E-Voting is the adoption of information and communication technology systems for supporting the casting, recording, and counting of votes in an electoral process. E-Voting systems introduce the advantages of increased voter convenience, electoral result accuracy as well as fast tabulation and counting of votes (for and Idea, 2011). For an e-voting system to be acceptable for use, it must satisfy some security and functional requirements, some of these requirements are (Liu and Wang, 2017; Bungale and Sridhar, 2016);

a. Confidentiality and Privacy: implies that the e-voting system must ensure that it is impossible to identify how or whom an electorate voted for.

Blockchain is a distributed ledger system of transactions, stored in blocks of data that are linked together cryptographically and are decentralized among various participants, resulting in the resistance to attack and immutability of such transactions (data) (Hanifatunnisa and Rahardjo, 2017) (Abuidris et al., 2019; Zheng et al., 2017). Blockchain technology has found wide adoption in e-voting systems. The various e-voting system implementations that are based on the blockchain can be classified into three categories (Yu et al., 2018);

Smart contracts in blockchain refer to computer programs written to facilitate contractual terms, which are deployed on the blockchain for execution when certain predefined conditions and requirements are satisfied (Hu et al., 2018). Various e-voting systems have been developed as smart contracts and deployed on various blockchain platforms, as in the case of (Yavuz et al., 2018; Hjálmarsson et al., 2018; Patidar and Jain, 2019) that put forward e-voting systems as smart contracts using the Solidity language provided by the Ethereum blockchain network. Also, (Kirillov et al., 2019; Kost’al et al., 2019; Vivek and Yashank, 2020), proposed e-voting systems deployed as smart contracts using the Hyperledger blockchain framework. Though the various systems offered integrity and immutability of records, however, the adoption of smart contracts entails having to maintain a public ledger that is visible to every participant in the blockchain network and translates to a privacy issue (Nzuva, 2019). The implication of this to its adoption in e-voting systems is the possibility of compromising voter privacy. Also, there is the issue of resistance of smart contracts to the amendment of contractual terms and conditions, which does not allow for the reflection of real-life dynamics and changing conditions (Nzuva, 2019). The translation of this to the various e-voting systems that have adopted this approach, is the inability to introduce into the system, the dynamics that may occur in electoral processes.

Cryptocurrencies are assets based on a blockchain system, which adopt cryptography for securing the exchange and transfer of such assets, between the participants of the blockchain system (Giudici et al., 2019). E-Voting protocols have also been developed to adopt the use of cryptocurrency in electoral processes, as in the case of (Zhao and Chan, 2016; Jason and Yuichi, 2016; Bao et al., 2018) that adopted the bitcoin for developing an e-voting system protocol, as well as (Fusco et al., 2018) that adopted cryptocurrency from a permissioned blockchain system to support e-voting. These systems achieved decentralization of authority, immutability as well as public verifiability of the electoral process. However, the adoption of cryptocurrency in e-voting does not protect the privacy of electorates (Fleder et al., 2015). Also, the adoption of cryptocurrency introduces a bottleneck in the number of transactions (votes) that can be processed at a time by the system, as in the case of bitcoin, due to its slow transaction rate (Sänger, 2019).

In traditional voting systems, ballot-box refers to storage boxes into which ballots are cast by electorates during an electoral process. There are e-voting systems that have adopted a seemingly ballot-box approach in the adoption of blockchain for securing the electoral process. In such e-voting systems, votes (ballots) are digital messages of a predefined structure, which are sent as transactions to the blockchain (Sheer Hardwick et al., 2018). Various researchers have adopted this approach in developing blockchain-based e-voting systems as in the case of (Sheer Hardwick et al., 2018; Wang et al., 2018). This approach allows for the blockchain-based e-voting system to be developed to reflect real-life features and dynamics of e-voting systems, as well as an avenue for coupling a privacy-oriented encryption scheme with the blockchain system (Bellini et al., 2020). As such this study goes on to adopt this approach in developing an e-voting system.

Several schemes have been developed to address the privacy concerns of the blockchain. These schemes are classified into two main categories (Feng et al., 2019):

This is the category of blockchain privacy preservation schemes are aimed at the protection of addresses (identity) of senders and receivers of assets (such as cryptocurrency) exchanged on a blockchain system (Feng et al., 2019). Schemes under this approach include; Ring signature, Non-interactive Zero-Knowledge proof (NIZKP), and Mixing services. This category of schemes is not suitable for the preservation of privacy in a blockchain as a ballot-box e-voting system.

