Cryptocurrencies and blockchain are growing in popularity across the internet, but one thing still keeps crypto from going mainstream: the anonymity and security it offers to holders, and criminals.

On the darknet, cryptocurrencies are ideal tender. They offer total anonymity, easy transactions across international borders, and can be easily laundered by exchanging for “clean” fiat currency. The darknet marketplace “Silk Road” operated for 2 years and made $1.2 billion dollars, mostly through bitcoin transactions.

When it comes to taxes, the IRS requires cryptocurrencies be reported as a property, rather than a currency. In 2015, 2.8 million people in the United States owned cryptocurrencies, but only 807 reported crypto on their taxes, that’s less than 0.03%. While some of these people were likely unaware of the IRS guidelines for cryptocurrency, it has become a haven for tax evasion.

At the same time, the promise of big dividends has made Bitcoin a tool for fraud and Ponzi schemes. From 2011-2012, Trendon Shavers operate Bitcoin Savings and Trust, collecting 764,000 bitcoin ($4.5 million, at the time) in exchange for a promise of 7% returns. When Shavers was sent to prison in 2016, half of investors lost their money.

Governments around the world are cracking down on crypto. The European Union, Mexico, Japan, and China have all promised to introduce regulations for monitoring and using cryptocurrencies. The global watchdog group Financial Action Task Force, which monitors international financial crime, is moving to increase their monitoring of cryptocurrency in money laundering schemes.

Meanwhile, government agencies are partnering analytics companies to track the use of Bitcoin in illicit activities, through the public ledger. While they’ve had some success, criminals have shifted to using other cryptocurrencies to commit crypto crimes, which use private ledgers to ensure that not only are users anonymous, but transactions are kept secret as well.