researchers wrote that while most of the content found on the blockchain is harmless, “there is also content to be considered objectionable in many jurisdictions, eg the depiction of nudity of a young woman or hundreds of links to child pornography. As a result, it could become illegal (or even already is today) to possess the blockchain.”

They added: “Although court rulings do not yet exist, legislative texts from countries such as Germany, the UK or the USA suggest that illegal content such as child pornography can make the blockchain illegal to possess for all users.

“This especially endangers the multibillion-dollar markets powering cryptocurrencies such as bitcoin.”

Private information uncovered

Other content found on the blockchain included wedding photos, chat logs, a pair of leaked cryptographic keys, emails and some personally identifiable information, including addresses, passwords and phone numbers. Researchers warned that some of the files could violate copyright and privacy laws.

As Mark Stockley of Naked Security noted, deleting content from the blockchain is difficult as every block of transactions is cryptographically connected to the blocks that preceded it. This presents some problems in terms of the EU data subject’s right to ask for their personal data to be deleted if it is not necessary; 51pc of the nodes on the bitcoin network would need to agree to the deletion of said data and then recalculate all the blocks that had been added to the blockchain after the data in question was added.

The researchers concluded: “The possibility to store non-financial data on cryptocurrency blockchains is both beneficial and threatening for its users. Although controlled channels to insert non-financial data at small rates opens up a field of new applications such as digital notary services, rights management or non-equivocation systems, objectionable or even illegal content has the potential to jeopardise a whole cryptocurrency.”