=Paper=
{{Paper
|id=Vol-3010/PAPER_01
|storemode=property
|title=A Survey of Privacy concerns in Blockchains and Information Retrieval
|pdfUrl=https://ceur-ws.org/Vol-3010/PAPER_01.pdf
|volume=Vol-3010
|authors=Archana Chhabra,Rahul Saha,Gulshan Kumar,Tai Hoon Kim
}}
==A Survey of Privacy concerns in Blockchains and Information Retrieval==
https://ceur-ws.org/Vol-3010/PAPER_01.pdf
A Survey of Privacy Concerns in Blockchains and Information
Retrieval
Archana Chhabra a, Rahul Saha b, Gulshan Kumar c, Tai Hoon Kim d
a
School of Computer Science and Engineering, Lovely Professional University, Phagwara, Punjab, India
b
School of Computer Science and Engineering ,Lovely Professional University, Phagwara, Punjab, India
c
School of Computer Science and Engineering,Lovely Professional University, Phagwara, Punjab, India
d
Glocal Campus, Konkuk University, 268 Chungwon-daero Chungju-si Chungcheongbuk-do, 27478, South Korea
Abstract
With the increase in population the number of internet users has also increased over the years,
protecting the privacy of user data has become an important issue. To maintain the privacy of
ones' information many techniques are there but, each of them has one or the other loophole.
Moreover, the emerging cross-chain technology to provide interoperability is another part of
concern for blockchains. In this paper, the concept of security and privacy is studied in relation to
blockchain technology and how the blockchain network is more secure as compared to other
networks. Also, the survey has been done on the advancement of blockchain in different
applications such as e-commerce, health care, etc. and techniques based on private information
retrieval to maintain privacy of data has also been studied. Further, the analysis has been done to
conjugate the blockchain and private information retrieval to provide better security and privacy
to the internet users.
Keywords 1
Privacy, Security, Blockchain, PIR, survey
1. Introduction
In this era of technological revolution, the blockchain is considered to be the new concept after the
internet since 2008, when it was first introduced by the researcher who implemented the concept of the
digital currency known as bitcoin. With time various cryptocurrencies came into existence such as
Ethereum which introduces smart contracts, but, the blockchain is emerging as one of the most promising
and innovative techniques of cyber-security. It provides a secure, transparent, and distributed framework
for sharing, exchanging, and integrating the information among several users and third parties. The idea
behind building blockchain was aiming at a network without any control of the central body. The
blockchain uses the consensus protocol to ensure the integrity of the data stored in the system. For
instance, one may assumes it an easy task to steal something from a box kept at an isolated place instead
stealing something from a box kept in open under many eyes. Thus, the blockchain can be considered as a
public ledger in which all committed transactions are stored in the list of blocks open to all and where
new blocks can be appended whenever required. The fundamental characteristics of the blockchain are
shown in Figure 1.
Algorithms, Computing and Mathematics Conference, August 19 – 20, 2021, Chennai, India.
EMAIL: rsahaaot@gmail.com (Rahul Saha)
ORCID: 0000-0003-3921-9512 (Rahul Saha)
© 2021 Copyright for this paper by its authors.
Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0).
CEUR Workshop Proceedings (CEUR-WS.org)
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Figure 1: Characteristics of Blockchain
1.1. Motivation behind Blockchain
Blockchain is an innovation to the financial transactions which are based on the involvement of trusted
third-party between the buyer and the seller such as e-bay/amazon, etc. that provides a trusted platform
for both the users. But trusting someone always remains an issue in the world of the internet. Therefore,
the blockchain network aimed to overcome this issue by providing a data structure to store and retrieves
data in a secure, distributed, and decentralized way. These features made the blockchain network popular
and gain acceptance in various domains over internet such as healthcare, IoT, supply chain management,
e-commerce, etc. It has been observed that transparency, openness, and decentralization are the biggest
motivating factors that lead to the development and success of blockchain technology by overcoming the
problem of a single point of system failure which is very common in centralized systems. Moreover, the
blockchain facilitates immutability of records which ensure the integrity of data even in the case of public
ledgers [1].
Though the blockchain has great potential to become a new engine over internet to conduct digital
commerce and share our personal data and life events in different domains but there are some serious
challenges in implementing blockchain. Firstly, it suffers from scalability problem as it cannot handle
large number of daily transactions in banking sector because it requires many updates in the single
transaction to complete which is not allowed by blockchain as the records once written in the chain of
blocks cannot be altered. To make any changes new blocks should be added which in turn further needs to
be verified and validated. This will not only decrease the transaction speed but also increases the cost of
the transaction. It could be handled with storage optimization and increasing the block size but, then there
will always be a trade-off between block size and optimization. Secondly, initial cost of implementing
blockchain network is quite high. Moreover, it may also suffer from privacy leakage in spite of the fact
that user are using their public and private keys. In this paper, we discuss the major advantages of using
blockchain in each application of digital world despite having certain challenges. Also, we shed some
light on benefits of using blockchain in conjunction with private information retrieval schemes in contrast
to the existing solutions and how privacy can be maintained in a better way over the internet.
1.2. What is Blockchain?
A blockchain consists of blocks connected in a chronological order. Each block in the chain contains
data to share or transact, hash of that data, and the previous hash which is open and accessible by all on
the network as shown in Figure 2. One of the biggest advantages of blockchain is that it is decentralized
and distributed in nature which makes it more secure. It uses a peer-to-peer network (decentralized) for
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managing the chain rather than the centralized network. Therefore it is open and public to anyone to join
the chain [2]. The blocks in blockchain are connected to each other in a chronological order. From this
architectural point of view, we can list some basic properties of blockchain:
1. Blocks can store any type of data.
2. Ensures data integrity.
3. Append-only.
4. Specific communication protocol is used because of decentralized network.
5. Uses a consensus algorithm, like proof-of-work, which reduces the chance of malicious node to
enter.
For example, if the blockchain is about bitcoins it will contain data for transactions, information about
receiver and sender, and the number of bitcoins present in the network. Each block contains a hash value
which is verified and if there is any change in the hash value then it means the hash does not belong to
that block. Moreover, each block also contains the hash of the previous block which facilitates the
connectivity of blocks to each other. Once a transaction is added to the block of a chain it cannot be
altered or tempered, hence, provides more security. Thus, a blockchain provides a secure way of
transaction among any two peers in an open network which could be verified if required.
Figure 2: Structure of blockchain
1.3. History and Evolution of Blockchain
Blockchain technology is based on the concept of decentralized and distributed computing. Blockchain
is not a new term rather it was practiced first in the 1990s to stop tampering with documents by a group of
researchers. But it got fame in the year 2009 when it was used and implemented by Santoshi Nakamoto to
create digital cryptocurrency also known as bitcoin [3]. The invention of bitcoin as blockchain helped in
overcoming the problem of double-spending over the internet without any intervention of a trusted third
party.
After bitcoin, the fame of blockchain on the internet keeps growing on in the form of cryptocurrency
as an application of cash till 2012, but sooner people started using digital currency as a secure way for
transferring payment electronically in the form of smart contracts by 2013. Before this many people
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believe that bitcoin and blockchain are similar terms. But after 2014 the research has been made that
blockchain can also be used for different applications such as health care systems, supply chain
management, e-voting, IoT, and many more. Nowadays, more work is being done on blockchain in
different fields which deals with the privacy of information retrieval over the network globally [4] [5].
The Figure 3 below show the growth in blockchain technology over the years.
Figure 3: History of Blockchain
Another new addition in this blockchain development is Cross Chain [6]. The blockchain alone cannot
provide the features such as interoperability among different connected entities on the network [7] [8].
