Nearly $64m in bitcoin has been stolen by hackers who broke into Slovenian-based bitcoin mining marketplace NiceHash.The marketplace suspended operations on Thursday while it investigated the breach, saying it was working with law enforcement as “a matter of urgency” while urging users to change their passwords.The hack was “a highly professional attack with sophisticated social engineering” that resulted in approximately 4,700 bitcoin being stolen, worth about $63.92m at current prices, said NiceHash head of marketing Andrej P Škraba.