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Curr. Opt. Photon. 2022; 6(5): 453-462

Published online October 25, 2022 https://doi.org/10.3807/COPP.2022.6.5.453

Copyright © Optical Society of Korea.

Privacy Information Protection Applying Digital Holography to Blockchain

Seok Hee Jeon1, Sang Keun Gil2

1Department of Electronic Engineering, Incheon National University, Incheon 22012, Korea
2Department of Electronic Engineering, The University of Suwon, Hwaseong 18323, Korea

Corresponding author: *skgil@suwon.ac.kr, ORCID 0000-0002-3828-0939

Received: July 20, 2022; Revised: September 4, 2022; Accepted: September 14, 2022

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Blockchain technology provides a decentralized and peer-to-peer network, which has the advantages of transparency and immutability. In this paper, a novel secure authentication scheme applying digital holography to blockchain technology is proposed to protect privacy information in network nodes. The transactional information of the node is chained permanently and immutably in the blockchain to ensure network security. By designing a novel two-dimensional (2D) array data structure of the block, a proof of work (PoW) in the blockchain is executed through digital holography technology to verify true authentication and legal block linkage. A hash generated from the proposed algorithm reveals a random number of 2D array data. The real identity of each node in the network cannot be forged by a hacker’s tampering because the privacy information of the node is encrypted using digital holography and stored in the blockchain. The reliability and feasibility of the proposed scheme are analyzed with the help of the research results, which evaluate the effectiveness of the proposed method. Forgery by a malicious node is impossible with the proposed method by rejecting a tampered transaction. The principal application is a secure anonymity system guaranteeing privacy information protection for handling of large information.

Keywords: Authentication, Blockchain technology, Digital holography, Hash, Optical encryption

OCIS codes: (060.4785) Optical security and encryption; (070.1170) Analog optical signal processing; (070.4560) Data processing by optical means; (090.1995) Digital holography; (090.2880) Holographic interferometry

If you are handling critical information in a specific application, you can access the information after a log-in process and may be asked to change specific information through authentication proof. However, simple authentication cannot be definitive proof because the log-in process itself may be wrongfully falsified, even if log-in actions are recorded chronologically. In a secure information system, it is a big problem if any piece of information can be substituted with false information by tampering. To prevent a tamper from falsifying information, the information must not be changed even if forged information asks to change the original true information. One method to keep the information from unchanging is that the majority of information loggers have truthful copies of information by a correct log-in scheme. The solution to this problem is that we have a protocol for recording the information in the log and distributing copies of it to network users, which composes a decentralized database. As one of the emerging methods in recent years, blockchain is essentially a distributed database that can overcome the drawbacks of a centralized database. The concept of blockchain technology was first proposed in 2008 by Nakamoto, who developed the first digital currency, Bitcoin [1], which is a decentralized peer-to-peer (P2P) cryptocurrency and has the characteristics of anonymity. Blockchain technologies are gradually developing in the world and their applications have expanded to other areas such as cybersecurity, smart contract, 5G technology, internet of things (IoT), artificial intelligence (AI), education, etc. Basically, blockchain technology is based on a digital method in an accessing network to ensure trustworthy authentication and security. Recently, some researchers have studied the adoption of blockchain technology by optical networks. Kou et al. [2] in 2017 proposed a blockchain mechanism based on enhancing the consensus for optical networks. Blockchain-based optical networks for 5G applications were presented by Yang et al. [3, 4] and Nag et al. [5], respectively. Fichera et al. [6] proposed blockchain-anchored failure responsibility management in disaggregated optical networks, and Zhang et al. [7] proposed a blockchain-based resources trusted consensus solution for multi-domain edge optical networks. While digital information processing uses electrical signals, optical information processing uses light as the basis of its operations, the advantage of which is fast optical computing. New optical methods for blockchain-based applications were studied to overcome the shortcomings of digital blockchain technology. In 2019, Lysenko et al. [8] studied a method for improving the capabilities of measuring large data arrays with the use of optically controlled transparent and blockchain technology, and Chaintoutis et al. [9] proposed a novel optical physical unclonable functions (o-PUFs) implementation that can be combined with private blockchains. In 2020, Dubrovsky et al. [10] reported their research on optical proof of work (oPOW) that uses the next generation of computer processing chips called optical processing units (OPUs) to lower the amount of energy consumed by cryptocurrency mining. In 2022, Yan et al. [11] proposed a blockchain-based optical communication security transmission system scheme to protect information leakage.

In this paper, we propose applying digital holography to blockchain technology for privacy information preservation and security, which is based on recent research and called an analog signature scheme using an RSA digital signature algorithm and phase-shifting digital holography (PSDH) [12]. This paper is organized as follows. In Section II, an overview of blockchain architecture is briefly presented and the proposed method to conduct blockchain technology using digital holography is described. The performance and feasibility of the proposed method are analyzed with research results in Section Ⅲ. Conclusions are summarized in Section Ⅳ.

2.1. Blockchain Overview

Blockchain is a chain of blocks, and it is essentially a distributed database of information or a distributed transaction ledger in a P2P network without a centralized authority. The blockchain network consists of nodes, and each node has a copy of the same transaction. Typically, a participant who connects to the blockchain is called a node. Each block includes a transaction (i.e. information) with a cryptographic hash and is linked to its previous block by verification. Figure 1 shows the structure of blockchain. As shown in Fig. 1, each block consists of a header section and a body section. The header may comprise information of the version, time stamp, hash of the previous block header, nonce, etc. The body may comprise a list of transactions. The main principal work of blockchain technology is to verify a newly generated block by a kind of consensus algorithm, and the most well-known one is the so-called proof of work (PoW). The body of a block is used in PoW and a time stamp, Merkle root, nonce, etc., are also used in PoW. In particular, the work of generating blocks in the network is called mining. A nonce is an arbitrary random number and can be useful as an initialization vector. Mining is done to find a nonce that satisfies a limited condition of a hash value. The blocks are linked together using the hash of previous blocks in a chronological sequence, which provides the blockchain with an immutability feature in a way that changing one block would require changing the headers of subsequent blocks. The transaction in the block is verified by examining its signature and checking for the existence of the previous hash in the same transaction. If the transaction is verified, it is permanently recorded in the blockchain and cannot be deleted or changed. Thus, the node creates a new block by consensus algorithm that is executed to reach an agreement by a majority of the nodes in the blockchain network without a centralized authority.

Figure 1.Structure of blockchain.