Cross chain attempts to solve all this issue [9]. One excellent example of a blockchain project which is
trying to explore cross chain transactions through banking services in cross-border networks is Ripple
[10]. Secondly, Polkadot allows the transfer of smart contract data through various blockchains [11].
Futher, the Blocknet are currently working on creating a decentralized exchange all in the effort of
enhancing interchange communication [12]. Aion online is another blockchain project that deals with
scalability and interoperability issues of blockchains. It is also coming up as a standard protocol used by
various blockchains [13]. All these projects of Cross Chains are in infancy stage and needs rigorous
feasibility studies in corresponding to standalone blockchains. However, they possess benefits of
scalability, interoperability and more decentralization and also leave behind various research questions.
1.4. Types of Blockchain
Blockchain networks are broadly classified into three type’s namely public blockchain, private
blockchain and consortium blockchain [14].
1. Public blockchain: In this type of blockchain there is no restriction on access by anyone.
Therefore, it is also known as a permissionless blockchain. Anybody having an internet
connection and wish to communicate can become a participant of the network by following the
consensus protocol (set of rules). Whenever a new node joins the network all the previous chain
history is shared with that node. It is extremely secure because of its redundant nature [15].
Moreover, the public system facilitates anonymity and transparency of data on the network. The
most common example of public blockchain technology is Bitcoin and Ethereum blockchain.
2. Private blockchain: A private blockchain is based on permission. One can only join the network
after getting an invitation request from the administrator. It is controlled by authorized users.
Therefore provides more privacy as compared to public blockchain and is more adaptable by
government and private sector companies because a central authority is required to govern which
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makes the network more secure, faster, and more efficient. But it has one downside also they do
not follow the concept of decentralization in contrast to public blockchain [16].
3. Consortium blockchain: It is also known as a hybrid blockchain because it is a mixture of both
private and public blockchain networks. Hence it provides the advantages of both the networks
that are it is partly centralized as it is not completely controlled by one organization rather it is
ruled by a dedicated group of people such as board members and also partly decentralized
because each node is given the right to make choice for their data or transaction to be kept open
or private in advance. Hence it plays a vital role in increasing the efficiency and transactional
privacy of the network [15].
1.5. Contribution and Organization
Blockchain is gaining popularity in various application domains. The transparency and
decentralization of blockchains are no doubt the advantages of this technology; the privacy concerns are
still an open question for blockchains. Therefore, we survey all the existing literature that is connected
with privacy issues or privacy solutions in blockchains or related applications. This survey also covers the
ways related to information retrieval with privacy preservation which is a good candidate for blockchains
for privacy issues of open and immutable transactions. This survey will be helpful for the privacy
researchers; blockchain researchers to develop privacy assured applications. The major contribution of
this survey is as follows:
1. We survey the privacy-based blockchain literature. To the best of our knowledge, this survey is
the first of its kind in this direction.
2. We categorize the blockchain privacy solutions based on applications.
3. We also consider the privacy information retrieval problems and solutions which is another
novelty of this survey.
The rest of this paper is organized as follows. Section II explains privacy perspectives. Section III
describes the literature work related to the importance of maintaining privacy using blockchain in
different fields. Section IV provides the analysis of the literature and limelight the problems that persist
related to privacy that can be worked upon. Further, Section V concludes the paper by discussing the
latest trend in technology and gives the outcome of the research work.
2. Privacy Perspectives
Privacy is the state of being free from public scrutiny or from having your secrets or personal
information shared. When you have your own room that no one enters and you can keep all of your things
there away from the eyes of others, this is an example of a situation where you have privacy. Privacy has
a quite long history it has its origin since ancient times. The protection of privacy from the technical
aspect is also very important because today with the advancement in science and technology dependency
of people on laptops, mobiles, desktop, etc for socializing themselves is highly increased. Daniel Solove
has defined privacy [17] as involving the right of mandating personal privacy concerning storing, re-
purposing, provision to third parties, and displaying of information pertaining to oneself via the internet.
Privacy of an individual could belong to financial privacy, internet privacy, medical privacy, sexual
privacy, and political privacy. Every individual wants their personal information is not to be shared or
used by others to avoid themselves from discrimination, personal embarrassment, or damage to one's
professional reputation [18].
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2.1. Privacy Parameters
There are following ways to assure privacy of an individual:
1. Identifiability: It is defined as the extent to which a person’s identity can be identified directly or
indirectly. The linking of information to a particular data set refers to a person’s identity [18].
2. Anonymity: It means hiding the real identity of the person due to which he will be non-
identifiable, unreachable, or untrackable by others. It is a technique by which one can feel free to
do whatever he wants by hiding himself. But on the darker side, it could also be misused by an
individual to lie easily to others [18].
3. Pseudonymity: It is derived from the word 'pseudonym' meaning 'false name', which allows a
person to use other names instead of using his real identity to communicate with others.
Pseudonymity has become essential on the internet on computer networks and also it is used in
conjunction with other features of privacy. A digital pseudonym consists of a bit of string that is
unique as Id and is used to authenticate the person and his data [18].
4. Unlinkability: It is a state of the system in which it is very difficult for the attacker or observer to
detect whether the two sets of data are related to each other or not. Unlinkability ensures that a
user may make multiple uses of resources or services without others being able to link these uses
together [19].
5. Unobservability: It is the property of privacy that makes an intruder unable to observe the
information being shared on the network indistinguishable. It means the attacker can notice who
is sending messages and who is receiving nut cannot relate who is sending data to whom.
Unobservabilty is considered as a desirable property in steganographic systems [19].
6. Transparency: It is the ability to easily access and work with data no matter where the data is
located or what kind of application has created the data. It also ensures that the data being
reported is accurate and is coming from an authorized source which builds the interest and trust of
people communicating over network [18].
7. Intervenability: It is the ability which allows an individual to raise a complaint whenever he feels
his privacy is harmed by someone [19]. For data controllers, intervenability allows them to have
efficient means to control their data processors as well as the respective IT systems to prevent
undesired effects. Examples for such means may be the ability to stop a running process to avoid
further harm or allow investigation, to ensure secure erasure of data including data items stored
on backup media, and manually overruling of automated decisions or applying breaking glass
policies.
2.2. Privacy Methods
For protecting an individual's personal information certain security measures are to be taken so that the
privacy of the data could be assured. To maintain the privacy of information it is very important to
determine what kind of data is collected (e.g. medical, financial or personal), where and how it is to be
gathered, who can access it, and so on. To protect the systems from data privacy breach following
measures can be taken [20] [21]:
1. Change management: People must keep on updating their information frequently on the internet
specifically social media sites so that their information cannot be tracked easily by a malicious
user.
2. Data loss prevention: It helps in monitoring and protecting the data in motion on the network,
data in storage, and also the data in use in end devices. Moreover, it also helps to block attacks,
privilege abuse, and unauthorized access, malicious web request, and unusual activities to prevent
data theft.
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3. Data masking: It helps in anonymizes data by encrypting or hashing, perturbation, and
generalization. Also, it facilitates data pseudonymization by replacing sensitive information with
realistic fictional data which helps in maintaining operational and statistical accuracy of
information.
4. Data Protection: It ensures the integrity and confidentiality of data by controlling change
reconciliation, data across borders, query whitelisting, etc.
5. Ethical walls: It creates a boundary between business groups to comply with M and A
requirements, government clearance, etc.
6. Information auditing and archiving: It provides monitoring of data by law and contractual
regulations. Whenever there is unauthorized access or change made to the data it activates
warning triggers. Further, it creates an audit trail for forensic visibility.
