# Key Characteristics to Create Optimized Blockchain Consensus Algorithms

J. Leo, Marié J. Hattingh

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J. Leo, Marié J. Hattingh. Key Characteristics to Create Optimized Blockchain Consensus Algorithms. 20th Conference on e-Business, e-Services and e-Society (I3E), Sep 2021, Galway, Ireland. pp.567-579, 10.1007/978-3-030-85447-8_47. hal-03648140

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# Key characteristics to create optimized blockchain consensus algorithms

J. Leo¹[0000-0003-1933-7298] and M.J. Hattingh²[0000-0003-1121-8892]

University of Pretoria, Private Bag X20, Hatfield, 0028
joshualeslieleo@gmail.com, marie.hattingh@up.ac.za

**Abstract.** Blockchain is a fairly new technology and still in its infancy. As a result, many research papers are creating optimized consensus algorithms. Therefore, a need for key characteristics to create optimized blockchain consensus algorithms has been identified. This research paper presents the results of a systematic literature on identifying the main blockchain consensus algorithms and their associated advantages and disadvantages. Papers from four different databases were retrieved and after exclusion criteria were applied, 44 papers were ultimately included in the review. Results indicated that the five main consensus algorithms were Proof-of-Work (PoW), Proof-of-Stake (PoS), Practical Byzantine Fault Tolerance (PBFT), and Delegated Proof-of-Stake. The results further indicated that efficiency was the main advantage of the PoS, PBFT, PoA and hybrid consensus algorithms. The main disadvantage was "energy wastage" and was attributed to the PoW algorithm. Security concerns were the main disadvantage of the PoS algorithm. These findings were used to present key characteristics that future researchers can have in mind when creating optimized blockchain consensus algorithms.

**Keywords:** Blockchain; Consensus Algorithm, Proof-of-Work (PoW); Proof-of-Stake (PoS)

## 1 Introduction

Satoshi Nakamoto first introduced blockchain technology when he developed Bitcoin in 2008 [1]. This technology allows for decentralization as a history of all the transactions are kept on each node on the network [2]. Any person with access to the network plays a role in maintaining the blockchain node. Blockchain has a strong advantage of being transparent as any party can acquire access to the transaction. Furthermore, other characteristics included security, immutability and transparency [3]. Due to these characteristics blockchain has played a disruptive role in major industries such as the supply chain, the medical industry, financial and energy industry [4]. For it to be carried out successfully in these industries, optimized blockchain consensus algorithms need to be utilized.

There have been several different consensus algorithms created. The most common ones being Proof-of-Work and Proof-of-Stake [5]. Both these algorithms come with their advantages and disadvantages. As a result, many new consensus algorithms are being created to deal with their shortcomings.
An initial scanning of research determined that numerous research papers were
creating improved consensus algorithms. As a result, a gap in research was determined
where it would be beneficial to have key characteristics that these new consensus
algorithms should have. Therefore, this research paper proposes the following main
research question: *What are the key characteristics to create optimized blockchain
consensus algorithms?*

To determine the answers to the main research, question the following sub-research
questions need to be answered: (1) What are the main blockchain consensus
algorithms? (2) What are the advantages of the main consensus algorithms found?, (3)
What are the disadvantages of the main consensus algorithms found?

This systematic literature review will be structured as follows. Section 2 details the
systematic review methodology. Section 3 and 4 presents the findings and discussion
of the findings of the systematic literature respectively. Section 5 offers up future
research whilst section 6 will conclude the study by summarizing the main findings.

## 2 Methodology

Blockchain technology is still in its infancy [15], as a result, the methodology proposed and utilized in this systematic literature review is designed specifically with the novel technology in mind. This research paper considers both qualitative and quantitative research. Both of these research types will be used as blockchain consortiums is exiting the Peak of Inflated Expectations and entering the Trough of Disillusionment in the Gartner Hype Cycle [17]. Its placement in the Gartner Hype Cycle means that there will be plenty of qualitative research into the topic but minimal quantitative research as there are not many applications for this type of research to be conducted on.