Blocks are validated using a computational process of PoW to ensure authentication. To carry out PoW, blockchain technology needs two cryptographic functions, hash and digital signature, which are used for signing and verifying information. A hash is represented as a one-way function that makes blockchain more secure and information in the blockchain immutable. A hash algorithm generates a fixed-length string of the input, where the output string is called an input hash value. Its features ensure that the blockchain cannot be tampered with. Figure 2 shows the blockchain linking through PoW. Network nodes interact with the blockchain using a hash and asymmetric cryptography with private/public key pairs. Each node converts block information including the previous hash and transactions into a hash using a hash algorithm. In most blockchains, Secure Hash Algorithm-256 (SHA-256) is used for the hash algorithm. This hash is converted to a digital signature by signing it with a private key. After transmitting the block to nodes in a network, the previous block’s hash is verified with a public key. If the node succeeds in verification, a new block is chained to its previous block and become a permanent block, and its information is not tampered with. Thus, the blockchain structure has a chronological chain of blocks from the genesis block to the current block. Blockchain technology can form a decentralized database in a secure way, providing the characteristics of robustness, transparency, and anonymity. Its principal applications range from cryptocurrency to business contracts, ensuring a secure digital identity. A major weakness of blockchain is its huge energy consumption and privacy information leakage.

Figure 2.Blockchain linking through proof of work.

2.2. Proposed Blockchain Scheme to Secure Privacy Information Using Digital Holography

No information communication system can avoid the problem of information insecurity. A well-known problem in blockchain schemes is transactional information privacy. Despite the advantages of blockchain with immutable and secure data management, it has still shortcomings in information privacy and confidentiality. Most users in the network are concerned about the traceability of individual information because block transactions are stored as public transactions that are propagated over the network. If individual privacy information is not preserved, hackers can easily track the target node by analyzing the transmitted transactions. Thus, privacy information protection becomes more important for information exchange and sharing. Several public key cryptographic algorithms can be adopted to protect privacy information, but these algorithms rely on their implementation. This paper is motivated by this challenge. Blockchain is a secure distributed information technology that involves network nodes, ensuring high anti-tampering reliability. So, we propose a blockchain-based privacy information protection authentication scheme to integrate digital holography technology with blockchain technology using a carefully designed block structure. It is effective to apply PSDH to blockchain. Essentially, to implement a privacy information protection scheme, the conventional digital signature algorithm used in PoW of blockchain is modified to a realizable asymmetric key algorithm using PSDH. Figure 3 shows the proposed blockchain-based information network consisting of nodes with PSDH. Schematically, the network consists of several nodes, which are participants in the blockchain system. Each node has PSDH hardware that is assumed to be implemented optically. PSDH consists of an optical Mach-Zehnder type interferometer to encrypt information. However, nodes should verify all the blockchain blocks, known as PoW consensus. These nodes (called full nodes) connect the blocks with transactions to the blockchain, and determine whether the blocks are valid. In contrast to public blockchain networks, the proposed blockchain network can be applied to a permissioned blockchain network because only the node with PSDH can validate the blocks and access the network. This means that the proposed blockchain is a kind of private blockchain network. Figure 4 shows a carefully designed block structure for the proposed blockchain algorithm. The proposed block structure consists of a header and a body. The header consists of a time stamp, version, number of transactions, Merkle root, nonce, and previous hash, while the body consists of a new transaction, three public key ciphers, and two digital signatures. In principle, a previous hash, a new transaction, a public key cipher, and a digital signature have the same format of 2D binary array data in this paper.

Figure 3.The proposed blockchain-based information network consisting of nodes with phase-shifting digital holography (PSDH).

Figure 4.Designed block structure for the proposed blockchain algorithm.

Blockchain mainly uses digital signatures to achieve consensus, verifying the proper transaction block generator, and preventing tampering with the wrongful transaction of unjustifiable nodes. Digital signatures use asymmetric cryptographic algorithms, that is, each node requires private/public key pairs. Figure 5 shows the PoW procedure for a new block linkage. As shown in Fig. 5, the digital signature algorithm includes two processes. The first operation is signature generation by signing the hash of transactions and the second operation is signature verification. To propose privacy information protection by applying digital holography to blockchain, we use the method reported in a previous paper [12], a proposal for an analog signature scheme based on RSA digital signature algorithm and phase-shifting digital holography. In [12], the term analog signature was used instead of digital signature because the encrypted digital signature forms a noise-like analog pattern. According to [12], which describes a method to modify the conventional digital signature algorithm into an optically realizable digital signature algorithm, the proposed solution would generate optical digital signatures of a hash and find an optically reconstructed hash. The optical architecture to create a new hash optimized for optical processing was not considered due to the complexity of optical implementation. This difficulty leads to a hybrid scheme that composes digital processing to produce hashes.

Figure 5.Proof of work (PoW) procedure for a new block linkage.

The PoW procedure for a new block linkage is described as follows. A block structure has a header and a body, as shown in Fig. 4. In this paper, a new transaction, public key ciphers, digital signatures in the body and only the previous hash excluding the left part in the header shown in Fig. 4 are assumed to be used in PoW for convenience. Two random numbers of 2D binary array P and Q are used in the private/public key pairs generation process. In the digital signature generation process, after performing a two-step PSDH function PSDH{∙}with these random numbers, three public key ciphers, PC1, PC2, and PC3, are obtained by

PC1,2,3=PSDHP,Q.

To generate private/public key pairs X, Y and a secret key S in binary data, a threshold function TH{∙} is applied to public key ciphers, giving

X=THPC1,PC2,PC3,Y=THPC3,S=THPC1,PC2.

On the other hand, a present block transaction T_pres is logically expressed by the combination of a previous hash H_prev, a previous transaction T_prev, and a new transaction T_new, which are stored in the block header and body.

Tpres=(TprevTnew)Hprev,

where ⴲ denotes XOR logic operation. Next, the signature execution node converts the present transaction T_pres to a present hash H_pres with the private key X and the signal key S by an agreed cryptographic hash algorithm Hash1{∙} as

Hpres=Hash1Tpres,X,S.

Originally, the private key X and the secret key S are generated from two random numbers P and Q by operating Eqs. (1) and (2). Mathematically, hash algorithm Hash1{∙} is one-way function because the PSDH function PSDH{∙} in Eq. (1) uses a Fourier transform function and the resultant public key ciphers are used to generate the private key X and the secret key S by a threshold function TH{∙} of Eq. (2). In generating digital signatures DS1, DS2, the node carries out the same PSDH function with the present hash H_pres and the private key X as

DS1,2=PSDHHpres,X

These digital signatures have a noise-like analog pattern so that a third party cannot deduce the original present hash while transmitting data. Now, the node disseminates two digital signatures DS1 and DS2 and releases three public key ciphers PC1, PC2, and PC3 to every node in the network. Similarly, public key ciphers have a noise-like pattern, which makes it difficult for a third party to find the binary data of public key Y. These ambiguity characteristics of the generated data guarantees an advantage to secure information transmission and privacy.

In the digital signature verification process, the node to verify digital signatures retrieves the public key Y and the secret key S from the three released public key ciphers PC1, PC2, and PC3. From the transmitted block, the previous hash H_prev and the present transaction T_pres are known to the verification execution node. With the retrieved public key Y and secret key S, the transmitted previous hash H_prev and the present transaction T_pres, another agreed cryptographic hash algorithm Hash2{∙} produces a computed hash H_com as

Hcom=Hash2Tpres,Y,S.

Next, to decrypt the present hash H_dec from two transmitted digital signatures DS1 and DS2, the verification execution node performs a PSDH function with the public key Y and the secret key S as

Hdec=PSDHDS1,DS2,Y,S.