7. User rights management and tracking: It keeps a track of end-user web application by mapping it
with the shared database/application and finally to the data accessed. Also, it identifies excessive,
inappropriate, and unused privileges.
2.3. Privacy requirements in Blockchain
To protect the privacy in blockchain-based applications two requirements have to be fulfilled that is
the links between transactions should not be visible or discoverable and the content of transactions is only
disclosed to the peers involved. These requirements could be set easily in case of private/permissioned
blockchain by keeping the transaction transparent to all. But, in case of public blockchain everyone has
access to the network with no restrictions so the privacy requirements should be considered on the basis
of following two factors:
1. Identity Privacy: It is the property which allows an individual to hide his real identity from others
on the network. But, even if a user apply random addresses (or pseudonyms) while making
transactions in blockchain network, they have only limited identity privacy because an adversary
who is observing the unencrypted network and traversal through the public blockchain, may
discovers who is using the network and for what purpose by utilizing certain behavioral analysis
strategies such as Anti-Money Laundering (AML) regulation [22] and Know Your Customer
(KYC) policy [23].
2. Transaction Privacy: This property allows a user to provide restrictive access to his transactional
details such as amount or the transaction pattern could only be accessed by specific users on the
network instead being visible to all in public blockchain network. For example, in electrical
health record management or big data’s anonymous authentication and authorization, users do not
want to disclose their sensitive information to all which raised a need for higher levels of privacy
to build an individual’s trust in the application.
2.4. Privacy threats for Blockchain
The protection of privacy is a critical issue in blockchain network because of its transparent and
publicly available nature one can easily make privacy breach while observing the network
communication. This section covers certain major privacy attacks that could be made in blockchain.
1. Privacy Leakage: This attack is mainly caused by reuse of addresses by the users in the network.
Public address of each peer is open to all in the network which could easily be traced by any
adversary via internet. Though, the peers are allowed to use different pseudonyms for different
transactions to hide their real identities but the adversary can link it to the actual user as all the
transactional details related to a node is visible to all the peers which is a major drawback of
blockchain [24].
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2. Selfish mining: In this, the selfish miners keep the mined blocks with themselves as secret blocks
rather broadcasting it on the network and continue doing this until their chain of secret blocks
become bigger than the actual public chain. The blocks generated by other miners are thus pruned
/ignored and are not considered for reward. So, selfish miners tend to get more revenue.
Therefore, the rational miners would be attracted to the selfish pool and want to join it which
makes the selfish miner power more than 51 percent quickly and increases the chances of attack
in the network [24].
3. Sybil attack: It is a kind of cyber-attack in which the adversaries try to create multiple fake
identities to gain large number of influencing nodes in the network. As per the PoW consensus
more than 50 percent power is required to gain control on the network, so the adversary try to
make as much fake identities as it can to rule the network and intrude personal information of the
users. This attack may further leads to strong privacy leakage if the blockchain network is
compromised [25].
4. Linking attack: These attacks are more common in blockchain-based IoT’s. For instance, an
anonymized data collected from two separate transactions which contains records of an individual
can be linked together using certain algorithms to fetch the details of the user maliciously and it
could happen because of the distributed nature of the blockchain network. Thus, the anonymized
data is not 100 percent private, so to maintain the privacy strong privacy-protection methods are
to be applied [26].
5. Transactional fingerprints: It is another threat to the anonymity of transactions. It could be
possible if one could gain access to any of the six mentioned aspects of the transaction namely,
Random time-interval (RTI), hour of day (HOD), time of hour (TOH), time of day (TOD), coin
flow (CF), and input/output balance (IOB). The extra knowledge about any of these may lead to
de-anonymize a user [27].
6. Distributed denial of service: In this attack a dishonest miner node may uses the large number of
peers in the network to send a lot requests to the victim node such as invalid transaction, invalid
blocks and so on in order to disrupt the victim’s transaction to complete [28].
3. Related work on privacy
In the field of computing, blockchain technology is the biggest innovation of the 21st century which
has shown major advances in many fields from financial to manufacturing as well as education and
medical. It is still unknown to many but the concept was there in existence since the 1990s and gained
popularity a few years back.
In early 1991 the famous scientists Stuart Haber and W. Scott Stornetta discovered the term
blockchain, which consists of a secured cryptographic chain of blocks containing documents whose
timestamps, could not be tampered with by anyone. After a year in 1992, the system was upgraded by
incorporating the concept of Merkle trees to improve efficiency by providing support for a large number
of documents in one block. The major change to the blockchain technique was made in the year 2008 by
Santoshi Nakamoto who worked as a team and invented Bitcoin-a digital ledger application also known
as cryptocurrency. Also, he published the first paper on Bitcoin technology in 2009, in which he provided
details about building trust in digital cash and the importance of decentralization in computing [29].
The blockchain is a peer-to-peer distributed network that is secured and used to record transactions on
a number of computers. It is transparent to all the connected nodes which enable all to access the shared
content over the network. It provides a secure way for people to make any kind of transactions without
the involvement of a trusted third party.
Many people have the misconception that Bitcoin and blockchain can be used interchangeably but the
fact is- Bitcoin is the first application of blockchain technology that came into existence in 2008. After
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digital cash with the advancement in technology, blockchain has gain popularity in other applications
such as smart contracts, real estate, etc by the end of the year 2015. The Most common application during
that period is the innovation of Ethereum which differs from Bitcoin as it provides the feature to record
other assets such as slogans and contracts in addition to timestamp. Ethereum was officially launched by
Buterin in 2015 and given great competition to the Bitcoin technique and gain popularity in the space of
cryptocurrency. Also in 2015, the concept of Hyperledger connecting digital ledger was come into
existence by LINUX foundation under the leadership of Brian Behlendorf which seeks for the
collaborative development of distributed ledgers. Its main focus is to improve the reliability and
performance of the current system to support global business transactions.
Blockchain does not stop rising after Ethereum and Bitcoin rather it has now gained popularity in
many fields including finance, insurance, medical, education, and so on. In recent years, many large
enterprises and government agencies are exploring blockchain technology applications to achieve
wonders in this era of the digital world. No doubt blockchain is now into every field but it still has many
loopholes in each application area which is to be addressed.
Potential applications of blockchain can be categorized as shown:
1. Blockchain in Finance: Blockchain was primarily designed for Bitcoin which is the most
commonly known digital currency based on the concept of decentralization. Other than Bitcoin, it
was later used for Ethereum, Peercoin, Altercoin, etc. [30]. Blockchain fulfills the necessary
security and privacy aspects of financial transactions between users so the concept of bitcoin is
adopted worldwide. Blockchain provides full confidentiality of the data being transact as it uses
encryption algorithms to gain access to the data. Further by mixing certain hashing techniques at
different levels of a transaction, provides data integrity that is once the data is written no one on
the network can change it. Also, it facilitates a non-repudiation feature by recording the time and
information of each transaction made between different users on the network.
2. Blockchain in healthcare: Maintaining health record manually is a very time consuming and
tedious task. To overcome this, many hospitals are now opting for online healthcare systems
provided publicly by some authorized organizations over the globe. These organizations
guarantee the security and privacy of an individual's health record shared on the network. The
blockchain technique for storing the health information of people works differently from the
Bitcoin concept as it is not to be accessed by all on the network, so there must be some central
body to govern all the data stored [31]. To provide better security services data should be
collected in a repository which could also be termed as data lakes. And to fetch data from that
lake one must be an authorized user of it. Also, to maintain the privacy of information each
individual or owner of the data must be given the authority to share his/ her information with
whom they want and not to be accessed by anyone who is part of the network. All these measures
are provided by blockchain technology so it is fruitful to use this technology worldwide.
