Blockchain as an innovative technology has increased application usage of distributed and decentralized systems in different fields. With the rapid growth of applications using blockchain technology, there is a need to develop consensus models that ensure data and system integrity. In decentralized systems, the integrity of data cannot be realized without proper and secured consensus models of protocols. In the blockchain era, many applications use different consensus protocols, each of them has its advantages and disadvantages. The decision to apply a specific protocol to any application depends on many factors. Selecting a proper consensus protocol it is a difficult task for the system analyst, the selection must be based on all pros and cons of different protocols compared to the selected protocol and application purpose.
In this paper, we present an overview of consensus protocols, especially focused on POW (Proof-of-Work), POS (Proof of Stake) and LBFTas most used protocols in cryptocurrencies such as Bitcoin, Etherum, etc. We reviewed the Bitcoin protocol POW, Etherum protocol POS, Libra Protocol LBFT and finally we summarized the open challenges and suggest future research on the known phenomenon 51% attack.
Keywords: Protocol, Bitcoin, Etherum, Cryptocurrency, synchronization, decentralized, distributed, integrity, probability.