Public Blockchain versus Private blockhain.

Summary In this document, we introduce some thinkings relative to the concept of blockchain, how it works and what are the issues for the banking system. Thus, first we recall what cryptography is, then we introduce the concept of blockchain as a protocol for transmitting information in a secure way, distinguishing two possible approaches: the decentralized public approach and the centralized private approach. The notion of cryptocurrency is introduced and two examples of applications of the public blockchains that are the bitcoin and the etherium are provided.