Moreover, as blockchain networks are distributed in nature so it also provides the built-in feature
of fault-tolerance across the network.
3. Blockchain in IoT: IoT is an interface that allows the nodes to connect and communicate to
transfer data over the network without any intervention of human to human or human to computer
interaction [32] [33]. Nodes could be any device, person, object, animal, etc. Using blockchain in
conjunction with IoT storage of data and accessing it a much simpler task from any remote
location while also ensuring security and privacy of user's information in the network. For
example, while creating an online account for accessing any application an individual has to go
through a long sequence of steps that are asked by the service provider to be fulfilled before
utilizing their services. Also, it asks for a certain set of questions to be answered such as What's
your pet's name? so that in case of verification of account because of forgot password or any
other issue it can guarantee a privacy check to the account holder by asking such questions. As
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the internet is growing day by day so the demand for secure IoT is also increasing which could be
provided with the help of Blockchain. The Major use of Blockchain-based IoT is in building
smart contracts, RFID, Digital wallets, and so on. The biggest challenge of implementing
blockchain in IoT is the scalability of the network because, with an increase in the number of
computing devices, request-response time will also increase.
4. Blockchain in other domains: In addition to the above-mentioned application areas of blockchain,
it could also be developed or used for security and privacy of other domains such as defense,
supply chain management, automobile industry, education, and government. Despite many
advantageous features, blockchain technology still takes 10 to 15 more years for establishing its
roots in these fields.
The increase in digitization of personal information and internet technologies poses several challenges
to consumer's private information being shared on the network. Moreover, the consumers are a great
source of providing data for web blogs and social networking sites so their private information is more
vulnerable on the internet. Therefore, privacy has always been a great topic to research for scholars for
many years. However, recently the research is more influenced by the threats to privacy and the ways to
overcome them. Further in this paper, privacy issues of blockchain technology in different areas are
discussed from the year 2015 till now. The literature is divided based on different application areas of
blockchain.
3.1. Privacy of Blockchain in financial market
After the innovation of bitcoin and its alternative cryptocurrencies such as Ethereum, Litecoin,
Primecoin financial services have affected for nearly a decade. Since then the banking industry is widely
affected by economic transformation, internet development, and financial innovations. Therefore, the
banking industry requires an urgent change and also needs to seek new growth of avenues. This change
could only be made possible with the adoption of blockchain technology in the financial sector which has
given a new direction to the finance market by inventing FinTech in 2016 [34]. Mu Qi-Guo in 2016 has
put light on the fact that blockchain will fundamentally revolutionize the existing operational models of
finance and economy which might further lead to great technological innovations in the field of FinTech.
In this paper, the author has discussed all the possible changes that could be made to the existing financial
market in collaboration with blockchain technology highlighting all the benefits and challenges in the
field.
Further, the new studies made by IBM have found that blockchain solutions are being adopted
worldwide. The survey data clear that 15 percent of banks and 14 percent of financial institutions intend
to implement blockchain commercially on large scale by 2017 [35].
Moreover, the survey results of another research known as "Blockchain Rewires Financial Markets:
Trailblazers take the lead", has taken a sample of 200 financial institutions from all over the globe and
concluded that 7 on the count of 10 financial companies are focusing on integrating blockchain
technology for assuring the following four areas namely, clearing and settlement of transactions,
wholesale payments, issuance of debt and equity, reference data. All this is made possible because the
blockchain promises a transparent, secure, and reliable platform to each participating entity in the
network.
Quan Khanh Nguyen in 2016 has studied the financial crisis in the banking economy and how they
have been overcome with the adoption of blockchain technology in the financial sector. It has made the
management of money by consumers very easy by providing relevant applications for smartphones,
laptops, and tablets and blessed the world of digitization to achieve wonders. The author concluded that
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blockchain opens a new era of opportunities where the nature of business among people will transform
from competition to cooperation [36].
3.2. Privacy of Blockchain in E-commerce
Xinping Min et al. in 2016 have studied the comparison between permissioned blockchain framework
(PBF) and bitcoin –derived blockchain in E-commerce and suggested the improvement in terms of
throughput, latency, and capacity. The author analyzed the pros and cons of both the techniques and
designed the permissioned blockchain framework to support the instant transaction and dynamic block
size in e-commerce. His framework is based upon peer inner blockchain Protocol (PIBP) provide better
throughput and overcome latency issue. Moreover, to prevent a dishonest peer to enter a network to
assure high credibility of transaction author used a permissioned trusted trading Network (PTTN) [37].
Yukin Xu and Xinping Min et al. in 2017 have derived a new consensus mechanism to provide better
credibility in comparison to previously existing consensus algorithms. The authors presented E-commerce
blockchain consensus mechanisms (EBCM) which do not work on the principle of computing power and
token rather works in a similar level of security and credibility as Nakamoto's consensus. EBCM is
capable to achieve high throughput in a real-time transaction. The author compares the EBCM with
Bitcoin to justify better throughput and latency. And finally, the author concludes that EBCM is best
suited to deal with the low credibility of transactions as it helps in building a safe and public autonomous
transaction network [38].
Yi-Hui Chen et al. in 2018 have applied the blockchain technique on one of the most used applications
of e-commerce over the internet is E-Auction which suffers from trust issues on the intermediary between
the buyer and the seller. And secondly, costs high to pay the trusted third party. The author applied a
decentralized peer to peer blockchain technique to resolve both the issues mentioned. The peer to peer
access structure guarantees the trusted communication from one point to another by authenticating
themselves before transferring the data. And the decentralization feature cuts own the cost of a centralized
trusted party. For this, the smart contract is created which hides the bidding cost from the lead bidder. To
assure these certain rules are created and as a smart deal which cannot be opened before the deadline. In
this paper, the author provides a mechanism based on blockchain for E-auction which deals with
confidentiality, non-repudiation, and tamper-proofing of the bid [39].
C.Liu et al. in 2018 have developed a normalized autonomous transaction system based on blockchain
facilitating IoT-based e-commerce. The author designed a three-layer NormaChain sharding blockchain
network that works to increase the efficiency of the transaction and also the scalability of the system.
Moreover, the author also uses the decentralized public-key encryption scheme (PEKS) to guarantee
illegal and unwanted user access to avoid crime transactions. Moreover, it also protects from ciphertext
attacks as stealing of secret keys is not possible which guarantees legitimate privacy to the user [40].
Y.Jiang et al. in 2019 have proposed a blockchain-based e-commerce system for preserving privacy
while shopping online. The author derived a protocol that prevents the user from any kind of identity
threat such as address or contact number, etc. The author uses blockchain technology to protect user's
information by implementing private smart contracts which act as a bridge between the buyer and the
seller at the time of transaction and hides the personal details of the user. Also, the author used the zero-
knowledge proof algorithm called zk-SNARKs which create and issue shield token to generate proof of
ownership among the users [41].
Table 1 shows the research gap in the area of maintaining privacy in e-commerce using blockchain.