If the computed hash value H_com is the same as the decrypted hash H_dec, the digital signature is valid and a created block with a new transaction can be linked in the blockchain permanently by consensus, ensuring that the original information of transactions has not been tampered with during data transmission. In other words, a malicious node cannot impersonate any legal node to substitute true transactions with forged transactions without true authentication.

On the one hand, a generated present transaction of Block N at Node N is generally obtained by

TN_new=(T0_newT1_newT2_newTN1_new)(H0H1H2HN1)

which is a sequential XOR logic operation of the previous transactions and the previous hashes of each block. To extract transaction information corresponding to each block, the following processing is operated as

TN_new=TNTN1,

where this processing is only allowed for authorized nodes in the network. The total number of new transactions in the body of Node N is stored in the header section of the block as shown in Fig. 4, and is referenced to recall the corresponding transaction.

3.1. Performance Evaluation of the Proposed Method

The performance of the PoW procedure for a new block linkage as shown in Fig. 5 is evaluated for the feasibility of the proposed scheme. In this paper, we assume a block structure as shown in Fig. 4. Binary data of size 256 × 256 pixels (8,192 bytes) are used for a component of the block header and body including a previous hash, a new transaction, public key ciphers, and digital signatures, while data of size 16 × 8 pixels (16 bytes) is enough to express year, month, day, hour, minute, and second as a group of time stamps, 16 × 2 pixels (4 bytes) for version, 16 × 2 pixels for number of transactions, 16 × 2 pixels for Merkle root, and 16 × 2 pixels for nonce, respectively. However, we exclude the left part of the header in Fig. 4 for evaluation convenience.

The blockchain architecture as shown in Fig. 3 must start from a special block (the genesis block), which contains the first transaction initiated by a network. However, only a single blockchain exists, that is, a single sequence of blocks including all the information from the first transactions to the present transactions. This consequent linkage happens by the PoW procedure shown in Fig. 5. Simply, it is assumed that each node in the network creates its own block to join in the blockchain. The block contains privacy information such as an ID for accessing the network by a log-in process. As shown in Fig. 5, let us suppose two nodes, Node 3 and Node 4, have the same blockchain from Block 0 to Block 3. Now, Node 4 (the truthful identity of David) wants to create a new block, Block 4, and connects it to the legal blockchain by PoW consensus. Figure 6 shows data of two random numbers, private/public key pairs and a secret key. Figures 6(a) and 6(b) show two random numbers of 2D binary array P4 and Q4 generated at Node 4. Three public key ciphers are obtained by PSDH function PSDH{∙}as Eq. (1), and binary private/public key pairs X4, Y4 and a secret key S4 are computed by a threshold function TH{∙} to three public key ciphers as Eq. (2). Figures 6(c)6(e) show private/public key pairs X4 and Y4, and a secret key S4, respectively.

Figure 6.Two random numbers, private/public key pairs and secret key: (a) a random number P4, (b) a random number Q4, (c) a public key X4, (d) a private key Y4, and (e) a secret key S4.

Figure 7 shows the generation of a transaction in the body of Block 4. For example, Node 4 makes its individual privacy information that is needed to register the log-in identity with the network, which is then encoded to QR code transaction Tx4_new. The data size of the QR code depends on the volume of information. Figures 7(a) and 7(b) show privacy information of Node 4 and its QR code denoting new transaction Tx4_new of Block 4, respectively. Figures 7(c) and 7(d) represent the previous transaction Tx3 in the body of Block 3 and the previous hash H3 in the body of Block 4, respectively. After an agreed XOR logic operation, a present transaction Tx4 is generated by Eq. (3) as Tx4 = (Tx3Tx4_new)ⴲH3. Figure 6(e) shows Tx4.

Figure 7.Transaction generation of Block 4: (a) privacy information of Node 4, (b) a new transaction Tx4_new of Block 4 [QR code of (a)], (c) the previous transaction Tx3, (d) the previous hash H3, and (e) a generated present transaction Tx4.

As described above, three public key ciphers are obtained by Eq. (1) and two digital signatures are obtained by Eq. (5) performing the function PSDH{∙}. Figure 8 shows intensity patterns of three public key ciphers (PC4_1, PC4_2, PC4_3) and two digital signatures (DS4_1, DS4_2), which form noise-like patterns by digital holography encryption.

Figure 8.Intensity patterns of three public key ciphers and two digital signatures: (a) PC4_1, (b) PC4_2, (c) PC4_3, (d) DS4_1, and (e) DS4_2.

In the digital signature verification process, the public key Y and the secret key S are obtained by retrieval processing from the three released public key ciphers PC4_1, PC4_2, and PC4_3. With these retrieved keys and the present transaction Tx4 given by the transmitted block, a hash H4_com is computed by another agreed cryptographic hash algorithm Hash2{∙} as Eq. (6). Also, another hash H4_dec is decrypted from two digital signatures DS4_1 and DS4_2 performing the function PSDH{∙} as Eq. (7). Figure 9 shows the verification of the hash and the reconstructed new transaction of Block 4. Figures 9(a) and 9(b) show a computed hash H4_com and a decrypted hash H4_dec of Block 4, respectively. As shown in Figs. 9(a) and 9(b), H4_com is equal to H4_dec. This means that Block 4 is ensured to connect it to the original legal blockchain. Figures 9(c) and 9(d) show the reconstructed QR code denoting new transaction Tx4_new of Block 4 and the decoded privacy information of Node 4 (the truthful identity of David) from the QR code of Fig. 9(d), respectively.

Figure 9.Verification of hash and the reconstructed new transaction of Block4: (a) a computed hash H4_com, (b) a decrypted hash H4_dec, (c) the reconstructed QR code denoting new transaction Tx4_new of Block 4, and (d) the decoded privacy information of Node 4.

Next, to evaluate the resistance to forgery from the perspective of protecting privacy information, it is assumed that a blockchain linkage from Block 0 to Block 4 was established by truthfully authorized nodes including all the information transactions. Now, an attacker tries to impersonate an authorized node, Node 4, to change privacy information of Block 4. The malicious node (the forged identity of Eve) makes a forged transaction of Block 4 and asks for a validation check. Figure 10 shows the generation of a forged transaction in the body of Block 4. Figures 10(a) and 10(b) show forged information of Node 4 and its QR code denoting a forged transaction Tx4_forged of Block 4, respectively. Figures 10(c) and 10(d) represent the previous transaction Tx3 in the body of Block 3 and the previous hash H3 in the body of Block 4, respectively, which are the same as those shown in Figs. 7(c) and 7(d). Figrue 10(e) shows a forged present transaction Tx4_f.

Figure 10.Forged transaction generation of Block 4: (a) forged information of Node 4, (b) a forged transaction Tx4_forged of Block 4 [QR code of (a)], (c) the previous transaction Tx3, (d) the previous hash H3, and (e) a forged present transaction Tx4_f.

