Blockchain is a decentralised, replicated, transparent and immutable data store. Blockchains are best described not as 'trustless', but on the basis of distributed trust: trusting everyone in aggregate. Consensus protocols are the heart and soul of the blockchains as they help in achieving this distributed trust. Blockchains are updated via the consensus protocols that guarantee their consistency and integrity over geographically distributed network nodes. Various algorithms can be applied to achieve a consensus based on the requirements like performance, security, scalability, consistency, and failure redundancy. Creating a global fair decentralised consensus protocol is of prime importance, in order to address above-mentioned requirements sufficiently. This paper focuses on analysing... the already proposed consensus protocols adopted by popular blockchain platforms to determine their feasibility and efficiency. Parameters that are critical in evaluating blockchain consensus protocol are also discussed. This paper also analyses the hardness of achieving the fair decentralised trust with proof.