As the economy moves online, digital currencies will be the future of financial systems, according to Russia’s central bank governor, Elvira Nabiullina.

There is a need for fast, cheap payment systems, and central bank digital currencies can fill that gap, she told CNBC’s Hadley Gamble in an exclusive interview.

“I think it’s the future for our financial system because it correlates with this development of digital economy,” she said.

Moscow published a consultation paper on a digital rouble in October, and aims to have a prototype ready by the end of 2021. Pilots and trials could start next year, Nabiullina said.

That could be a concern for the U.S., according to a former U.S. Treasury official, Michael Greenwald.

“What alarms me is if Russia, China, and Iran each creates central bank digital currencies to operate outside of the dollar and other countries followed them,” he told CNBC’s Hadley Gamble on Wednesday. “That would be alarming.”WATCH NOWVIDEO09:59Why central banks around the world want to get into digital currencies

Central bank digital currencies are not the same as cryptocurrencies, such as bitcoin. They are issued and controlled by authorities, and the value of one digital rouble will equal one cash rouble, the Bank of Russia said last year.

Cryptocurrencies were illegal in Russia until last year, and still cannot be used to make payments.

“We will go step by step, because it’s [a] very difficult, technological, legal … project,” Nabiullina said.