In the Bitcoin blockchain found a picture similar to child porn

Left: methods for embedding data in the Bitcoin blockchain (the corresponding services are shown in italics). Right: payload volume, cost and efficiency of methods

Blockchain is a continuous sequential chain of blocks containing information about all operations ever performed in the database, starting with the very first one. Each block consists of a header and a list of transactions. The block header includes its own hash, the hash of the previous block, transaction hashes, and additional overhead information. The information in the blocks is not encrypted and is available in clear text.

A group of specialists from the Faculty of Communications and Distributed Systems of the Rhine-Westphalian Technical University of Aachen (Germany) has published the results of the blockchain research of the most popular Bitcoin cryptocurrency. It turned out that many interesting things are stored there. But the most dangerous thing is that the researchers found a photograph similar to child porn. This finding can have long-term consequences, because for storing such information on a personal computer criminal liability is provided in 112 countries of the world. And all the full-fledged Bitcoin nodes download and store on local computers a chain of blocks from beginning to end, about 180 gigabytes today.

The problem is that removing prohibited content from the blockchain is impossible by definition. He will stay there forever. Changing a specific block will require recalculation of the hash of not only the modified block, but of all subsequent ones, which is almost impossible.

In principle, such a situation could be expected. Experts have previously warned that the nature of the blockchain as an open and irrevocable repository of content opens the door to abuse.

German researchers have studied in detail the entire array of non-financial information included in the blockchain. This is the first ever comprehensive and systematic study of extraneous content in a blockchain. Most of the information is innocent, but there are also very dubious things. For example, the image of a naked young girl and hundreds of links to child pornography. Researchers fear that because of such content, the blockchain may become illegal for storage (or it is already now). Thus, the "pranks" of some attackers can undermine the foundation of the world's largest cryptocurrency.

The authors describe as fully as possible the possible ways of introducing extraneous content into the blockchain, their advantages and risks, and also assess the implications for the financial cryptosystem.

As shown in the illustration above, extraneous data is included in the blockchain at a low level in small portions of 80-96 bytes. For this, input and output scripts are used (these are peculiar conditions for unlocking input and output addresses, from where and where funds are transferred, for more details on scripts, see here ). Existing content embedding services in the blockchain systematically exploit low-level scripts, breaking up larger pieces of data into many fragments. For their work, they charge about two dollars per byte. This is a small fee for the opportunity to spread important information to millions of computers around the world and, perhaps, to preserve it forever in the history of human civilization.

Every month about 10-20 files are embedded in the blockchain. As of August 2017, extraneous data was contained in 3,535,855 transactions (1.4% of all transactions in the blockchain), totaling 118.53 MiB.


Left: the number of files embedded in the blockchain monthly. Right: total transaction size from each data injection service

What extraneous information has been accumulated in the blockchain so far? Overwhelmingly, this text information. Occasionally there are images and HTML files, in rare cases - source code, archives, sound files and PDF documents.



At least eight sexual content files were found in the blockchain. Five of them are soft pornography, and the other three, according to researchers, may be illegal in almost all jurisdictions. Two files are collections of links to child pornography (274 links, including 142 links to hidden Tor services). And one image of a naked young girl. On the forum, this image is accompanied by the description "child porn".

The results of the research make one wonder whether the blockchain's property to allow the recording of arbitrary information in the chain is a blessing.