11
Table 1: Privacy of Blockchain in E-commerce
Ref-no Year Contribution Privacy Blockchain type Research gap
Parameter
37 2016 Peer inner Identifiabliblity Private/permissioned Storage of
blockchain and blockchain transactions
protocol for unlinkability and blocks
trusted
transaction
38 2017 E-commerce Identifiability, Consortium Storage of
blockchain credibility and Blockchain transaction and
consensus unobservability blocks, Data
mechanism consistency
39 2018 Blockchain Anonymity, Consortium Complex
based Smart unlinkability blockchain Architecture
Contract for and and
Bidding unobservability Implementation
System cost
40 2018 Normachain Anonymity, Consortium Trasaction
unlinkability blockchain handling
and
unobservability
41 2019 Blockchain- Anonymity and Public/permission- Scalability
based e- pseudonymity less blockchain
commerce
system
3.3. Privacy of Blockchain in healthcare
Xiao Yue et al., in 2016 built an application named Healthcare Data Gateway (HDG) is a blockchain-
based framework that allows patients to own, control, and share their information securely without
hampering their privacy. Two protocols are used in the model to safeguard patient’s data, first is
Indicator-centric schema (ICS) which helps patients to organize all kinds of personal health records easily
and the second is secure multi-party computing (MPC) which protects the patient information from an
untrusted third party to access. The author designed three-layer architecture to build his mobile app for
supporting the mentioned features and making it easy for an individual to install it on their phone and can
access it from anywhere. The HDG supports anonymization, secure communication, and data backup and
recovery [42].
Xueping Liang et al., in 2017 discussed the privacy and security issues in the existing electronic health
care mobile applications and tried to cover some of them in a better way by proposing a new framework
that integrates the feature of decentralized and permissioned blockchain to preserve the individual's
identity by allowing him to manage his data manually as the data is shared and synchronized via the cloud
with the healthcare providers and health insurance companies. The author also worked on improving the
scalability and performance by adopting a tree-based data processing algorithm to handle large sets of
data at the same time. The author developed a mobile app works on hyper ledger fabric blockchain
12
technology which validates the nodes on the network to assure the privacy of the healthcare system that is
between the end-user and the cloud [43].
Christian Esposito et al., in 2018 put the limelight on the limitations to the existing cryptographic
framework used to address security and privacy in cloud-based health care systems and also suggested
better solutions to overcome the current issues to host and share data within the cloud. Manual
maintenance of health records has become a hectic job that everybody is bored off. Therefore, certain
techniques have been designed to store the medical records and personal health data of an individual on
an electronic platform which has various drawbacks such as identity theft for personal benefits, etc are
mentioned in this paper. The author proposed a conceptual blockchain-based health care ecosystem to
provide better security and privacy for sharing data within the cloud. This system facilitates the following
four benefits as described in the paper that there is no requirement of the third party to sign the agreement
which protects the system from a single point of failure. Secondly, each patient is allowed to access and
control their details. Thirdly, medical history is recorded as a chain of blocks that is consistent, accurate,
complete, and timely distributed among all on the network. Lastly, in case any changes made to the data
on blockchain will be visible to all patients or members on the network. But on the darker side, the author
also mentioned some of the challenges that have to be considered while its practical implementation such
as the data on the blockchain is immutable that is the data once added cannot be altered or removed, it
should be dealt with in while implementing [44].
Table 2 describes the gap in the area of health-care implementation using Blockchain.
Table 2: Privacy of Blockchain in Healthcare
Ref-no Year Contribution Privacy Blockchain type Research gap
Parameter
42 2016 Build an app Identifiabliblity Private/permissioned High
HDG using and blockchain computational
blockchain unlinkability cost and
scalability
43 2017 Blockchain Identifiability, Private/Permissioned Access control
for data unlinkability Blockchain and large
sharing and and database
collaboration- pseudonymity
a mobile
application
44 2018 A conceptual Identifiability, Consortium Complex
blockchain- Anonymityand blockchain Architecture
based to unlinkability and
protect Implementation
health data cost
stored in a
cloud
13
3.4. Privacy of blockchain in supply chain management
Feng Tiang et al., in 2016 studied the agri-food supply logistics in China and find out the problems
that persist in the existing logistics pattern of the market which is traceability of the product supplied. To
overcome these issues the author discussed the advantages and disadvantages of RFID and blockchain
technology and discovered how it can benefit the supply of the food chain market in China. The author
developed a conceptual framework to design an agri-food supply chain traceable system which helps in
enhancing food safety, quality and also helps in reducing the loss incurred during the logistics process.
And finally, compare the results with the traditional food chain system and proves it better in terms of
monitoring and tracing of food quality and safety from the farm to the fork [45].
In comparison to the above-mentioned conceptual framework Miguel Pincheira et al., in 2018 has
practically implemented a system for agri-food supply chain management with some changes and
analyzed the performance of the system on different factors. The authors designed and deployed a system
called 'AgriBlockIot' based on decentralized blockchain and integrated it with IoT sensor devices for
better results. This system assures the transparent and auditable asset traceability of the food along the
whole supply chain from production to consumption. The author implemented the system on two different
blockchains that is Ethereum and hyper ledger sawtooth and compares the results of both the techniques
based on latency, CPU, and network usage, and also lime lighted its pros and cons. The author concluded
that the performance of hyper ledger system is better than that of one implemented with Ethereum but in
certain scenarios, it is better to use Ethereum technique instead of the hyper ledger. He discovered it is
more convenient to use Ethereum blockchain with IoT device in case of more scalability and reliability is
needed, otherwise, it is better to use hyper ledger blockchain because it is considered as a mature
implementation at the level of Ethereum [46].
Yash Madhwal and Peter B. Panfilov, in 2017 has mentioned the importance of SCM in the aviation
industry and discussed the loopholes in the existing supply chain of spare parts of the aircraft's globally.
The author showcases the use of blockchain in managing the inventory of aircrafts spare parts and also
monitoring its performance and usage. This will help in achieving a transparent network for the supply of
different parts and reduce the risk of black marketing the product. The author concluded that these new
data-driven distributed techniques will help the SCM managers to analyze the supply, demand, and source
of availability of the parts and also protect them from any illegal access [47].
Table 3 highlights the gaps in the field of maintaining privacy in supply chain management using
Blockchain.
Table 3: Privacy of Blockchain in Supply Chain Management
Ref-no Year Contribution Privacy Blockchain type Research gap
Parameter
45 2016 RFID and Traceability Public/permission- High
blockchain- and less blockchain implementation
based agri- authenticity cost, Storage
food chain and
synchronization
46 2018 AgriBlockIoT Transparency Public/Permission- Single language
and traceability less Blockchain used for smart
contracts and
Computation
14
cost
47 2017 Blockchain- Transparency Public/Permission- Authentication
based SCM and availability less Blockchain of spare parts,
for spare RFID tags and
parts of an smart contracts
aircraft can be used for
better results
3.5. Privacy of Blockchain in IoT
Ali Dorri et al., in 2017 has proposed an optimized blockchain for IoT systems to provide better
security and privacy in IoT devices connected over the network. Though the blockchain technique is
expensive and suffers from high overlay bandwidth and delays which does not complement IoT devices,
they suggested a lightweight blockchain-based architecture in conjunction with IoT which provides most
of the security and privacy features of IoT devices at low overhead and delay. To examine his idea he
tried to implement it on a smart home but can also be applied to other IoT applications. The author
followed a three-tier hierarchical structure to provide optimize resource usage and also increase the
scalability of the network but still has to work upon consensus for better results [48].
Moreover, Ali Dorri et al., later in 2017 has proposed a better or modified lightweight scalable (LSB)
blockchain framework to assure security and privacy in IoT devices. In this paper, the author tried to
overcome certain drawbacks of IoT systems such as limited resource consumption, centralization, and
lack of privacy. The author proposed a comprehensive tired LSB framework that will meet all the major
requirements of the IoT devices and applications. Also, the blockchain technology will help in preserving
the security and privacy of the network. The architecture works on the principle of lightweight consensus
algorithm which proved better in terms of latency, overhead, and scalability [49].
