Some protocols have been proposed to support online payment, but most don't preserve the anonymity of the users. There is much concern about this lack of privacy because with the transactional information one would be able to construct a picture of the habits of a particular user. There are some protocols which are anonymous, but they require a central bank that is involved in all transactions. As electronic currency is just data, it can easily be copied. Illegal use of electronic cash should be unconditionally or computationally impossible.
Some systems rely only on post-fact detection and punishment of double spending of coins.
The identity of the individual using electronic currency should be protected. It should be impossible to find out his spending habits or even his income. In traditional transaction systems such as checks and credit cards, an individual is traceable. At least traceability should be made difficult enough to make the effort too huge in comparison to the benefit from tracking down an individual. Some currency servers record all coins that have been spent to prevent double spending by checking a unique coin identifier before every new transaction.