The process of selecting the research papers to be included in this systematic
literature review was as follows:

1. The following keywords were initially inserted into the chosen databases: (blockchain AND consensus algorithm) OR proof of work OR proof of stake. These keywords were selected as blockchain consensus algorithms is the core components of the research paper. Proof-of-Work and Proof-of-Stake were included in the search terms as they are two of the most popular blockchain consensus algorithms, therefore it would yield better results.

2. Blockchain technology was first introduced in a paper by Satoshi Nakamoto in 2008. Since it was introduced in 2008, the search filter data range for all the databases were from 2008-2020. The reasoning behind the source types that will be listed below is because formal research papers that have gone through a rigorous process to be published, is desired. The following is the filter parameters that were used for each database:

* Science Direct – source type: review articles and research articles

* IEEE – source type: journals and conferences

* Ebsco Host – source type: academic journals

* Emerald – source type: article
3. The following step is to select relevant research papers by reading through the title and abstract of the research paper. The paper will be included if the author mentions relevant research containing consensus algorithms in the title and abstract. The research paper must also be downloadable immediately for it to be included. The results can be seen in the "Title and Abstract".

4. The final step in selecting research papers to be included in the study will be analyzing the research papers to determine if it adheres to the following inclusion criteria:

* Advantages of blockchain consensus algorithms

* Disadvantages of blockchain consensus algorithms

* Future research of blockchain consensus algorithms

The process mentioned above resulted in 44 research papers being included in this study. Content analysis will be the chosen analysis technique.

## 3 Results – background

Analysis of the resultant papers indicated that 18 papers were based on qualitative research and 19 papers were based on quantitative research. Science Direct yielded 13 papers, and IEEE produced 23 papers respectively followed by five papers from EBSCO Host and 3 papers from Emerald Insight.

In answering sub-research question one, results further indicated that there was a total of 30 different blockchain consensus algorithms mentioned in the 44 articles that were included in the study. The four most common algorithms that the 44 included research papers mentioned, were Proof-of-Work (PoW), Proof-of-Stake (PoS), Practical Byzantine Fault Tolerance (PBFT), and Delegated Proof-of-Stake (DPoS).

In answering the second sub-research question the main advantages of blockchain consensus algorithms were efficiency, scalability and security. Efficiency was the advantage that was mentioned in 60% of the papers. The second type of advantage, scalability was associated with the PoW, PoS, and DPoS having the advantage of scalability.

Security as an advantage was associated with PoW and PBFT consensus algorithms [1],[2], [3].

In answering the sub-research questions, three findings indicated that “energy wastage” was mentioned in 25 papers as the most often occurring in the PoW consensus algorithm. Security was identified as the biggest disadvantage of the PoS algorithm by six papers [4]–[9].

## 4 Discussion

The following sections discuss the findings in terms of the three sub-research questions posed in section 1.
## 4.1 S-RQ1 What are the main blockchain consensus algorithms?

According to the results in section 3, out of the 44 research papers considered, 30 different consensus algorithms were identified. Of the 30 listed consensus algorithms, four of them were mentioned significantly more than the rest. These four algorithms included Proof-of-Work, Proof-of-Stake, Delegated Proof-of-Stake, and Practical Byzantine Fault Tolerance. This section will allow the reader to gain insights into how these consensus algorithms work. A stronger argument can be made for the advantages and disadvantages which will be further explored.

**Proof-of-Work.** Bitcoin uses the Proof-of-Work consensus algorithm. Proof-of-Work is essentially nodes putting in a rigorous computational effort to keep the blockchain network secure. This consensus algorithm involves nodes on the network competing with each other to solve a cryptographic problem which is easily verifiable by other nodes on the network. These nodes that are competing against each other are known as miners and the process of solving this cryptographic problem is known as mining. A miner's responsibility is to verify transactions, validate, create and add blocks to the chain [14]. This process of mining will be explained below.

