Bitcoin is the best known of these currencies, especially after hackers this month instructed victims to pay ransoms in the anonymous digital cash in order to get their computer files decrypted. Not that many bitcoins exist: there are about 16.3m of them, with only 1,800 new ones minted every day. But growing demand has pushed bitcoin’s price to a record recent high of about $1,830, up from $450 a year ago.