The advantages of Bitcoin, such as immutability and decentralization, can create problems for the cryptocurrency.

A team of researchers analyzed 1,600 files stored in the blockchain and found there images of a pornographic nature and links to videos with child pornography. Thus, anyone who installs a full Bitcoin node can automatically become the owner of the illegal content.

In addition to information about transactions, small notes or files can also be stored in the blockchain. For example, the very first block in the Bitcoin chain contains a text message left by its creator Satoshi Nakamoto "The Times 03 / Jan / 2009 Chancellor on brink of second bailout for banks." This is the headline of the article from the front page of the British newspaper The Times, released on the same day on January 3, 2009.

Thus, Satoshi Nakamoto wanted to confirm the actual date of Bitcoin launch and perhaps hint at the problems of the banking sector, which should be eliminated by decentralized cryptocurrencies. In the same way, people who posted links and photos of illegal content in the blockchain did the same, but their goals are not so clear and sublime.

In a study, entitled "Quantitative Analysis of the Impact of Arbitrary Blockchain Content on Bitcoin," a team from the Aachen University (Germany) speaks of at least eight pornographic materials found, including one allegedly with a child abuse scene. Two more files contain 274 links to third-party sites with similar content, and 142 of them lead to pages in the darknet. As a result of this finding, Bitcoin can become illegal in most countries and jeopardize the entire cryptocurrency industry.

The document lists other types of content, the distribution of which may also violate the laws of some states as these materials violate copyright, affect the privacy of some individuals, have politically sensitive content, and even contain old malicious programs.

Of course, the Internet also has a lot of illegal content, but to make the Internet work, you do not need to store this content on your computer. Although ordinary users can easily use cryptocurrencies with the help of so-called "light wallets" that do not require the download of all the blockchain software to your computer, if some restraining measures are still developed, this will affect the miners and people who work with full nodes. And without them, Bitcoin cannot exist.

It should be noted that programmers and regulators have long discussed the likelihood of placing such files in the blockchain. In his blog in April 2013, the former Bitcoin Core developer Jeff Garzik described the scenario as a "terrible situation" that "is difficult to solve." And in March 2015, the Interpol warned about the possibility of using blockchain to store malicious programs or images of a pornographic nature. But as far as we know, until now there has been no working solution to this problem.

Some experts believe, however, that the problem with storing unwanted files in the blockchain is contrived. The fact is that even if you download the entire Bitcoin blockchain, you will not find Satoshi Nakamoto mentioned there. Instead, you will find a hash where the transaction data and a text message are included. It looks like this:

000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f

This is the coinbase block parameter (not to be confused with the Coinbase exchange), written in hexadecimal format. The size of the data entered here does not exceed 100 bytes, and only miners can add data. There are five more ways to enter information into the block, but the size of these files can be less than 100 bytes. Thus, when saying that the Bitcoin blockchain contains pornography, we are only talking about links to similar videos. And to find these links, the user who downloaded the blockchain will have to perform a long procedure of decryption and verification of all addresses. In the same way, you can insert an encoded link inside any database, including Facebook, Amazon, or Wikipedia.