Yogachandran Rahulamathavan et al., in 2017 have developed an IoT ecosystem based on blockchain
using the Attribute-based encryption (ABE) technique for maintaining privacy in the network. The author
sheds light on basic properties provided by blockchain such as data integrity, non-repudiation but he also
mentioned that confidentiality and privacy are the two major issues that are still to be overcome in a better
while appending blockchain to IoT. This novel architecture provides both confidentiality and access
control and also identified as an effective technique for data communication in decentralized networks.
The ABE algorithm provides end-to-end privacy in the ecosystem because only the trusted miners in the
network can decrypt the encrypted data by the sender [50].
Lizing Zhou et al., in 2018 has proposed a novel system to provide better security and privacy features
for IoT applications called 'Beekeeper' which involves the concept of threshold servers and homomorphic
computation on the data stored in blockchain. The author used Ethereum blockchain and threshold secure
multi-party protocol to perform computations (TSMPC) on the server's data. The system allows any node
to become a leader if it desires to be. Moreover, the homomorphic computation helps the server to process
user's data easily without learning anything from it. The performance of the Beekeeper system can be
increased rapidly as it allows or attracts external computing devices to join the network. Further, it also
protects from any malicious node to enter as TSMPC helps in verifying the threshold number of its
15
servers is active or not. Therefore, the proposed model guarantees the decentralization, confidentiality,
anonymity, homomorphic threshold, and credibility of the transaction [51].
Md. Abdur Rahman et al., in 2019 has designed and implemented a Mobile edge computing (MEC)
framework based upon blockchain and IoT for facilitating secure sharing economic services in smart
cities. The author worked upon one of the major issues faced by the sharing economy that is the
management of the unique identity and verification of each stakeholder securely and anonymously. The
author mentioned the pros and cons of using cognitive edge computing with blockchain to support the
sharing economy application of IoT and come up with an architecture that supports decentralization on
and off storage blockchain, identity management, and smart contract services to transact the data [52].
Abdallah Zoubir Ourad et al., in 2018, have discussed how the use of blockchain technology in
conjunction with IoT provides better access and authentication of user data on the network. The authors
reviews the basic IoT authentication model and also the blockchain-based authentication model and based
on their pros and cons a new architecture is using Ethereum smart contracts to address the challenges in
the existing model [53].
Nachiket Tapas et al., in 2018 has proposed an enhanced model based upon blockchain to overcome
the drawbacks of existing IoT cloud-based data storage applications. The author implemented his model
using smart contracts over Ethereum platform. It mainly focuses on providing better access control and
audit operations over the network. The author took into consideration the concept of a smart city to prove
his idea as smart cities are well suited for research in multiple areas nowadays. The author highlighted
that the infrastructure of smart cities can be viewed as a heterogeneous network of cyber resources that
leads to deal with the problem of access control, authorization, and delegation of IoT-cloud resources
[54].
Table 4 shows the research gap in the area of IoT using blockchain technology.
Table 4: Privacy of Blockchain in IoT
Ref- Year Contribution Privacy Blockchain type Research gap
no Parameter
48 2017 Optimized Anonymity and Public/permission- Overhead and
Blockchain for unlinkabilty less blockchain overlay can be
IoT improved
49 2017 Blockchain- Identifiability, Consortium Computation and
based smart anonymity and blockchain implementation
home unlinkabilty cost is high
framework
50 2017 IoT ecosystem Anonymity, Private/Permissioned Scalability and
using unlinkability Blockchain computational cost
ABE technique and
unobservability
51 2018 IoT system Anonymity, Public/permission- Scalability
based on unlinkability less blockchain
TSMPC protocol and
16
unobservability
52 2019 MEC-based Identifability Private/Permissioned System can be
economy and anonymity blockchain further tested for
system sharing economies
at a large scale
53 2018 Oauth Identifability, Public/permission- Consumption(gas)
implementation anonymity and less blockchain cost and scalability
via smart unobservability can be improved
contract
54 2018 IoT system for Anonymity and Public/permission- Usage of Smart
Cloud unlinkabiltiy less blockchain contracts for
Authorization SmartMEEcosystem
and Delegation
3.6. Privacy preserving strategies for blockchain
Undoubtedly, the blockchain technology has achieved milestone in a diversity of applications over
internet assuring better privacy and security in contrast to the various other centralized and distributed
techniques but it is still vulnerable to certain privacy attacks because of its openness and transparency
among all the peers in the network. Therefore, surveys has been conducted which highlights the available
techniques used for enhancing privacy of blockchain in a diversity of applications.
Qu Feng et al., in 2019 has studied the weaknesses of existing wireless communication channel for
VANETs (Vehicular ad-hoc Network) and come up with a novel framework based on blockchain
technology known as BPAS (Blockchain-assisted Privacy-preserving Authentication System) which
facilitates the auto authentication of the vehicles while preserving the privacy at same time. The authors
uses hyperledger fabric platform for deploying the framework and evaluating the results to verify. To hide
the real identity of the vehicle owner and providing the traceability of the vehicle at the same time is a
tedious task which is successfully achieved by the authors with this architecture. The various algorithms
are used namely, fuzzy extractor which will enhance the security at the time of authenticating the vehicle,
ABE (Attribute-based Encryption to ensure the privacy of user, and blockchain and smart contracts which
will help in maintaining access controls and provides ABIs (application binary interfaces) which supports
inserting, uploading and revoking of public keys with respect to the vehicles [55].
Aiqing Zhang and Xiadong Lin, in 2018 has made a review on critical issues that persists in the
existing EHRs and analyzed that in the recent years blockchain has provided quite promising solutions in
this field of health-care to achieve privacy of patient data sharing on the network. The authors proposed a
framework based on blockchain for preserving privacy while diagnosis of a patient. Two blockchains
were used namely, private and consortium blockchain; the private blockchain is used to store the actual
PHI (personal health information) of the patient which is encrypted while consortium blockchain keeps
the records of the PHIs indexes. The BSPP (blockchain-based secure and privacy-preserving PHI)
protocol was implemented on JUICE and the performance of the architecture is evaluated in terms of
privacy and security [56].
Mengmeng Yang et al., in 2018 conducted a survey in the field of IoT applications that is
crowdsensing. The crowdsensing is a technique which enables the people with mobile devices to travel to
specific locations and collect data to submit it back to the requesters and earn reward in return. The
authors analyzed that the existing system suffers with a problem of privacy infringement and therefore,
17
proposed a novel solution based on blockchain to address the privacy issues. The framework is designed
in such a way that it overcomes the problem that is caused by the transparency property of blockchain that
is user’s real identity cannot be disclosed by linking to his payment transactional history. Moreover, the
system also facilitates the location privacy to the workers in the crowdsensing system without the
involvement of any third party. To implement the proposed architecture the authors used a combination of
public and private blockchain in their model [57].
Yiming Wu et al., in 2019 has identified that task matching is a very critical issue in crowdsensing
application of IoT. Thus, they proposed a blockchain-based privacy-preserving task matching (BPTM)
system to resolve the privacy and reliability related issues in the existing solutions. The authors
highlighted that identity anonymity is a very sensitive issue in crowdsensing application; therefore, it is to
be handle with care, otherwise, in case if the workers identity is disclosed and distributed publically then
it may lead to many malicious things. A searchable encryption technique is used to achieve the privacy-
preserving in task matching to protect requirements of the task and preferences of the worker’s. Also, the
concept of encrypted index is used to provide matching services. The combination of smart contract with
searchable encryption guarantees the identity anonymity and reliability of the system without any
involvement of third party [58].