In checking the validity of the digital signature, changed decrypted public key ciphers and the forged transaction Tx4_forged are used to retrieve two hashes H4_fcom and H4_fdec. Figure 11 shows the verification of the hash and the reconstructed forged transaction of Block 4. Figures 11(a) and 11(b) show a computed forged hash H4_fcom and a decrypted forged hash H4_fdec of Block 4, respectively. As shown in Figs. 11(a) and 11(b), H4_fcom is equal to H4_fdec. However, these hashes are not the authenticated previous hash H4 shown in Fig. 11(c), which was represented and stored in the genuine header of Block 4 of the legal blockchain. This disagreement forces the forged transaction to be refused to join in the blockchain. Figures 11(d) and 11(e) show the reconstructed QR code denoting forged transaction Tx4_rf of Block 4 and no reconstructed information of forged Block 4, respectively.

Figure 11.Verification of hash and the reconstructed forged transaction of Block 4: (a) a computed forged hash H4_fcom, (b) a decrypted forged hash H4_fdec, (c) original previous hash of Block 4 H4, (d) the reconstructed QR code denoting forged transaction Tx4_rf of Block 4, and (e) no reconstructed information of the forged Block 4.

In this paper, we propose a decentralized blockchain-based authentication scheme providing very high security and anonymity by the method of encrypting the individual transaction information of each block. Therefore, the real identity of each node cannot be revealed through data transmission and the transaction of the true block cannot be forged by a malicious hacker. The proposed method can be applied to a field handling the privacy identity of the node such as secure sensor data in a ubiquitous sensor network (USN) and state data in robot network nodes.

3.2. Analysis of the Proposed Scheme and Future Research

One of the emerging applications in blockchain technology is Bitcoin. In the case of the Bitcoin blockchain, the header data size of the block consists of 80 bytes. The details are as follows: Time stamp (4 bytes), version number (4 bytes), nonce (4 bytes), difficulty target (4 bytes), Merkle root (32 bytes), and hash of previous block header (32 bytes). In principle, the hash of the block header in the Bitcoin blockchain results in a 32-byte output using the SHA-256 hash algorithm. Compared to this algorithm, the proposed method is assumed to have a hash value of 8,192 bytes due to the 2D data of size 256 × 256 pixels. This means that the proposed hash algorithm is 256 times larger than the SHA-256 algorithm, and it is much more difficult to find a hash of the block header verifying it. Also, the weakness of the current blockchain technology is the small storage size in the body of a block, that is, the limitation of transaction descriptions in the blockchain. In a view of storage volume, the proposed method uses the block structure shown in Fig. 4, which is much more sufficient than conventional blockchain technology, to describe information of the block. If we expand data of size 512 × 512 pixels or 1024 × 1024 pixels for example, the hash value becomes too complicated to be hacked and the blockchain can store many transactions. In addition to this storage capacity, a very effective way is introduced in the method. We use a tool to convert the original information to a QR code. The text information shown in Fig. 7(a) is encoded to the QR code transaction shown in in Fig. 7(b). To protect privacy information and reduce the information size of all the transaction descriptions, we adopt an XOR logic operation between the transactions and the previous hash as Eq. (3). Despite this concise transaction expression, the privacy information of Node N is extracted sequentially by Eqs. (8) and (9). Next, we focus on considerations for a practical optical implementation of the proposed scheme. The most important part in the network shown as in Fig. 3 is the optical PSDH hardware to be used for encryption. The optical architecture of PSDH consists of spatial light modulators (SLMs), which are key components to represent input data. In the proposed method, the data size for processing blockchain technology is dependent on the displaying capability of the SLM. The commercial SLM can display the proposed data of size 256 × 256 pixels easily, and can even display an expanded data array. The limiting problem is that a precise phase type of SLM used in PSDH is very expensive to manufacture. Optical experiment for the proposed scheme will be needed in future work. A Merkle root is small-sized data in a block header summarizing all transactions in the block and a nonce is used for mining the blockchain system. Advanced research to consider the Merkle root and nonce in the block header will follow.

In this paper, we apply digital holography to blockchain technology and propose a novel secure authentication scheme for privacy information protection in a network. The proposed scheme features immutability, identity information privacy, and authentication security. A PoW consensus algorithm in the blockchain is executed to prove genuine authentication through digital holography encryption technology, resulting in legal block linkage. With a novel 2D array data structure design of the block, the PSDH technology generates a hash that is a random number of 2D array data. The proposed method provides a hash value of 8,192 bytes due to data of size 256 × 256 pixels, which is 256 times larger than the conventional SHA-256 algorithm. This means that it is much more difficult to find a hash of the block header verifying it with the proposed method. Also, the designed block structure of the 2D array allows much more sufficient storage to represent a large amount of transaction information of the block than that of the conventional blockchain. In addition, an effective way converting original information to QR code is introduced, and a combinational XOR logic operation between the transactions and the previous hash is adopted to protect privacy information. The real identity of each node cannot be revealed by a hacker while transmitting data, because the privacy information of nodes is encrypted using digital holography and stored in the blockchain. Also, a tamper-proof person cannot falsify privacy information by a forged transaction. The feasibility of the proposed scheme is analyzed by its performance evaluation. The principal application is a secure anonymity system guaranteeing privacy information protection for handling large amounts of information.

Data underlying the results presented in this paper are not publicly available at the time of publication, and may be obtained from the authors upon reasonable request.

This work was supported by an Incheon National University (International Cooperative) Research Grant in 2020.

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Article

Article

Curr. Opt. Photon. 2022; 6(5): 453-462

Published online October 25, 2022 https://doi.org/10.3807/COPP.2022.6.5.453

Copyright © Optical Society of Korea.

Privacy Information Protection Applying Digital Holography to Blockchain

Seok Hee Jeon1, Sang Keun Gil2

1Department of Electronic Engineering, Incheon National University, Incheon 22012, Korea
2Department of Electronic Engineering, The University of Suwon, Hwaseong 18323, Korea

Correspondence to:*skgil@suwon.ac.kr, ORCID 0000-0002-3828-0939

Received: July 20, 2022; Revised: September 4, 2022; Accepted: September 14, 2022

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Blockchain technology provides a decentralized and peer-to-peer network, which has the advantages of transparency and immutability. In this paper, a novel secure authentication scheme applying digital holography to blockchain technology is proposed to protect privacy information in network nodes. The transactional information of the node is chained permanently and immutably in the blockchain to ensure network security. By designing a novel two-dimensional (2D) array data structure of the block, a proof of work (PoW) in the blockchain is executed through digital holography technology to verify true authentication and legal block linkage. A hash generated from the proposed algorithm reveals a random number of 2D array data. The real identity of each node in the network cannot be forged by a hacker’s tampering because the privacy information of the node is encrypted using digital holography and stored in the blockchain. The reliability and feasibility of the proposed scheme are analyzed with the help of the research results, which evaluate the effectiveness of the proposed method. Forgery by a malicious node is impossible with the proposed method by rejecting a tampered transaction. The principal application is a secure anonymity system guaranteeing privacy information protection for handling of large information.

