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Data science and Big data research paper

Challenges and Issues in an e-Healthcare system based on IoT

Problem statement:

Data security and privacy, as in all systems, are also the most significant challenges in a health care system based on IoT. All IoT devices capture data in real-time, Thus the need of adhering to standards and data protocols is of utmost importance.

Fraudulent claims, misuse of IoT device data including data ownership, and regulation of data are some of the additional issues in such a healthcare system.

Research question:

How do we ensure IoT devices are secure in this system?

How to maintain data integrity and ensure user data is not compromised in a Health care Organization.

Purpose Statement:

To identity and document available solutions for maintaining the security and data integrity in an IoT based e-healthcare system.

2. Draft a 5-6 page research prospectus (a short version of what will become your “research proposal”) that is a shortened description of a possible research project you’re looking into for your dissertation, including a description of the problem area, the purpose of your research project, and research question(s) that you would answer in your dissertation.

Either paper needs to be approximately 5-6 pages long, including both a title page and a references page (for a total of 7-10 pages). Be sure to use proper APA formatting and citations to avoid plagiarism.

Your paper should meet the following requirements:

• Follow APA7 guidelines. Your paper should include an introduction, a body with fully developed content, and a conclusion.

• Conclude your paper with a detailed conclusion section.

• Support your answers with the readings from the course, the course textbook, and at least three scholarly journal articles from the UC library to support your positions, claims, and observations, in addition to your textbook. Of course, I start with Google Scholar, and the UC Library is a great place to find resources.

• Be clearly and well-written, concise, and logical, using excellent grammar and style techniques. You are being graded in part on the quality of your writing.

Research Prospectus Outlined

First Floor William T. Jerome Library
TLC@bgsu.edu

www.bgsu.edu/LearningCommons/

Phone: (419) 372-2823

A research prospectus is a preliminary plan for conducting a study. This is not a detailed, technical research
proposal, but, rather, a considered analysis of the issues you are likely to confront in such a study. In
essence, it is a preliminary proposal. In completing this task, you should be sure to consider at least the
following:

Research Problem. What is the research problem you are trying to solve? [A problem is a situation that,
left untreated, produces a negative consequence for some group, institution or individual(s). “Girls score
lower on technology attitude scales than boys” isn’t necessarily a problem; “girls are less inclined to pursue
careers in technology-related fields” is.] What makes it a problem? For whom? Who says so?

Assumptions. On what assumptions are you basing your work? Which of them seem to be verifiable in the
literature? Which are more speculative?

Theoretical Issues. What theoretical issues arise in your proposed study? For example, “theoretically,”
how would you explain this problem and the results you suspect you might get to another scholar? (Do you
take a behavioral view? Social systems view?) Are there other theoretical orientations that should be
considered in the design of your study?

Literature Review. What, in general, does the literature say about your topic? [This need not be a
complete review, but you should cite some of the major theory, research and writers in the field.]

Research Questions. Based on your problem, what are the research questions you are trying to answer?
Why and how will answering these questions contribute to solving the research problem? Remember…a
research question can be answered ONLY with data or information.

General Research Plan. In general, how would you propose to conduct this research study so that it
answers your research questions? What kind of data will you gather (specify type, such as surveys,
observations, interviews…or some combination of these types)? From whom will you gather it? Why them?
How will you reduce the data – make sense of it? How will you assure that the data are of high quality?

Anticipated Difficulties and Pitfalls. What kind of difficulties and pitfalls might you expect in doing a study
of this type? What will you do to prevent them or minimize their effects?

Anticipated Benefits. Who will benefit from the fact that this research is undertaken? How? Why? Who
might be disturbed this proposed study? How? Why?

This should be a thoughtful, reflective paper that presents a balanced view of the proposed study – both its
problems and its opportunities. It should serve as a first, solid communication with your committee about
the kind of thinking you have been doing on an anticipated area of inquiry that might comprise at least a
portion of your thesis/dissertation work. In addition, be aware that individual disciplines look for different
components in a prospectus. Consult with your committee to make certain you have all the information you
need in order to begin compiling your prospectus.

