What are the differences between cryptocurrencies and digital currencies backed by CBs? Given all the hype around the cryptocurrencies like bitcoin, ethereum, etc., I'm wondering what are the main differences between those and a possible digital currency, backed by a central bank, in a hypothetical 'cashless society'?

Any help would be appreciated.
Two more differences 
(1) You can pay your taxes in a government controlled digital currency, and you can't do that in cryptocurrency.  According to Modern Monetary Theory, that is an essential element to the fundamental value of a currency
(2)  even though an individual cryptocurrency may have the feature of a limited supply, there's nothing to stop multiple cryptocurrencies being created, potentially diluting each other's value.  That is not the case with the single currency of the realm.