Bitcoin is back. It has tripled in value since it crashed at the end of 2018, while this week the oldest central bank in the world, Sweden's Riksbank, started trialling its own official digital currency.
Cryptocurrencies have been tipped as the future of the financial system, winning admiration among a diverse audience, from intellectuals and tech evangelists to organised crime bosses.
Cryptocurrencies are digital assets that can be used in the same way as mainstream currencies, to buy and sell goods and services.
Unlike the dollar or euro, they are not backed by central banks and they use strong cryptography - digital codes - in their creation and to record their exchange.
The oldest and best-known is Bitcoin, first mined (created) in 2009 by Satoshi Nakamoto (a pseudonym) in response to the financial crisis, which he said showed banks could not be trusted with our money because they lent it on and created financial storms.
The story of Bitcoin is a financial rollercoaster - in January 2017, it was worth roughly $1,000 (€907) but by the end of the year it hit a peak of $19,499.
Fast-forward 12 months and Bitcoin was worth just $3,156 in a crash that wiped 84pc off its value. Its extreme volatility was on display again this week when it plunged by 8pc to $9,327 from $10,168 in just 45 minutes.
Those extreme price moves illustrate a fundamental flaw in all cryptocurrencies, according to new research from the Bank of England.
"Speculative buying can drive cryptocurrency prices down," Peter Zimmerman, a senior economist in the Bank's Advanced Analytics Division, wrote in a research paper.
This occurs because Bitcoin's blockchain technology limits how quickly crypto transactions can be settled - around seven transactions per second - so the system becomes congested. Congestion caused Microsoft to stop accepting Bitcoin in its online store and even the North American Bitcoin Conference wouldn't take it to pay for event tickets.
"People speculate on cryptocurrency because they expect it to be adopted as a means of payment, or because they expect to sell it to someone who believes adoption will occur. But the act of speculation makes it harder for adoption to happen," Mr Zimmerman wrote.
It takes a lot to shake the faith of crypto enthusiasts, however, and the currency survived the bankruptcy of Japanese exchange Mount Gox, which handled 70pc of all Bitcoin transactions across the world when it went broke in 2014 as hackers broke into its vaults and stole $480m.
So why the interest from the world's central banks, who started off with the view that cryptocurrencies were the preserve of fringe groups, who simply didn't trust the state on anything, and criminals?
In March 2018, Agustin Carstens, who heads the central bank for central banks, the Bank for International Settlements (BIS), told the Irish Independent in an interview that crypto was a "Ponzi scheme".
Facebook changed all of that when it announced its own currency, Libra.
This digital currency differs dramatically from crypto in that it is backed by so-called 'fiat money', like the dollars and euro that are issued by regular central banks. And so it has an intrinsic value of its own and is called a 'stablecoin'.
The BIS has had a change of heart and now finds that central banks covering a fifth of the world's population are likely to issue their own digital currencies in the coming three years.
While we are all used to electronic cash replacing physical money when we swipe, central bank digital currencies aim to be a replacement for all coins and notes - a true cashless society. Their argument goes that costly and inefficient payment systems could be sped up and made a lot cheaper, a measure that could boost economic growth. The Bank of England said in a 2016 research paper that central bank digital currencies could increase economic output by almost 3pc by cutting costs and boosting the efficiency of finance.
While Sweden, where the amount of cash in circulation is a tiny 1pc of the economy, looks to be ready for the Riksbank's e-krona, attitudes are deeply sceptical elsewhere.
A poll released on Thursday by OMFIF, an economic think-tank, showed that people in Germany, France and Italy mistrusted not only private currencies, but also those issued by central banks.
The root of this mistrust appears to be the European Central Bank's (ECB) policy of negative interest rates, which is viewed as confiscation by savers.
In Germany, negative rates have been dubbed 'punishment rates' and the popular 'Bild' newspaper labelled ex-ECB chief Mario Draghi 'Count Draghila'.
"The similar and remarkably high levels of distrust across the three euro-area economies surveyed suggest that monetary policy might be a factor behind lower trust in central banks," the study said.
Some economists have made the case that issuing central bank digital currency would help put a floor under interest rates, but the Bank of England found ordinary citizens would react to such steps with considerable ill will.