Bitcoin is the most widely known distributed, peer-to-peer payment network without existence of a central authority. In Bitcoin, users do not use real names; instead, pseudonyms are used. Managing and verifying transactions and issuing of bitcoins are performed collectively by peers in the network. Since pseudonyms are used without providing any identity, it is perceived that Bitcoin provides anonymity. However, it is one of the most transparent payment networks since all transactions are publicly announced. Blockchain, which is the public ledger of Bitcoin, includes all transactions to prevent double-spending and to provide integrity. By using data in the blockchain, flow of bitcoins between transactions can be observed and activities of the users can be traced. When the implications obtained from the blockchain are combined with external data, identity and profile of a user can be revealed. This possibility has undesirable effects such as spending history of a user becomes accessible to other people, or cash flow of a merchant becomes exposed to competitors. There are several proposals as extensions or alternatives to Bitcoin, which improve anonymity and privacy. This survey presents an overview and detailed investigation of anonymity and privacy in Bitcoin-like digital cash systems. We examine the studies in the literature/Web in two major categories: 1) analyses of anonymity and privacy in Bitcoin and 2) extensions and alternatives to Bitcoin, which improve anonymity and privacy. We list and describe methods and outcomes for both categories and group studies according to these methods and outcomes. We also present relationships between outcomes of analyses and the improvement methods. We compare performances of the methods and show relationships between the proposals. Moreover, we present guidelines for designing an anonymity/privacy improvement and discuss future research directions.