Adapted from: www.coedu.usf.edu/

Blockchain Challenges and Security Schemes:

A Survey

Sirine SAYADI
MEDIATRON Laboratory

Higher School of
Communication of Tunis,

Sirine.sayadi@supcom.tn

Sonia BEN REJEB
MEDIATRON Laboratory

Higher School of
Communication of Tunis,

sonia.benrejeb@supcom.tn

Ziéd CHOUKAIR
MEDIATRON Laboratory

Higher School of
Communication of Tunis,

Z.choukair@supcom.tn

Abstract With the increasing number of connected devices
and the number of online transactions today, managing all
these transactions and devices and maintaining network
security is a research issue. Current solutions are mainly based
on cloud computing infrastructures, which require servers
high-end and broadband networks to provide data storage and
computing services. These solutions have a number of
significant disadvantages, such as high maintenance costs of
centralized servers, critical weakness of Internet Of Things
applications, security and trust issues, etc. The blockchain is
seen as a promising technique for addressing the mentioned
security issues and design new decentralization frameworks.
However, this new technology has a great potential in the most
diverse technological fields. In this paper, we focus on
presenting an overview of blockchain technology, highlighting
its advantages, limitations and areas of application.

The originality of this work resides in the comparison
between the different blockchain systems and their security
schemes and the perspective of integrating this technology into
secured systems models for our comfort and our private life.

Keywords Blockchain, Security, Technology, Smart
Contracts, Consensus

I. INTRODUCTION

The current network model connects multiple computing

devices and will continue to support small-scale Internet of
Things networks that will not be able to meet the growing
needs of tomorrow’s large ecosystems. Centralized cloud
servers will remain a bottleneck. throttling and a point of
failure that can disrupt the network.

In this context, Blockchain technology appeared in 2009
by Nakamoto [1] “Bitcoin Developers” as a storage
technology serving decentralized large registers and as a
security technique for authenticating, authorizing and
verifying data generated.

With blockchain technology the concept of consensus has
emerged as a mechanism that ensures trust in communication
between two entities without the intervention of an
intermediary. We can use blockchain in cryptocurrency,
smart contracts, digital identity management, internet of
things, access control applications, automated peer-to-peer
insurance, in banks and in many other applications [2].

Since its inception, from the initial cryptocurrency to the
current smart contract, blockchain technology has shown
promising prospects in many areas of application.

This proposed paper will be a state-of-the-art study on
blockchain technology. Section 2 will present an overview of
blockchain technology. Section 3 will describe a semantic
study of the potential of blockchain technology. We present
in Section 4 some cases of use of this technology. Then we
will examine the security threats, some real attacks for this
technology, and its security enhancement solutions in Section
5 and finally we will conclude our paper by suggesting future
directions.

II. OVERVIEW OF BLOCKCHAIN TECHNOLOGIES

This section presents a complete visualization of
blockchain technology, how it works, its structure and
existing types.

A. Blockchain Process

The Blockchain process is described as a transaction

between users on the network that are grouped into blocks.
The block is validated and saved on the network by a
«minor » according to cryptographic techniques that depend
on the rules of the type of blockchain used.

In the bitcoin blockchain this technique is called the
“Proof-of-Work“, (POW), and “proof-of-stake” (POS) in the
blockchain ethereum. If the block is validated, it is time
stamped and added to the block chain. The transaction is then
visible to the receiver as well as the entire network. This
process takes some time depending on the blockchain (about
10 minutes for bitcoin, 15 seconds for Ethereum) [8].

Each blockchain is identified by its cryptographic hash
and carries a list of transactions and a hash to the previous
block.