Chao Lin et al., in 2020 have proposed a blockchain-based conditional privacy-preserving
authentication (BCPPA) protocol for VANETs. The authors shed light on the drawbacks on the existing
solutions based on blockchain, PKI (private key integration) protocols, or ID-based protocols. The authors
stated that somehow or the other these protocols lacks in guarantying the privacy such as key/certificate
preloading and revocation, intraceability or frequent interactions in the VANETs. Therefore, the authors
designed a novel framework by combining blockchain and PKI-based technique to address the mentioned
challenges. Moreover, the framework also resolves the escrow problem and provides periodically updated
private information which means it works well with realistic on-board units (OBUs) [59].
Webin Jhang et al., in 2018 have built up a naïve blockchain-based voting protocol which not only
preserves end-to-end privacy in the system but also provides mechanism for detectability and
correctability against cheating. The authors conducted a research on existing online platforms for voting
that are centralized and even those which are based on blockchain suffers from certain confidentiality and
privacy breach issues. Therefore, they designed a new receipt-free, easily verifiable and privacy-
preserving peer voting protocol that helps the existing peer on the networks to vote without any
interference of third-party for identifying the voters or tallying the votes [60].
Wenbo Jiang et al., in 2019 have discussed the privacy and security problems that currently exist in
various public key infrastructures (PKI) IoT applications based on centralized networks such as single
point of failure, privacy breach and so on. The authors also studied the various blockchain-based solutions
which are decentralized in nature and have desirable properties such as cryptography, immutable records,
etc. are proved to be a great solution for developing blockchain-based PKIs which are well suitable for
many IoT applications but it does not behave well in handling thin-clients. The thin-clients are those who
have limited storage on their device and hence cannot keep a copy of complete database. To deal with this
problem a naïve framework is designed known as privacy-preserving thin-client authentication scheme
(PTAS) based on blockchain in collaboration with private information retrieval (PIR) protocols. It will
allow the thin-clients to act normally as full nodes and identity of the user that who is authenticating with
the thin-client will be hidden among k indistinguishable identities because of PIR technique [61].
Ashutosh Dhar Dwivedi et al., in 2019 has discussed the shortcomings in the field of maintain health-
care records over internet. The authors studied the existing techniques such as RPM (remote patient
monitoring) and many other based on IoT and blockchain; but they stated that each of these suffers from
certain privacy issues in one way or the other. Moreover, the authors also highlighted that implementing
18
the blockchain in its actual could be disadvantageous as it demands high computational cost, high
bandwidth and large computational power; thus, are not suitable for most resource-constrained IoT
devices. To resolve these issues, the authors come up with a novel framework which works on the
concept of modified blockchain in which they have eliminated the concept of PoW making it suitable for
IoT devices. They used a combination of light-weight digital signature which provides tamper-proofing of
the documents, ring signature which allows a signer to sign a message anonymously making it impossible
for others to guess who has signed the message except the actual signer and lastly the concept of double
encryption of data using lightweight encryption and public encryption schemes. Using these techniques
together the author’s guarantees the privacy and anonymity of user’s data in small IoT devices for health-
care [62].
The Table 5 below describes the various strategies that are provided by different authors to preserve
privacy in blockchain network.
Table 5: Privacy preserving strategies for Blockchain
Ref-no Year Framework/ Privacy Application area
Architecture Parameter
55 2019 BPAS Unlinkability, VANETS
Traceability
and
identifiability
56 2018 BSPP Anonymity Healthcare
and
identifiability
57 2018 Blockchain-based Anonymity, Crowdsensing
privacy preserving Unlinkability,
crowdsensing and
system Transparency
58 2019 BPTM Identity Crowdsensing
anonymity ,
Unlinkability,
and User
privacy
59 2020 BCPPA Unlinkability VANETS
and 51
percent attack
60 2018 Blockchain-based Privacy to Online Voting
privacy preserving voter,
voting framework detection and
correction
against
cheating
61 2019 PTAS Privacy to Thin-clients
user and data
19
62 2018 Cluster-based Anonymity of Healthcare
blockchain user data
3.7. Privacy Information Retrieval
In cryptography, private information retrieval (PIR) is a protocol that allows a user to retrieve an ith
bit of the information from the n bit database server without revealing which data is retrieved. This can be
done in two ways, first by using information-theoretic PIR and second, by using computational PIR.
Information-theoretic PIR based on replication of server allows the user to access the complete
database to fetch the queried information which not only helps in leaking the other information to the user
rather also reduces the communication efficiency between the client and the server.
On the other hand, computational PIR based on the single server uses cryptographic algorithms to
maintain the privacy of the data needed by the client while communicating with a server which leads to
computational overhead. Therefore, a survey has been done which discusses the various ways of assuring
the privacy of data using PIR schemes.
Ali khoshgozara et al., in 2009 has discussed all the existing solutions to maintain the privacy of
location using PIR schemes and proposed a new technique that is best among all to guarantee the location
privacy of the user. The authors developed a scalable private information retrieval information approach
to location privacy known as SPIRAL. It overcomes the issue that exists in previously existing techniques
that blur the user's location when the spatial distance is extended and also suffers from revealing the user's
identity to others in the nearby region. The SPIRAL assures privacy which is invariant to the total number
of users, size of the enclosed region, and querying pattern of the user. It provides a blind evaluation of the
queries in the range [63].
Wesam Al Amiri et al., in 2019 has proposed a system for smart parking for drivers in crowded cities
where parking is a major issue because of traffic congestion and air pollution. In contrast to the existing
systems, he built a system using the concept of consortium blockchain in conjunction with PIR. It allows
the driver to search for the nearest available parking lot which is provided by the blockchain and without
sharing his location with anyone which is provided by PIR. He implemented his work using a python
charm cryptographic library that runs on Raspberry Pi 3 devices. Moreover, his work was also financially
supported by a central body known as NSF grant [64].
Li weng et al., in 2015 proposed a new PCBIR protocol over existing schemes for protecting privacy
in large content-based information systems. The author designed a framework that provides a double
layer of protection. Firstly, it uses robust has values for queries which prevent from disclosing original
content and features. Second, the client can further reduce some bits to increase the ambiguity in the
server which makes it difficult to understand by the server what kind of information the client wants. The
authors tested the proposed work on different sizes of the dataset and experimented with it on Matlab and
get the desired results which proved that the system can be used for any CBIR system. The author also
proposed a k diversity privacy-preserving scheme for the system under which a device can make a policy
depending upon its capability because a stronger policy will not only increase the load on the server rather
20
it will also increase the cost of bandwidth and data processing. This is very useful for heterogeneous
systems [65].
Razane Tajeddine et al., in 2017 have studied all the techniques suitable for colluded databases and
find out certain problems such as download cost, the capacity of a query, and so on. Thus, the author
comes up with a naïve solution that employs an extended t-PIR scheme for a minimal of t server may
collude, for any given pattern of the query. The author experimented with some special collusion patterns
and proves that the retrieval rate is significantly high. The proposed work of coded data with arbitrary
collusion pattern works well when the size of t (a subset of the server) is less [66].
Karam Banawan et al., in 2019 has discussed the problem regarding PIR through wiretaps and
provided a better solution that provides both security and privacy to the user trying to retrieve the
information. The authors designed an algorithm based on the asymmetric key generation which
maximizes the retrieval rate and also helps the client to retrieve the mth message privately from the N
copies of the database without leaking any information to the eavesdropper in the communication
channel. It focuses on the two major problems of the PIR, first is to protect the identity of the desired data
from the public databases and second is protecting the requested message from the external
eavesdrop(wiretap) observing the communication. In this paper, to overcome the problem mentioned
above the authors implemented the secrecy constraint in conjunction with the usual privacy constraint in
which a secret key is generated for each database with the artificial noise vector using an MDS code [67].
