# A Survey on Consensus Protocols and Attacks on Blockchain Technology
## 1 Introduction  
[PERSON] introduced the initial blockchain in his published whitepaper \"Bitcoin: A Peer-to-Peer Electronic Cash System\" in 2008 [1]. In this, he described the cryptocurrency's technical specifications and motivations and also suggested how Bitcoin provides a strong case for the invention of a new payment system. He also explored how transactions work, how individuals are rewarded for their efforts [1], the use of a network, and also outlines how privacy is key to Bitcoin in the same way it is for the banking industry. Decentralization is the most important feature of blockchain technology, which means that the storage of data is distributed and stored on a number of blocks in the network. This type of distributed system provides more robustness to blockchain technology. In this distributed system, if some nodes have faults, they do not affect the other nodes. This is one of the best features of this technology and it is very useful for cyber-physical systems and other systems. Currently, the manufacturing of cyber-physical systems is facing issues such as security, privacy, flexibility problems [2; 3; 4; 5; 6; 7] and data integrity, transparency, and many more. The industry has various problems related to security [8; 9; 10; 11; 12]. Blockchain technology hasvarious features to overcome these issues [13; 14]. Many industries now use blockchain technology to secure their system. Blockchain technology is very efficient, not only in financial technology but also [7; 8; 9] in industry. In [8], the five-level architecture known as 5C-CPS was anticipated for evolving CPSs in manufacturing sectors. The use of blockchain technology in real-time applications is widespread; however, we have found that issues remain in networking, data privacy, data security, and centralization, paving the way for further research and development. Additional security and privacy features are required for use in secure distributed storage. To improve the privacy and security of blockchain technology, we must use hybrid cryptography or advanced cryptography [15; 16; 17]. For blockchain scaling, we have proof of luck for efficiency in the consensus protocols arena. However, following an in-depth study on major consensus algorithms, there is a need for future case-specific applicability of these consensus algorithms. The security and privacy of blockchain needs to be upgraded because with the enhancement of technology, the protection of data is a major issue. Many hackers are updating their computational power and therefore the security of blockchain can be broken down in the future [18; 19; 20]. These consensus algorithms [21; 22; 23; 24; 25; 26; 27] are delegated Byzantine fault tolerance (dBFT), tangle (IOTA), proof of stake (PoS), delegated proof of stake (DPoS), proof of weight, proof of elapsed time, proof of history, proof of stake velocity, proof of importance, proof of reputation, proof of burn, proof of identity, proof of activity, proof of time, outowbors, proof of retrievability, proof of capacity, Byzantine fault tolerance, delayed proof of work, RAFT, stellar consensus, proof of believability, directed acyclic graphs, Hashgraph, proof of work (PoW), holochain, block lattice (nano), proof of existence, SPECTRE, proof of authority, ByteBall, etc. [28; 29; 30; 31; 32].In this paper, we focus on consensus protocols and the attacks on them. Detailed security features need to be improved, as recently we have suffered attacks on consensus algorithms, such as DDoS attacks and sharding, in a permissionless blockchains, and with the presence of Byzantine adversaries such as these, many attacks are happening. A consensus mechanism is a fault-tolerant technique used in blockchain systems to obtain the necessary agreement among distributed processes or multi-agent systems, such as cryptocurrencies, on a single data value or a single state of the network. It is useful for keeping records, among other things.