Once a block is filled with transactions, the miner can initiate the verification process. The block contains a header that includes the hash pointer (the hash of the data of the previous block), the network difficulty, a timestamp, the version of the block, a list of the transactions that they think should be added to the network, and nonce. A nonce is a 4-byte adjustable number [5]. The miner must continuously change the nonce so that the outcome of the hash results in it is below the threshold that is set by the network difficulty level included in the header. After the miner finds a hash that is below the threshold, the block will be propagated onto the blockchain network using flooding algorithms [28]. The other nodes on the network will verify this block by taking the nonce that was used for validation and will hash the block with the same cryptographic function used by the miner that proposed that the block is valid. If the resulting hash results in a value lower than the threshold, then the miner will deem the block to be valid. The majority of the nodes on the network must deem this block valid for it to be added onto the chain of the blockchain network. If the block is successfully added to the chain, the miner will be rewarded with a certain amount of currency of the network, 12.5 coins in the Bitcoin context [9]. The longest chain is deemed the most valid one as it required the majority of the network's computational power [11].

This process of mining has some advantages but result in more disadvantages according to the research papers analyzed. These advantages will be discussed in section 4.2. As a result of the disadvantages stemming from the proof-of-work consensus algorithm, there have been algorithms that have used Proof-of-Work as a building foundation but improvements have to be made

**Proof-of-Stake (PoS).** The proof-of-stake is an alternative consensus algorithm that was created to deal with the inefficiencies and disadvantages of Proof-of-Work [11]. According to the paper, *Analysis of the main consensus protocols of blockchain*, Ethereum is planning on moving away from Proof-of-Work and transitioning towards
the proof-of-stake consensus algorithm [12]. 81.82% of the included research, either mentioned or elaborated on this algorithm.

This consensus algorithm involves validators that have the responsibility of ensuring
transactions and blocks are authenticated and valid. Stakeholders stake a certain amount
to be considered to validated and add blocks to the chain. The stake is a certain amount
of the digital currency that is stored in a vault to ensure that the validator does not carry
out any malicious actions. This ensures that those who have staked more are less likely
to carry out malicious actions as they will lose what they have staked [1]. The validator
will be selected on a random selection basis with the validators staking more, having a
higher chance of being selected as the one to validate the block. The validator that is
selected will ensure that the transactions in the block are valid. If the transactions are
deemed to be valid, the validator will add the block onto the existing chain. They will
then will be rewarded in transactions fees instead of coins as in the Proof-of-Work
consensus algorithm [13].

The creation of this has resulted in some advantages such as the reduction of
computational power required. Although there are benefits as a result of this algorithm,
there are disadvantages that also occur. Further advantages and disadvantages of this
consensus algorithm will be discussed further in section 4.3 Similar to Proof-of-Work,
additional consensus algorithms use the foundation of proof-of-stake to create new and
improved versions of this algorithm.

<figure><img src="image_3.png" /><figcaption>Fig. 1. Comparison of PoW vs PoS</figcaption></figure>

**Delegated Proof-of-Stake (DPoS).** Delegated Proof-of-Stake was mentioned in ten of the research papers included in the study [11], [15], [27], [30], [31], [24], [48], [35], [36] and [58]. This consensus algorithm according to the research paper, *Blockchain technology in the energy sector: A systematic review of challenges and opportunities*, is described as a stakeholder voting consensus scheme [11]. The stakeholders, the
people that own coins, elect nodes(witnesses) to validate, authenticate and create blocks [11]. The witnesses that receive the greatest number of votes will be given the authorization to create blocks. They will take turns in validating the blocks [14]. If the witnesses do not adhere to their responsibilities the stakeholders can remove them as a witness and elect a different node. In return for creating these new blocks, the witnesses are awarded the associated fees. Nodes on the network are also able to elect delegates who determine the rules and protocols of the network [11].