Iordanis Kerenidis et al., in 2004 developed a Quantum symmetric privacy information retrieval
(QSPIR) in comparison to the previously existing PIR and SPIR schemes for protecting the privacy of
data as well as the user without shared randomness among the servers which is not possible in any of the
existing SPIR models. The different datasets are tried and tested by the authors to prove that QPIR
requires less communication than the best known classical PIR schemes. A review is made by the authors
on the existing SPIR models in the quantum world, where users and servers have quantum computers and
can communicate qubits to retrieve information [68].
Terence H. Chan et al., in 2015 proposed a new PIR scheme for coded data storage on the servers over
the existing schemes which are based on uncoded data storage. The author experiments with his proposed
work using MDS codes for storing the data and guarantees the error-free and private retrieval of
information to the user. The author also shows in his results that the coded PIR technique provides the
optimal trade-off between retrieval cost and storage cost in comparison to the existing schemes which
does not give the best results when the data size is large [69].
Hsuan-Yin Lin et al., in 2019 have studied the trade-offs that can be achieved if some information
leakage is allowed as demanded by the user while retrieving data from the server using PIR protocols. HE
named his work as weakly-private information retrieval (WPIR) as it allows to leak some identity of the
data being retrieved as asked by the client and also compares the other two parameters that are upload
cost and access complexity with the existing PIR schemes. To achieve WPIR the author partitioned the
complete database into n equal parts and allowed leakage of particular part on demand. Under WPIR the
authors proposed two schemes; first, one assures the minimum upload and downloads cost of the data,
and another works upon privacy factor. The authors' work has proved that relaxing the perfect privacy
factor not only improves the download and upload cost but also gives a higher access rate [70].
Kaihua Qin et al., in 2019 have worked upon the privacy of lightweight bitcoin transactions and came
up with the drawback that a large bandwidth is required if privacy is to be assured using simple payment
verification (SPV) protocol. Moreover, if bloom filters are used for the transaction then privacy cannot be
guaranteed. Thus, the authors come up with a naïve solution in which they have used SPV protocol in
21
conjunction with the PIR scheme which not only reduces the bandwidth rather it also decreased the
latency factor and guarantees privacy to the user [71].
Jayneel Vora et al., in 2018 have put a limelight on the issues in the existing electronic health records
(EHRs) and proposed a blockchain-based framework for Securing Electronic Health Records known as
BHEEM. The existing EHRs schemes are unable to guarantee security to the patient's data and these are
unable to maintain the balance between providing data to patients, providers, and third-party access.
Therefore, the author introduced a new concept of BHEEM which overcomes the existing issues and also
helps in maintaining the security and privacy of EHRs. Moreover, the authors have shown that the
framework is highly scalable [72].
Table 6 describes the research gap in the field of maintaining privacy of data using PIR.
Table 6: Privacy Information Retrieval
Ref-no. Year Architecture/ Privacy Blockchain Research gap
Framework Parameter type
63 2009 Scalable PIR Anonymity NA Not suitable for
for location large subset of
privacy database
server
64 2019 Blockchain Anonymity Consortium Not feasible for
based PIR and blockchain mobile
unlinkability application
65 2015 Robust hash Privacy of NA Does not work
algorithm for person and if client and
preserving data server has
content- different
based privacy architecture
66 2017 PIR scheme on Privacy of NA Not suitable
coded data for data when colluding
arbitrary sets are large in
collusion number
patterns
67 2019 PIR through Identifiability NA Can be
wiretap and experimented
channel II unlinkability further in the
presence of
Byzantine
adversaries
68 2004 Quantum Privacy of NA Communication
computing to person and complexity
maintain PIR data needs to be
improved
69 2015 PIR scheme for Privacy of NA Work well for
coded data data fixed storage
size
22
70 2019 Weakly PIR Information NA Privacy of data
leakage is compromised
71 2019 SPV based PIR Privacy of Public/ Downloading
protocol for user permission- bandwidth can
lightweight less be improved
Bitcoin blockchain further
72 2018 Blockchain Identifiability Private/ More execution
based and prmissioned time and large
framework for unlinkability blockchain computational
EHRs power
4. Open Research Problem
The main objective of this paper is to study the existing PIR and blockchain schemes that are used by
many researchers to guarantee privacy in different domains over the internet and highlight the gaps in the
literature work. After reviewing all the above-mentioned papers it has been observed that PIR is of two
types, namely, information-theoretic PIR and computational PIR. Both types have their pros and cons, so
it completely depends on the user which type of PIR he wants to depend upon the nature of the
information. Moreover, it is also analyzed that if blockchain and PIR if applied together than privacy of
data and user can be handled in a more convenient manner.
From the review made in section III, we have observed some open research opportunities in the
direction of blockchain privacy and PIR.
1. Though the blockchain networks have gained popularity in different domains over internet but it
still faces some challenges because of it open to all and transparent feature.
2. The problem of linkability of transactional data which may help a dishonest node to use it for its
own benefits.
3. Also in case of public blockchain the anonymity of peers could also be disadvantageous and may
lead to malicious attacks.
4. The problem in retrieving data if a subset of the database server is large in number because it
leads to complex computation as the data being queried has to be searched on more than one
database server.
5. Downloading bandwidth is low in most of the solutions that are only based upon informational
PIR because it sends a copy of the entire database to the client when a request is made by him.
6. Computational cost is higher because in the case of computational PIR many complex algorithms
are used which increases the transaction's complexity rate with a polynomial value.
7. Storage or caching of data is a big issue because upon requesting the ith bit from the database the
whole database is sent to maintain the privacy of data.
8. More execution time is needed for computation in the case of computational PIR which increases
the access time cost.
9. In many PIR models, adversaries may attack while communication is done because it mainly
focuses on protecting the privacy of data.
It has been observed that both blockchain and PIR are applicable individually for different applications
to maintain privacy in their ways but, they still face certain challenges despite having various advantages
23
and their unique features. The diagram below in Figure 4 highlights the major characteristics of
blockchain and PIR and also focus on common features that could be advantageous to guarantee privacy
in various applications. Moreover, in case of cross chains when multiple blockchains integrate to provide
interoperability the privacy concerns are also high. For example, if a blockchain application is developed
to trace the supply of vaccinations in countries and another blockchain is introduced to collect the COVID
patients and their vaccination process and they are connected, in that case the privacy levels are also
different and PIR can help in processing the information.
Figure 4: Blockchain and PIR
To tackle all the above-mentioned issues it is advisable to use blockchain in conjunction with PIR to
assure privacy and security in various domains applicable to all decentralized and distributed system
based applications of IoT, SCM, healthcare, finance, e-commerce, etc. It is a fact that both blockchain and
PIR schemes have their advantages and disadvantages but if these two are implemented together can
bring a major change in the history of technological advancement. For instance, their merger can lead to
the development of a prototype which supports non anonymous user to make unlinked transactions
providing privacy to both user's identity and the information being shared or retrieved. And to prove this,
the results of this new prototype can be compared with the existing blockchain-based protocols.
5. Conclusion
In this paper, we have discussed privacy in different aspects specifically concerning blockchain and
PIR. Blockchains are the main focus in this survey where we review the various blockchain applications
that deal with different privacy parameters. Moreover, we also discuss the PIR concept and review the
existing literature based on the same. The main idea behind this survey is to focus on the collaboration
aspect of blockchains and PIRs as they are two different parts; however, they both are having the
potentials to be conjugated. The research problems identified during the study opens up various important
directions of blockchains and PIR handshaking.
6. Acknowledgement
I would like to thank Dr. Rahul Saha, Gulshan Kumar and Tai Hun Kim for their expertise and
assistance throughout all aspects of our study and for their help in writing the manuscript.
24
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