Keywords: Authentication, Blockchain technology, Digital holography, Hash, Optical encryption

I. INTRODUCTION

If you are handling critical information in a specific application, you can access the information after a log-in process and may be asked to change specific information through authentication proof. However, simple authentication cannot be definitive proof because the log-in process itself may be wrongfully falsified, even if log-in actions are recorded chronologically. In a secure information system, it is a big problem if any piece of information can be substituted with false information by tampering. To prevent a tamper from falsifying information, the information must not be changed even if forged information asks to change the original true information. One method to keep the information from unchanging is that the majority of information loggers have truthful copies of information by a correct log-in scheme. The solution to this problem is that we have a protocol for recording the information in the log and distributing copies of it to network users, which composes a decentralized database. As one of the emerging methods in recent years, blockchain is essentially a distributed database that can overcome the drawbacks of a centralized database. The concept of blockchain technology was first proposed in 2008 by Nakamoto, who developed the first digital currency, Bitcoin [1], which is a decentralized peer-to-peer (P2P) cryptocurrency and has the characteristics of anonymity. Blockchain technologies are gradually developing in the world and their applications have expanded to other areas such as cybersecurity, smart contract, 5G technology, internet of things (IoT), artificial intelligence (AI), education, etc. Basically, blockchain technology is based on a digital method in an accessing network to ensure trustworthy authentication and security. Recently, some researchers have studied the adoption of blockchain technology by optical networks. Kou et al. [2] in 2017 proposed a blockchain mechanism based on enhancing the consensus for optical networks. Blockchain-based optical networks for 5G applications were presented by Yang et al. [3, 4] and Nag et al. [5], respectively. Fichera et al. [6] proposed blockchain-anchored failure responsibility management in disaggregated optical networks, and Zhang et al. [7] proposed a blockchain-based resources trusted consensus solution for multi-domain edge optical networks. While digital information processing uses electrical signals, optical information processing uses light as the basis of its operations, the advantage of which is fast optical computing. New optical methods for blockchain-based applications were studied to overcome the shortcomings of digital blockchain technology. In 2019, Lysenko et al. [8] studied a method for improving the capabilities of measuring large data arrays with the use of optically controlled transparent and blockchain technology, and Chaintoutis et al. [9] proposed a novel optical physical unclonable functions (o-PUFs) implementation that can be combined with private blockchains. In 2020, Dubrovsky et al. [10] reported their research on optical proof of work (oPOW) that uses the next generation of computer processing chips called optical processing units (OPUs) to lower the amount of energy consumed by cryptocurrency mining. In 2022, Yan et al. [11] proposed a blockchain-based optical communication security transmission system scheme to protect information leakage.

In this paper, we propose applying digital holography to blockchain technology for privacy information preservation and security, which is based on recent research and called an analog signature scheme using an RSA digital signature algorithm and phase-shifting digital holography (PSDH) [12]. This paper is organized as follows. In Section II, an overview of blockchain architecture is briefly presented and the proposed method to conduct blockchain technology using digital holography is described. The performance and feasibility of the proposed method are analyzed with research results in Section Ⅲ. Conclusions are summarized in Section Ⅳ.

II. PROPOSED BLOCKCHAIN-BASED SECURING SCHEME

2.1. Blockchain Overview

Blockchain is a chain of blocks, and it is essentially a distributed database of information or a distributed transaction ledger in a P2P network without a centralized authority. The blockchain network consists of nodes, and each node has a copy of the same transaction. Typically, a participant who connects to the blockchain is called a node. Each block includes a transaction (i.e. information) with a cryptographic hash and is linked to its previous block by verification. Figure 1 shows the structure of blockchain. As shown in Fig. 1, each block consists of a header section and a body section. The header may comprise information of the version, time stamp, hash of the previous block header, nonce, etc. The body may comprise a list of transactions. The main principal work of blockchain technology is to verify a newly generated block by a kind of consensus algorithm, and the most well-known one is the so-called proof of work (PoW). The body of a block is used in PoW and a time stamp, Merkle root, nonce, etc., are also used in PoW. In particular, the work of generating blocks in the network is called mining. A nonce is an arbitrary random number and can be useful as an initialization vector. Mining is done to find a nonce that satisfies a limited condition of a hash value. The blocks are linked together using the hash of previous blocks in a chronological sequence, which provides the blockchain with an immutability feature in a way that changing one block would require changing the headers of subsequent blocks. The transaction in the block is verified by examining its signature and checking for the existence of the previous hash in the same transaction. If the transaction is verified, it is permanently recorded in the blockchain and cannot be deleted or changed. Thus, the node creates a new block by consensus algorithm that is executed to reach an agreement by a majority of the nodes in the blockchain network without a centralized authority.

Figure 1. Structure of blockchain.

Blocks are validated using a computational process of PoW to ensure authentication. To carry out PoW, blockchain technology needs two cryptographic functions, hash and digital signature, which are used for signing and verifying information. A hash is represented as a one-way function that makes blockchain more secure and information in the blockchain immutable. A hash algorithm generates a fixed-length string of the input, where the output string is called an input hash value. Its features ensure that the blockchain cannot be tampered with. Figure 2 shows the blockchain linking through PoW. Network nodes interact with the blockchain using a hash and asymmetric cryptography with private/public key pairs. Each node converts block information including the previous hash and transactions into a hash using a hash algorithm. In most blockchains, Secure Hash Algorithm-256 (SHA-256) is used for the hash algorithm. This hash is converted to a digital signature by signing it with a private key. After transmitting the block to nodes in a network, the previous block’s hash is verified with a public key. If the node succeeds in verification, a new block is chained to its previous block and become a permanent block, and its information is not tampered with. Thus, the blockchain structure has a chronological chain of blocks from the genesis block to the current block. Blockchain technology can form a decentralized database in a secure way, providing the characteristics of robustness, transparency, and anonymity. Its principal applications range from cryptocurrency to business contracts, ensuring a secure digital identity. A major weakness of blockchain is its huge energy consumption and privacy information leakage.

Figure 2. Blockchain linking through proof of work.

2.2. Proposed Blockchain Scheme to Secure Privacy Information Using Digital Holography

No information communication system can avoid the problem of information insecurity. A well-known problem in blockchain schemes is transactional information privacy. Despite the advantages of blockchain with immutable and secure data management, it has still shortcomings in information privacy and confidentiality. Most users in the network are concerned about the traceability of individual information because block transactions are stored as public transactions that are propagated over the network. If individual privacy information is not preserved, hackers can easily track the target node by analyzing the transmitted transactions. Thus, privacy information protection becomes more important for information exchange and sharing. Several public key cryptographic algorithms can be adopted to protect privacy information, but these algorithms rely on their implementation. This paper is motivated by this challenge. Blockchain is a secure distributed information technology that involves network nodes, ensuring high anti-tampering reliability. So, we propose a blockchain-based privacy information protection authentication scheme to integrate digital holography technology with blockchain technology using a carefully designed block structure. It is effective to apply PSDH to blockchain. Essentially, to implement a privacy information protection scheme, the conventional digital signature algorithm used in PoW of blockchain is modified to a realizable asymmetric key algorithm using PSDH. Figure 3 shows the proposed blockchain-based information network consisting of nodes with PSDH. Schematically, the network consists of several nodes, which are participants in the blockchain system. Each node has PSDH hardware that is assumed to be implemented optically. PSDH consists of an optical Mach-Zehnder type interferometer to encrypt information. However, nodes should verify all the blockchain blocks, known as PoW consensus. These nodes (called full nodes) connect the blocks with transactions to the blockchain, and determine whether the blocks are valid. In contrast to public blockchain networks, the proposed blockchain network can be applied to a permissioned blockchain network because only the node with PSDH can validate the blocks and access the network. This means that the proposed blockchain is a kind of private blockchain network. Figure 4 shows a carefully designed block structure for the proposed blockchain algorithm. The proposed block structure consists of a header and a body. The header consists of a time stamp, version, number of transactions, Merkle root, nonce, and previous hash, while the body consists of a new transaction, three public key ciphers, and two digital signatures. In principle, a previous hash, a new transaction, a public key cipher, and a digital signature have the same format of 2D binary array data in this paper.