This category of blockchain privacy preservation schemes aims at protecting the actual content of the transactions that take place on a blockchain system (Feng et al., 2019). Privacy preservation schemes in this category are:

a. Non-Interactive Zero-Knowledge Proof (NIZKP): is a scheme that allows for a party (prover) to prove to another party (verifier), the validity of a statement, without having to reveal any other information, through a process that does not involve interaction between the two parties (Blazy, 2012; Partala et al., 2020). NIZKP has been adopted in several blockchain-based e-voting systems (Sallal, 2019; Hjálmarsson et al., 2018). Though these systems guaranteed security, transparency as well as protection of voter privacy in e-voting, however, these approach suffers from scalability issues due to the enormous and massive computation cost requirements of generating the proofs (George and Samman, 2016; Bhardwaj, 2020).

(Jabbar and Alsaad, 2017) proposed a remote electronic voting system using the ElGamal cryptosystem for ensuring the security of votes. The system ensured the preservation of ballot privacy. However, the adopted ElGamal cryptosystem was, slow in performance during the encryption of votes. Also, (Mustafa and Waheed, 2021), developed an e-voting system using the permissioned Hyperledger Fabric (HLF) platform. The system adopted a blind signature scheme and Zero-Knowledge Proof (ZKP) to establish security, transparency, and privacy. However, the adoption of multiple identities obscuring schemes meant that the system traded performance for voter privacy. Also, an e-voting system with a universal verifiable voting vector was proposed by (Zou et al., 2017) to allow for the verification of ballots. The system ensured transparency and auditability of the entire ballot process. The system however failed to put in place modalities for the protection of voter privacy and confidentiality. (Pawlak et al., 2018) proposed a schema for the development of auditable and end-to-end verifiable e-voting systems through the adoption of multiple intelligent agent nodes, namely; super-node, polling stations, and trusted nodes. At the end of the election, the votes at the trusted and super-node chain are tallied and counted accordingly. The system allowed for suitability of the electoral process, however, it prioritized certain nodes in the blockchain network over others, causing some nodes to have outsized influence over the entire network, leaving room for manipulation of the electoral process. Wu, (Wu, 2017), implemented an e-voting system based on a private blockchain system with a three-entity model, namely the voters, registration authority, and election authority, all subject to public supervision. It also implemented ring signature together with RSA cryptosystem for unforgeability of ballots. The system ensured ballot anonymity. However, due to the decentralized nature of the blockchain system, it would be possible for the permitted participants of the blockchain to view the ledger of the blockchain and find out how voters had voted. Consequently, Liu and Wang, (Liu and Wang, 2017), proposed an e-voting system based on Proof-of-Stake (PoS) consensus Blockchain, with a blind signature. The system ensured, transparency and immutability of ballots. The adoption of Proof-of-Stake consensus by the system meant the need to prioritize some nodes of the blockchain over others, meaning that some nodes would have an outsized influence on the network (Zamostin, 2019). Similarly, Zhang et al. (Zhang et al., 2019) introduced an e-voting system based on the public Ethereum Blockchain system with message authentication and transmission mechanism, to prevent forging of ballots, together with Blind signature, for validation of message authenticity. The system ensured the prevention of vote manipulation with proper decentralization of authority. However, the adoption of public Ethereum meant that payments had to be made (in gas) equivalent to the amount of work done in mining the e-voting smart contract, thus, incurring additional expenses for the conduct of elections.

Yi, (Yi, 2019), also designed an e-voting system based on a private blockchain using a synchronized model of distributed ledger technology (DLT) for the prevention of ballot forgery and elliptic curve encryption of voter credentials, to provide authentication and non-repudiation. The distributed nature of the blockchain ledger however meant that the permitted participants of the blockchain were able to view how voters have voted, which is a privacy flaw. Consequently, Mohammedali and Al-Sherbaz, (Mohammedali and Al-Sherbaz, 2019), put forward an e-voting framework based on a private blockchain, coupled with an elliptic curve algorithm. The system achieved transparent balloting (casting of votes), there was, however, no proper decentralization of authority which gives room for manipulation of the integrity of ballots. Arun et al., (Arun, 2019), developed an e-voting system based on the currency transaction approach on the Ethereum blockchain, by the allocation of a digital coin to cryptographic wallets assigned to each voter. The system achieved audibility and transparency with the immutability of votes, however, failed to put in place considerations for the protection of the confidentiality of votes. Thereafter, Park, (Park, 2019), put forward the adoption of a decentralized Proof-of-Work (PoW) consensus-based blockchain for decentralization in an e-voting system. Though the system achieved immutability of votes, the permissioned nodes on the blockchain could probe the ledger of transactions to see how electorates had voted, thereby compromising the privacy of the electorates.