**Practical Byzantine Fault Tolerance (pBFT).** There are times where nodes do not come to a consensus as a result of a block acting in a malicious manner or the communication between them is not successful. This causes a delay in the blocks being added to the chain. This is known as a Byzantine fault. A Byzantine Fault Tolerance system allows a certain amount of these “malicious nodes” to be tolerated [11]. As a result, blocks can be added to the chain as per usual, without any delay being caused by these nodes.

The Practical Byzantine Fault Tolerance consensus algorithm is developed from the foundation of the Byzantine Fault Tolerance characteristic. It requires that 2/3 of the nodes of the network are to behave accordingly [11]. 36.36% of the included research papers mentioned this algorithm in its contents [1]–[3], [5], [12], [20]–[30].

Practical Byzantine Fault Tolerance two types of nodes, a primary node and secondary nodes (backup nodes). PBFT has five stages (note: m represents the maximum nodes that can be tolerated in the network) [12]:

1. Request: a block is created by the primary node and distributed on the network.

2. Prepare: a PRE-PARE message will be broadcasted by the primary node and the backup nodes need to verify this message.

3. Prepare: the backup nodes receive the PRE-PARE message and the block and then will broadcast the PREPARE message to the network. To move onto the next stage, the backup node must receive 2m + 1 of the same PREPARE message from the other backup nodes.

4. Commit: the nodes broadcast the COMMIT message to all nodes on the network. It also must wait for 2m+1 of the identical COMMIT message from the other blocks.

5. The primary node can then append the block to the network.

## 4.2 S-RQ2 What are the advantages of the main consensus algorithms found?

The advantages discussed in the research papers stemmed from the consensus algorithms that were mentioned in section 4.1. In this section, the advantages of efficiency, security and scalability will be discussed and how they are achieved by the different blockchain consensus algorithms.

**Efficiency.** There are three different advantageous efficiencies identified namely efficiency as a general term, transaction throughput and energy efficiency. These three advantages will be discussed and how they are obtained differently in the four consensus algorithms:
***Efficiency.*** An advantage of the Proof-of-Stake consensus algorithm is increased efficiency. One of the included research papers stated that Proof-of-Stake is efficient, but none of them provides a clear explanation on why they stated it was efficient [55]. Efficient or efficiency is not an appropriate characteristic to describe Proof-of-Stake. It should rather be paired with another term to more accurately describe this consensus algorithm, for example, energy-efficient.

***Energy efficiency.*** Six of the research papers included stated that the benefit of Proof-of-Stake is its reduced power consumption or energy efficiency [5], [12], [14], [25]–[27]. The way that this reduced power consumption is achieved, is by replacing the computational effort with a randomly weighted selection [23]. Instead of many nodes on the network competing to validate blocks, a node on the network is randomly chosen to become the validator. This eliminates the nodes needing to brute force the correct nonce thus reducing the computational power required as only one node is doing the work instead of all the nodes on the network.

Of the 44 research papers included in the analysis, 9.09% of them agreed that the implementation of the Practical Byzantine Fault Tolerance results in energy-efficiency benefits [1]–[3], [12], [18]. The key to its energy efficiency is by achieving consensus, and not solving complex mathematical problems like Proof-of-Work. However, the speed and scalability of the algorithm will be affected by the message overhead as the network grows in size [11].

***Transaction throughput.*** The advantage of the Hybrid consensus algorithm is its high transaction throughput. It is the combination of the Proof-of-Work and Byzantine Fault Tolerance consensus algorithms. It makes use of the Byzantine Fault Tolerance protocol to come to a consensus. This results in increased energy efficiency as complex mathematical problems don't have to be solved.

***Scalability.*** Two research papers listed Delegated Proof-of-Stake as being scalable [12], [14]. Due to the voting scheme and the process to achieve consensus in Delegated Proof-of-Stake, this algorithm benefits from both increased efficiency and reduced energy wastage [14]. These 2 benefits will allow the application to be scalable with bigger networks.