Figure 3. The proposed blockchain-based information network consisting of nodes with phase-shifting digital holography (PSDH).

Figure 4. Designed block structure for the proposed blockchain algorithm.

Blockchain mainly uses digital signatures to achieve consensus, verifying the proper transaction block generator, and preventing tampering with the wrongful transaction of unjustifiable nodes. Digital signatures use asymmetric cryptographic algorithms, that is, each node requires private/public key pairs. Figure 5 shows the PoW procedure for a new block linkage. As shown in Fig. 5, the digital signature algorithm includes two processes. The first operation is signature generation by signing the hash of transactions and the second operation is signature verification. To propose privacy information protection by applying digital holography to blockchain, we use the method reported in a previous paper [12], a proposal for an analog signature scheme based on RSA digital signature algorithm and phase-shifting digital holography. In [12], the term analog signature was used instead of digital signature because the encrypted digital signature forms a noise-like analog pattern. According to [12], which describes a method to modify the conventional digital signature algorithm into an optically realizable digital signature algorithm, the proposed solution would generate optical digital signatures of a hash and find an optically reconstructed hash. The optical architecture to create a new hash optimized for optical processing was not considered due to the complexity of optical implementation. This difficulty leads to a hybrid scheme that composes digital processing to produce hashes.

Figure 5. Proof of work (PoW) procedure for a new block linkage.

The PoW procedure for a new block linkage is described as follows. A block structure has a header and a body, as shown in Fig. 4. In this paper, a new transaction, public key ciphers, digital signatures in the body and only the previous hash excluding the left part in the header shown in Fig. 4 are assumed to be used in PoW for convenience. Two random numbers of 2D binary array P and Q are used in the private/public key pairs generation process. In the digital signature generation process, after performing a two-step PSDH function PSDH{∙}with these random numbers, three public key ciphers, PC1, PC2, and PC3, are obtained by

PC1,2,3=PSDHP,Q.

To generate private/public key pairs X, Y and a secret key S in binary data, a threshold function TH{∙} is applied to public key ciphers, giving

X=THPC1,PC2,PC3,Y=THPC3,S=THPC1,PC2.

On the other hand, a present block transaction T_pres is logically expressed by the combination of a previous hash H_prev, a previous transaction T_prev, and a new transaction T_new, which are stored in the block header and body.

Tpres=(TprevTnew)Hprev,

where ⴲ denotes XOR logic operation. Next, the signature execution node converts the present transaction T_pres to a present hash H_pres with the private key X and the signal key S by an agreed cryptographic hash algorithm Hash1{∙} as

Hpres=Hash1Tpres,X,S.

Originally, the private key X and the secret key S are generated from two random numbers P and Q by operating Eqs. (1) and (2). Mathematically, hash algorithm Hash1{∙} is one-way function because the PSDH function PSDH{∙} in Eq. (1) uses a Fourier transform function and the resultant public key ciphers are used to generate the private key X and the secret key S by a threshold function TH{∙} of Eq. (2). In generating digital signatures DS1, DS2, the node carries out the same PSDH function with the present hash H_pres and the private key X as

DS1,2=PSDHHpres,X

These digital signatures have a noise-like analog pattern so that a third party cannot deduce the original present hash while transmitting data. Now, the node disseminates two digital signatures DS1 and DS2 and releases three public key ciphers PC1, PC2, and PC3 to every node in the network. Similarly, public key ciphers have a noise-like pattern, which makes it difficult for a third party to find the binary data of public key Y. These ambiguity characteristics of the generated data guarantees an advantage to secure information transmission and privacy.

In the digital signature verification process, the node to verify digital signatures retrieves the public key Y and the secret key S from the three released public key ciphers PC1, PC2, and PC3. From the transmitted block, the previous hash H_prev and the present transaction T_pres are known to the verification execution node. With the retrieved public key Y and secret key S, the transmitted previous hash H_prev and the present transaction T_pres, another agreed cryptographic hash algorithm Hash2{∙} produces a computed hash H_com as

Hcom=Hash2Tpres,Y,S.

Next, to decrypt the present hash H_dec from two transmitted digital signatures DS1 and DS2, the verification execution node performs a PSDH function with the public key Y and the secret key S as

Hdec=PSDHDS1,DS2,Y,S.

If the computed hash value H_com is the same as the decrypted hash H_dec, the digital signature is valid and a created block with a new transaction can be linked in the blockchain permanently by consensus, ensuring that the original information of transactions has not been tampered with during data transmission. In other words, a malicious node cannot impersonate any legal node to substitute true transactions with forged transactions without true authentication.

On the one hand, a generated present transaction of Block N at Node N is generally obtained by

TN_new=(T0_newT1_newT2_newTN1_new)(H0H1H2HN1)

which is a sequential XOR logic operation of the previous transactions and the previous hashes of each block. To extract transaction information corresponding to each block, the following processing is operated as

TN_new=TNTN1,

where this processing is only allowed for authorized nodes in the network. The total number of new transactions in the body of Node N is stored in the header section of the block as shown in Fig. 4, and is referenced to recall the corresponding transaction.

III. FEASIBILITY EVALUATION

3.1. Performance Evaluation of the Proposed Method

The performance of the PoW procedure for a new block linkage as shown in Fig. 5 is evaluated for the feasibility of the proposed scheme. In this paper, we assume a block structure as shown in Fig. 4. Binary data of size 256 × 256 pixels (8,192 bytes) are used for a component of the block header and body including a previous hash, a new transaction, public key ciphers, and digital signatures, while data of size 16 × 8 pixels (16 bytes) is enough to express year, month, day, hour, minute, and second as a group of time stamps, 16 × 2 pixels (4 bytes) for version, 16 × 2 pixels for number of transactions, 16 × 2 pixels for Merkle root, and 16 × 2 pixels for nonce, respectively. However, we exclude the left part of the header in Fig. 4 for evaluation convenience.