After a careful review of the literature, it can be seen that the adoption of blockchain in e-voting systems guarantees integrity with the possibility of compromising the confidentiality of electorates. There is therefore the need to adopt a technique that ensures the protection of the confidentiality of the blockchain system. The following section, therefore, describes the technique adopted in this study for achieving the goal of protection of confidentiality with integrity in e-voting.

Figure 1 gives an illustration of the structure of the ballots which will be passed as transactions to the blockchain serving as a ballot box. Consider an election with contestants A, B, … , P and electorates, W, X, Y, Z.

The ballot cast by each of the electorates is structured such that a voter cast a value of one for the contestant of their choice, and casts a value of zero for all of the other contestants in an array. Meaning that the ballot cast by each of the electorates is an array with a length equal to the number of contestants, P in the election. Taking an electorate, W that cast a ballot, B _WN, for a candidate, C _S then; B WN    =   [ V WA ,   V WB ,   … ,   V WP  ] (1) such that; | B WN | = P (2) and; B WN =   { 0   ;   N   ≠ C s 1   ; N = C s (3)

After a voter has cast their ballot, B _WN, there is a need to preserve the confidentiality of the ballot through the process of encryption. This is done by the adoption of Paillier homomorphic encryption, which provides additive homomorphism needed to tally and sum together the encrypted votes. Section 3.2 provides insight into how this is achieved.

Paillier homomorphic encryption is a partially homomorphic encryption scheme that supports additive homomorphism, which is an operation that allows two ciphertexts to be multiplied, resulting in a ciphertext whose decryption is a sum of the corresponding plaintexts. Consider a scenario with two data, D ₁ and D ₂ , both subjected to Paillier encryption operation, E, using a public encryption key (n, g), thereby producing two separate ciphertext values, C_D1 and C_D2, such that; C D 1 = E ( D 1 ,   S 1 ) (4) and; C D 2 = E ( D 2 ,   S 2 ) (5) where; S ₁ and S ₂ are two randomly selected integer values, such that; 0   <  S  <  n (6)

This encryption protocol is adopted for the encryption of each voter ballot, B _WN , as specified in Eq. 1, such that encryption of the ballot of an electorate, W is given by;   C DWN = E  ( B WN   ,  S ) (7) C DWN = [ E ( [   V WA ] ,  S ) ,  E ( [   V WB ] ,  S ) ,   … ,  E ( [   V WP ] ,  S ) ] (8)

The value of C _DWN, as specified in Eq. 8 is the ballot transaction that is then sent by each electorate to the blockchain system which serves as a ballot box for e-voting.

After encrypting the ballots and committing the encrypted ballots to the blockchain, at the end of the electoral process, there is a need to tally and count the casted votes. Since the votes have been encrypted homomorphically, the Paillier cryptosystem provides additive homomorphism which allows computations to be carried out on encrypted data without the need for initial decryption. Section 3.2.1 provides further insight into how additive homomorphism is achieved.

Open source blockchain repository provided by (Emiceli, 2019; Traub, 2018), both implemented using Node. js (a back-end and cross-platform runtime environment), were adopted for developing a private blockchain system on which the e-voting system is based. In the blockchain, transactions are placed in blocks that contain;

a. A pseudonymous blockchain address of an electorate, with the prefix, 0x00FUTMCPE.

The blockchain system verifies the authenticity of transactions being carried out through the adoption of a consensus protocol. This consensus protocol is the backbone of the blockchain which helps prevent bad actors (nodes) from tampering with the blockchain. The consensus protocol in use in the adopted blockchain is further described in Section 3.3.1.

Figure 4 shows the architecture of the system. At the core is the blockchain private blockchain system, which maintains a ledger. At the polling unit, the voters (electorates) cast their ballots, in the form specified in Eq. 1, after which the ballots are encrypted homomorphically, thereby transforming the ballots into the structure specified in Eq. 8. A new block of the transaction is then created, containing; the encrypted ballots, the pseudonymous address of the voter and the admin, the timestamp of the block creation, the hash of the previous block of the transaction, as well as the hash of the current block. Then the new block of the transaction is mined using the consensus mechanism as illustrated in Figure 3. After mining, the new block is committed to the ledger of the blockchain.

This process continues, until the end of the election when the electoral authority decides to end the election. Then the admin, by additive homomorphism, tallies the encrypted votes, thereby obtaining a final encrypted sum that can be decrypted to obtain the results of the election. The electoral authority can then proceed to make the result of the election public, by sending it to the email address of all the electorates that voted in the election.

This section explains how the performance of the system was evaluated in terms of the transaction throughput of the blockchain, as well as the system’s response time to various user actions. The sequence of actions taken to test and evaluate the performance of the system is given below;

Step -1: An API endpoint was developed on the blockchain, to allow for simulation of a varying number of transactions, which is accessed through a webpage interface as can be seen in Figure 5.