Two papers deemed scalability as a benefit as a result of the Proof-of-Stake-consensus algorithm [12], [14]. This algorithm involves randomly selecting a validator to create and add a block to the chain. As there is only one agreement (picking the validator) before adding the block to the chain, energy demand is decreased and the general efficiency increases. As stated in the text above, these benefits are a good indication that it will be able to handle a larger load as the network size increases and thus making it scalable.

The general consensus of scalability of the Proof-of-Work consensus algorithm in the analysed papers is inconsistent. Two papers identified it as being scalable [12], [14], whilst two papers deemed it not scalable [22], [23]. Based on the numbers above, Proof-of-Work is not scalable. All the reasons listed for good scalability above, have
energy efficiency as a benefit. The results indicated 35 energy wastage as a
disadvantage of the Proof-of-Work consensus algorithm.

**Security.** Practical Byzantine Fault Tolerance and Proof-of-Work having the advantage of security. Security in the Proof-of-Work algorithm is a result of including the hash pointer in the block [1]. As discussed in section 2, this hash pointer is what links blocks together. Any modification to a block will essentially change its hash and it will not match one of the hash pointer [6].

Practical Byzantine Fault Tolerance is said to be secure by two of the analyzed papers. It was shown that it would tolerate a certain number of malicious and would ignore these nodes. The security is increased as malicious nodes have no say in the network.

## 4.3 S-RQ3 What are the disadvantages of the main consensus algorithms found?

This section will identify and elaborate on the common disadvantages identified of the main consensus algorithms as identified in section 4.1.

**Common disadvantages of Proof-of-Work.** Proof-of-Work is the consensus algorithm that Bitcoin uses in its blockchain architecture and it's the original consensus algorithm [25]. The most common disadvantages experienced by Proof-of-Work is security issues, wastage of energy, transaction throughput, and high latency.

The most common disadvantage of the Proof-of-Work consensus algorithm is that it is not energy efficient. As many as 35, (79.55%) of the included research papers, listed this as one of the disadvantages of the algorithm. The energy wastage occurs as many miners compete with each other to validate blocks. They compete with each other by solving complex mathematical problems by using brute force to determine the correct nonce that would solve these problems[27]. Brute forcing requires a lot of computational effort which leads to energy wastage[20]. This process of brute-forcing the correct nonce doesn't even guarantee that the miner will be the one chosen to validate the block and their effort could all in vain.

Transaction throughput is the number of transactions that can be processed in a certain period [6]. Low transaction throughout was another recurring disadvantage of the Proof-of-Work consensus algorithm. Four of the research papers agreed on this disadvantage [6] and [9]. The low throughput is a result of the creation and addition of blocks. As stated in section 1, miners validate and include transactions in a block. Once the block with the transaction is inserted into the chain, transactions have to wait for several additional blocks to be added to the chain before the transaction is confirmed [6]. The slow process of adding blocks to the chain delays the transaction confirmation and thus lowering the transaction throughput.

Seven out of the 44 research papers analyzed agreed that high latency was a recurring challenge of the Proof-of-Work consensus algorithm [1], [5], [6], [9], [22], [40], [31]. It is the period between a transaction and the time it takes for the transaction to be processed [5]. Block intervals are what determines the latency of a consensus
algorithm[32]. The above text identified that it is a very slow process for blocks to be
added to the chain in the Proof-of-Work consensus algorithm. This increases the block
interval time and thus increasing the latency in the network.

Six of the research papers included in the research study, all had security issues as a
disadvantage for the Proof-of-Work consensus algorithm [24], [25], [36], [42], [47] and
[48]. Security is one of the challenges as numerous attacks were developed to target
and penetrate this consensus algorithm. Some of the attacks that were included in the
included research paper are as follows:

*   **51% attack** – In the Proof-of-Work algorithm, a block can only be added to the chain if the majority of nodes in the network deem it is valid. To verify the validity of the block, miners need large amounts of computational power, also known as hashing power. This hashing power helps determine the performance of the specific miner. The 51% attack is when an organization or an entity is in control of the majority of the hashing power [29]. As they have the majority of the hashing power, they can carry out the following actions [22]:
    *   Control if blocks get validated or not.
    *   The ability to exclude or modify the ordering of transactions.
    *   It also gives them the ability to prevent the confirmation of transactions.
    *   Proof-of-Work makes this attack difficult to carry out as an organization would need an enormous amount of hashing power which would not be feasible [24].