The blockchain architecture as shown in Fig. 3 must start from a special block (the genesis block), which contains the first transaction initiated by a network. However, only a single blockchain exists, that is, a single sequence of blocks including all the information from the first transactions to the present transactions. This consequent linkage happens by the PoW procedure shown in Fig. 5. Simply, it is assumed that each node in the network creates its own block to join in the blockchain. The block contains privacy information such as an ID for accessing the network by a log-in process. As shown in Fig. 5, let us suppose two nodes, Node 3 and Node 4, have the same blockchain from Block 0 to Block 3. Now, Node 4 (the truthful identity of David) wants to create a new block, Block 4, and connects it to the legal blockchain by PoW consensus. Figure 6 shows data of two random numbers, private/public key pairs and a secret key. Figures 6(a) and 6(b) show two random numbers of 2D binary array P4 and Q4 generated at Node 4. Three public key ciphers are obtained by PSDH function PSDH{∙}as Eq. (1), and binary private/public key pairs X4, Y4 and a secret key S4 are computed by a threshold function TH{∙} to three public key ciphers as Eq. (2). Figures 6(c)6(e) show private/public key pairs X4 and Y4, and a secret key S4, respectively.

Figure 6. Two random numbers, private/public key pairs and secret key: (a) a random number P4, (b) a random number Q4, (c) a public key X4, (d) a private key Y4, and (e) a secret key S4.

Figure 7 shows the generation of a transaction in the body of Block 4. For example, Node 4 makes its individual privacy information that is needed to register the log-in identity with the network, which is then encoded to QR code transaction Tx4_new. The data size of the QR code depends on the volume of information. Figures 7(a) and 7(b) show privacy information of Node 4 and its QR code denoting new transaction Tx4_new of Block 4, respectively. Figures 7(c) and 7(d) represent the previous transaction Tx3 in the body of Block 3 and the previous hash H3 in the body of Block 4, respectively. After an agreed XOR logic operation, a present transaction Tx4 is generated by Eq. (3) as Tx4 = (Tx3Tx4_new)ⴲH3. Figure 6(e) shows Tx4.

Figure 7. Transaction generation of Block 4: (a) privacy information of Node 4, (b) a new transaction Tx4_new of Block 4 [QR code of (a)], (c) the previous transaction Tx3, (d) the previous hash H3, and (e) a generated present transaction Tx4.

As described above, three public key ciphers are obtained by Eq. (1) and two digital signatures are obtained by Eq. (5) performing the function PSDH{∙}. Figure 8 shows intensity patterns of three public key ciphers (PC4_1, PC4_2, PC4_3) and two digital signatures (DS4_1, DS4_2), which form noise-like patterns by digital holography encryption.

Figure 8. Intensity patterns of three public key ciphers and two digital signatures: (a) PC4_1, (b) PC4_2, (c) PC4_3, (d) DS4_1, and (e) DS4_2.

In the digital signature verification process, the public key Y and the secret key S are obtained by retrieval processing from the three released public key ciphers PC4_1, PC4_2, and PC4_3. With these retrieved keys and the present transaction Tx4 given by the transmitted block, a hash H4_com is computed by another agreed cryptographic hash algorithm Hash2{∙} as Eq. (6). Also, another hash H4_dec is decrypted from two digital signatures DS4_1 and DS4_2 performing the function PSDH{∙} as Eq. (7). Figure 9 shows the verification of the hash and the reconstructed new transaction of Block 4. Figures 9(a) and 9(b) show a computed hash H4_com and a decrypted hash H4_dec of Block 4, respectively. As shown in Figs. 9(a) and 9(b), H4_com is equal to H4_dec. This means that Block 4 is ensured to connect it to the original legal blockchain. Figures 9(c) and 9(d) show the reconstructed QR code denoting new transaction Tx4_new of Block 4 and the decoded privacy information of Node 4 (the truthful identity of David) from the QR code of Fig. 9(d), respectively.

Figure 9. Verification of hash and the reconstructed new transaction of Block4: (a) a computed hash H4_com, (b) a decrypted hash H4_dec, (c) the reconstructed QR code denoting new transaction Tx4_new of Block 4, and (d) the decoded privacy information of Node 4.

Next, to evaluate the resistance to forgery from the perspective of protecting privacy information, it is assumed that a blockchain linkage from Block 0 to Block 4 was established by truthfully authorized nodes including all the information transactions. Now, an attacker tries to impersonate an authorized node, Node 4, to change privacy information of Block 4. The malicious node (the forged identity of Eve) makes a forged transaction of Block 4 and asks for a validation check. Figure 10 shows the generation of a forged transaction in the body of Block 4. Figures 10(a) and 10(b) show forged information of Node 4 and its QR code denoting a forged transaction Tx4_forged of Block 4, respectively. Figures 10(c) and 10(d) represent the previous transaction Tx3 in the body of Block 3 and the previous hash H3 in the body of Block 4, respectively, which are the same as those shown in Figs. 7(c) and 7(d). Figrue 10(e) shows a forged present transaction Tx4_f.

Figure 10. Forged transaction generation of Block 4: (a) forged information of Node 4, (b) a forged transaction Tx4_forged of Block 4 [QR code of (a)], (c) the previous transaction Tx3, (d) the previous hash H3, and (e) a forged present transaction Tx4_f.

In checking the validity of the digital signature, changed decrypted public key ciphers and the forged transaction Tx4_forged are used to retrieve two hashes H4_fcom and H4_fdec. Figure 11 shows the verification of the hash and the reconstructed forged transaction of Block 4. Figures 11(a) and 11(b) show a computed forged hash H4_fcom and a decrypted forged hash H4_fdec of Block 4, respectively. As shown in Figs. 11(a) and 11(b), H4_fcom is equal to H4_fdec. However, these hashes are not the authenticated previous hash H4 shown in Fig. 11(c), which was represented and stored in the genuine header of Block 4 of the legal blockchain. This disagreement forces the forged transaction to be refused to join in the blockchain. Figures 11(d) and 11(e) show the reconstructed QR code denoting forged transaction Tx4_rf of Block 4 and no reconstructed information of forged Block 4, respectively.

Figure 11. Verification of hash and the reconstructed forged transaction of Block 4: (a) a computed forged hash H4_fcom, (b) a decrypted forged hash H4_fdec, (c) original previous hash of Block 4 H4, (d) the reconstructed QR code denoting forged transaction Tx4_rf of Block 4, and (e) no reconstructed information of the forged Block 4.

In this paper, we propose a decentralized blockchain-based authentication scheme providing very high security and anonymity by the method of encrypting the individual transaction information of each block. Therefore, the real identity of each node cannot be revealed through data transmission and the transaction of the true block cannot be forged by a malicious hacker. The proposed method can be applied to a field handling the privacy identity of the node such as secure sensor data in a ubiquitous sensor network (USN) and state data in robot network nodes.

