A blockchain is essentially a digital ledger of transactions that is duplicated and distributed across the entire network of computer systems on the blockchain. Blockchains can implement cryptocurrency systems, because of their ability to maintain a secure and decentralized record of transactions. This technology guarantees fidelity and security of a record of data and generates trust without the need for a trusted third party. Ethereum is a blockchain considered a "smart contract platform" because it was one of the first blockchains allowing their development. Smart contracts are programs which are self-verifying programs running on top of the blockchain. Those are public, distributed and immutable, consequently, developers cannot design security by layers, such as a Firewall or a virtual private network.