*   **Double-spending** – the double-spending attack involves spending the same currency twice. This can be carried out by taking a conflicting transaction from another branch and transferring the funds back to the attacker [35].

**Common disadvantages of Proof-of-Stake.** The common disadvantages of Proof-of-Stake is the threat of centralization, and the rich nodes having the ability to take advantage of the network. Both of these disadvantages all relate to the security of the consensus algorithm and it is the most common disadvantage identified in the research papers included in this study. Six is the number of analyzed research papers that identified security as one of Proof-of-Stake’s disadvantage [4]–[10].

As security is the biggest disadvantage, this section aims to provide light on some of
the attacks that were identified in the included research papers. This will be done by
listing and explaining the most common attacks that would be possible on the Proof-
of-Stake consensus algorithm.

There are three different types of long-range attacks: simple, posterior corruption and Stake Bleeding. All three of these long-range attacks aim to do one thing, replace the existing chain with a new chain that begins from the Genesis block (the initial block. In this research article, we will only be discussing the Stake Bleeding attack.

As mentioned above, the Stake Bleeding attack occurs when another chain is created
from a genesis block. Each new node is provided with the Genesis block. This new
chain becomes replaces the original one. A new node to the network always begins with
the Genesis block. They try to build this chain up until it's longer or more valid than
the valid chain. They are still a validator in the original chain, but when they get chosen
to validate a block (become the slot leader), they skip their turn. This is called a
Liveness Denial [6]. Because of the mechanics of Proof-of-Stake, no block is generated
in this phase. The attacker's stake does decrease as the process goes on, which makes it less likely that they will get chosen. At the same time, they begin validating and adding blocks to their chain. The malicious validator also copies the transactions that occur on the main chain and includes them in their chain. As they are validating transactions, they receive a stake that allows them to compete in the original chain. Once the chain outpaces the original one, they make one more stake to other validators and then publish this other branch.

Another security issue of the Proof-of-Stake is possible centralization. The following papers have identified this as a common security issue of the Proof-of-Stake consensus algorithm. Nodes are selected to be validators by a weighted random selection process. The more a node stakes, the higher the probability that they will be selected as a validator. This means that the rich will get richer and centralization will start occurring [24].

## 5 Future work and implications for researchers

This systematic literature review provides a researcher with key characteristics to take into consideration when creating optimized blockchain consensus algorithms. It identified these characteristics to be security, scalability, and efficiency. As a result of the findings of this research paper, the following areas of research are proposed:

* Quantitative research into the scalability, security and efficiency of blockchain solutions on a larger scale.

* Quantitative research on how efficiency can affect the security and scalability of blockchain solutions.

* Quantitative research into the efficiency of blockchain consensus algorithms. A common pattern noticed in the analysed papers was that they relied on comparing the consensus algorithms to Proof-of-Work and logically deducing that it's more energy efficient. This may be the case, but statistical real-world evidence needs to be researched to determine the true energy consumption of a consensus algorithm.

## 6 Conclusion

Blockchain will disrupt many industries and the core component of this technology is consensus algorithms. Currently, Proof-of-Work and Proof-of-Stake are two of the most popular algorithms, but they yield many disadvantages [24]. As a result, this systematic literature review identified the characteristics to create optimized blockchain consensus algorithms. These findings indicated that there were many different advantages and disadvantages for the different consensus algorithms. From these findings, key characteristics identified were scalability, security, and efficiency. It was determined that these three key characteristics are key to developing optimized solutions as they will affect real-world applications. Due to the findings of this research paper, further research areas related to these characteristics were proposed.
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