# A Survey on Consensus Protocols and Attacks on Blockchain Technology
## 1 Introduction
Many researchers reviewed and surveyed consensus protocols, and some of them surveyed popular consensus protocols, such as PoW (proof of work), PoS (proof of stack), PoW (proof of weight), and PoET (proof of elapsed time) [1; 3; 5; 6]. Some studies surveyed on the basis of PoW (proof of work), and the IoT, surveying popular consensus protocols and discussing their performance, vulnerabilities, and their properties. We include 30 consensus protocols in this paper. We surveyed 30 consensus protocols on the basis of security because this blockchain technology is adopted by many sectors for keeping records secure. Recently, blockchain technology has been proliferating technology [21; 22; 23; 24; 25; 26] in academia, industry, and government sectors around the world. This technology has numerous application domains in almost all spheres of human life [27; 28; 29; 30; 31; 32]. This technology's potential has encouraged research and industry communities to explore its usefulness in different application domains. As a result, we have a plethora of blockchain systems omnipresent in several domains with real-time use. Presently, security and privacy in blockchain scenarios are in four groups:  
* Contract privacy
* Auditability and transparency
* Transactional privacy
* Accountability and non-repudiation
* However, many of these need advanced cryptographic protocols. The main intention of this survey is to provide a brief study on existing consensus protocols and the attacks on blockchain technology that occurred until 2022. To achieve this, we have surveyed recently published referenced articles from various sources, such as IEEE, Scopus, Hindawi, Elsevier, Springer, ACM, and others sources. We have used some keywords to identify referenced publications, such as \"Blockchain consensus Protocols,\" \"Security issues of Blockchain Technology,\" \"Attacks on Blockchain,\" and \"Privacy issues of Blockchain Technology.\" By using these queries, we identified many existing reference articles and conducted our survey. We have represented the attacks on blockchain and consensus protocols in tabular format in Tables 1-4 and at the end, we have discussed some recent surveys of blockchains in Table 5. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Authors, Year** & **Algorithm** & **Blockchain Type** & **Mining** & **Scalability** & **Latency** \\ \hline [PERSON] et. al. 2016 [33] & ELASTICO (2016) & Permissionless & Computational power & Scalable & Low \\ \hline Z. [PERSON] et. al. 2017 [34] & Implicit consensus (2017) & Permissioned & Proof-based & Not Scalable & High \\ \hline [PERSON] et al. 2017 [35] & Proof of trust (PoT) (2018) & Permission-based consortium & Probabilistic and voting mining & Scalable & Low \\ \hline Neo white paper [36] & DBFT consensus algorithm (2018) & Permissioned & Random selection of miner & Not Scalable & Very Low \\ \hline [PERSON] et al. 2014 [37] & Ripple (2014) & Permissioned & Voting mining & Scalable & Low \\ \hline [PERSON], [PERSON] et al. 2017 [38] & Proof of vote (2017) & Consortium & Voting mining & & Very Low \\ \hline [PERSON] et al. 1992 [39] & Proof of work (2008) & Permissionless & Computational power & Not Scalable & Very High \\ \hline [PERSON] et al. 2016 [40] & Proof of stake (2011) & Permissioned and permissioned & Node wealth and staking age & Scalable & High \\ \hline [PERSON] 2014 [41] & Proof of stake velocity (2014) & & Stake and amount (velocity) & Scalable & Low \\ \hline [PERSON] et al. 2014 [42] & Proof of activity (2014) & Permissionless & Effectiveness of work by the miner & Scalable & Low \\ \hline Wiki [43] & Proof of burn (2014) & Permissioned and permissioned & Coin burning (probabilistic lottery) & Scalable & Average \\ \hline [PERSON] et al. 1999 [44] & Practical Byzantine consensus algorithm (1999) & Permissioned & Round of mining & Not Scalable & Very Low \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison of the consensus algorithms with respect to generic parameters.