Bitcoin is the core of decentralized cryptocurrency systems. The underlying data structure of Bitcoin is called the blockchain in which transactions of digital coins between accounts are batched in so-called blocks, where each block is appended to the last one in a cryptographic way to make the malicious/accidental change of blocks content very hard. Participants following this protocol can create together a distributed, economical, social and technical system where anyone can join/leave and perform transactions in-between without neither needing to trust each other nor having a trusted third party. It is a very attractive technology since it maintains a public, immutable and ordered log of transactions which guarantees an auditable ledger accessible by anyone.