3.2. Analysis of the Proposed Scheme and Future Research

One of the emerging applications in blockchain technology is Bitcoin. In the case of the Bitcoin blockchain, the header data size of the block consists of 80 bytes. The details are as follows: Time stamp (4 bytes), version number (4 bytes), nonce (4 bytes), difficulty target (4 bytes), Merkle root (32 bytes), and hash of previous block header (32 bytes). In principle, the hash of the block header in the Bitcoin blockchain results in a 32-byte output using the SHA-256 hash algorithm. Compared to this algorithm, the proposed method is assumed to have a hash value of 8,192 bytes due to the 2D data of size 256 × 256 pixels. This means that the proposed hash algorithm is 256 times larger than the SHA-256 algorithm, and it is much more difficult to find a hash of the block header verifying it. Also, the weakness of the current blockchain technology is the small storage size in the body of a block, that is, the limitation of transaction descriptions in the blockchain. In a view of storage volume, the proposed method uses the block structure shown in Fig. 4, which is much more sufficient than conventional blockchain technology, to describe information of the block. If we expand data of size 512 × 512 pixels or 1024 × 1024 pixels for example, the hash value becomes too complicated to be hacked and the blockchain can store many transactions. In addition to this storage capacity, a very effective way is introduced in the method. We use a tool to convert the original information to a QR code. The text information shown in Fig. 7(a) is encoded to the QR code transaction shown in in Fig. 7(b). To protect privacy information and reduce the information size of all the transaction descriptions, we adopt an XOR logic operation between the transactions and the previous hash as Eq. (3). Despite this concise transaction expression, the privacy information of Node N is extracted sequentially by Eqs. (8) and (9). Next, we focus on considerations for a practical optical implementation of the proposed scheme. The most important part in the network shown as in Fig. 3 is the optical PSDH hardware to be used for encryption. The optical architecture of PSDH consists of spatial light modulators (SLMs), which are key components to represent input data. In the proposed method, the data size for processing blockchain technology is dependent on the displaying capability of the SLM. The commercial SLM can display the proposed data of size 256 × 256 pixels easily, and can even display an expanded data array. The limiting problem is that a precise phase type of SLM used in PSDH is very expensive to manufacture. Optical experiment for the proposed scheme will be needed in future work. A Merkle root is small-sized data in a block header summarizing all transactions in the block and a nonce is used for mining the blockchain system. Advanced research to consider the Merkle root and nonce in the block header will follow.

IV. CONCLUSIONS

In this paper, we apply digital holography to blockchain technology and propose a novel secure authentication scheme for privacy information protection in a network. The proposed scheme features immutability, identity information privacy, and authentication security. A PoW consensus algorithm in the blockchain is executed to prove genuine authentication through digital holography encryption technology, resulting in legal block linkage. With a novel 2D array data structure design of the block, the PSDH technology generates a hash that is a random number of 2D array data. The proposed method provides a hash value of 8,192 bytes due to data of size 256 × 256 pixels, which is 256 times larger than the conventional SHA-256 algorithm. This means that it is much more difficult to find a hash of the block header verifying it with the proposed method. Also, the designed block structure of the 2D array allows much more sufficient storage to represent a large amount of transaction information of the block than that of the conventional blockchain. In addition, an effective way converting original information to QR code is introduced, and a combinational XOR logic operation between the transactions and the previous hash is adopted to protect privacy information. The real identity of each node cannot be revealed by a hacker while transmitting data, because the privacy information of nodes is encrypted using digital holography and stored in the blockchain. Also, a tamper-proof person cannot falsify privacy information by a forged transaction. The feasibility of the proposed scheme is analyzed by its performance evaluation. The principal application is a secure anonymity system guaranteeing privacy information protection for handling large amounts of information.

DISCLOSURES

The authors declare no conflicts of interest.

DATA AVAILABILITY

Data underlying the results presented in this paper are not publicly available at the time of publication, and may be obtained from the authors upon reasonable request.

ACKNOWLEDGMENT

This work was supported by an Incheon National University (International Cooperative) Research Grant in 2020.

FUNDING

Incheon National University (International Cooperative) Research Grant in 2020.

Fig 1.

Figure 1.Structure of blockchain.
Current Optics and Photonics 2022; 6: 453-462https://doi.org/10.3807/COPP.2022.6.5.453

Fig 2.

Figure 2.Blockchain linking through proof of work.
Current Optics and Photonics 2022; 6: 453-462https://doi.org/10.3807/COPP.2022.6.5.453

Fig 3.

Figure 3.The proposed blockchain-based information network consisting of nodes with phase-shifting digital holography (PSDH).
Current Optics and Photonics 2022; 6: 453-462https://doi.org/10.3807/COPP.2022.6.5.453

Fig 4.

Figure 4.Designed block structure for the proposed blockchain algorithm.
Current Optics and Photonics 2022; 6: 453-462https://doi.org/10.3807/COPP.2022.6.5.453

Fig 5.

Figure 5.Proof of work (PoW) procedure for a new block linkage.
Current Optics and Photonics 2022; 6: 453-462https://doi.org/10.3807/COPP.2022.6.5.453

Fig 6.

Figure 6.Two random numbers, private/public key pairs and secret key: (a) a random number P4, (b) a random number Q4, (c) a public key X4, (d) a private key Y4, and (e) a secret key S4.
Current Optics and Photonics 2022; 6: 453-462https://doi.org/10.3807/COPP.2022.6.5.453

Fig 7.

Figure 7.Transaction generation of Block 4: (a) privacy information of Node 4, (b) a new transaction Tx4_new of Block 4 [QR code of (a)], (c) the previous transaction Tx3, (d) the previous hash H3, and (e) a generated present transaction Tx4.
Current Optics and Photonics 2022; 6: 453-462https://doi.org/10.3807/COPP.2022.6.5.453

Fig 8.

Figure 8.Intensity patterns of three public key ciphers and two digital signatures: (a) PC4_1, (b) PC4_2, (c) PC4_3, (d) DS4_1, and (e) DS4_2.
Current Optics and Photonics 2022; 6: 453-462https://doi.org/10.3807/COPP.2022.6.5.453

Fig 9.

Figure 9.Verification of hash and the reconstructed new transaction of Block4: (a) a computed hash H4_com, (b) a decrypted hash H4_dec, (c) the reconstructed QR code denoting new transaction Tx4_new of Block 4, and (d) the decoded privacy information of Node 4.
Current Optics and Photonics 2022; 6: 453-462https://doi.org/10.3807/COPP.2022.6.5.453

Fig 10.

Figure 10.Forged transaction generation of Block 4: (a) forged information of Node 4, (b) a forged transaction Tx4_forged of Block 4 [QR code of (a)], (c) the previous transaction Tx3, (d) the previous hash H3, and (e) a forged present transaction Tx4_f.
Current Optics and Photonics 2022; 6: 453-462https://doi.org/10.3807/COPP.2022.6.5.453

Fig 11.

Figure 11.Verification of hash and the reconstructed forged transaction of Block 4: (a) a computed forged hash H4_fcom, (b) a decrypted forged hash H4_fdec, (c) original previous hash of Block 4 H4, (d) the reconstructed QR code denoting forged transaction Tx4_rf of Block 4, and (e) no reconstructed information of the forged Block 4.
Current Optics and Photonics 2022; 6: 453-462https://doi.org/10.3807/COPP.2022.6.5.453

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