In this paper we explain the basics of Bitcoin protocol and the state of the art of the main attacks to it. We first present an overview of digital currencies, showing what they are and the social need they aim to satisfy. We then focus on the main digital currency up to date, Bitcoin. We treat the basics of the protocol showing what are addresses and transactions and how they are used in a distributed consensus protocol to build the blockchain. After that the main part of this paper presents the state of the art of the three main attacks on the protocol: fraudulent mining techniques, double spending attempts and deanonymization attacks