The exception to this is the first block in the chain, called
“genesis”, which is common to all clients in a blockchain
network and has no parent. This establishes a link between
the blocks, thus creating a chain of blocks, or blockchain.
Any node having access to this ordered and back-linked
block list can read it and understand what is the current
global state of the data exchange on the network.

Figure 1: Blockchain Process

B. Blockchain Structure

A block is composed of two main parts which are the

Block Header and the transactions (see Figure 1). The Block
header contains several fields, the most important among
them are the block version, the Merkle tree Root Hash, time
stamp, nBits, Nonce and parent block hash. Transactions are
the data saved in the block [46].

These fields will be detailed below (see figure 2):

Block Version: Specifies the set of block validation
rules to follow [46].

Merkle tree Root Hash : is a condensed digital
fingerprint of all transactions in the block. The slightest
modification of a transaction in the block modifies this root.
Its principle is to calculate the hash of a node from a hash of
his sons [3].

Timestamp: current time in seconds in universal
time since January 1, 1970 [46].

nBits: target threshold of a valid block hash.

Nonce: A 4-byte field, which usually starts with 0
and increases for each hash calculation. On receipt of the new
block, the complete nodes compute the header hash only
once, to see if the Nonce is valid [37].

Parent block hash: The nodes save the data of the
block’s. Thus, all the nodes have the hash of the block 31, if
the block 32 is received by a node, it will determine that the
block 32 is the child of 31 by checking this field [37].

Figure 2: Simplified Block Structure

C. Type of Blockchain

There are three types of Blockchain technologies
presented in the following table :

Public blockchain from which everyone can
participate in the process of reaching consensus and
verifying the transaction. Like Bitcoin [4] and Ethereum [5].

Consortium blockchains: In this type, the node
can be chosen in advance if the data in blockchain can be
public or private. They can be considered as partially
decentralized like Hyperledger [6].

Private blockchain has strict management
authority over access to data. Nodes are restricted, not all
nodes can participate in this blockchain like Ripple [7].

Table 1: Comparative table of blockchain types [46]

All types of blockchain have advantages. The choice of
blockchain type depends on our needs and our proposed
services.

III. POTENTIELS OF BLOCKCHAIN

Blockchain technology is not only a technique, but it is a

technological revolution with very important security
features, its operating model using consensus and its shared
ledger to solve the problems of traditional centralized
models.

A. Basic Security Techniques

We detail in this section the different basic security

principle by specifying how Blockchain technology can
perfectly guarantee them.

Integrity: it to ensure that the information has not
been changed, only by those authorized to make
changes. Blockchain uses cryptographic mechanisms
to guarantee that operations are immutable with the
purpose of verifying integrity.

Availability: it ensures the availability of data for
every need. the service is always active at the request
of a legitimate users. Blockchain allows users to
maintain blocks in a decentralized manner with
various copies on the blockchain.

Pivacy: is the guarantee that only authorized persons
can access to the information. The Blockchain uses a
pseudo-anonymization mechanism (hash functions) to
hide user identities to ensure confidentiality.

Authentication: a procedure by which a computer
system certifies the identity of a person or a computer
to allow that person to access certain secure resources.
The Blockchain technology provides this function by
providing private keys for users who are authorized to
carry out transactions.

Non-repudiation: Is the impossibility, for a person or
any other entity engaged in a communication, to deny
having received or sent a message, and this is ensured
by blockchain technology.

B. Shared Ledger

This is the basic feature of blockchain technology, it

means that blockchain does not have a centralized node, data
is processed, stored and updated in a decentralized way. This
avoids the problems of single deffain point and offers a peer
to peer communication such that all nodes are interconnected
and all participants in the network are equal without a central
node.

C. Smart Contract

The smart contract is autonomous computer programs

that once started, automatically execute pre-defined
conditions with conditional statements of the type « if ….
Then …. » Using the information available on the blockchain.

These contracts must be able to reduce audit costs,
execution, arbitration and fraud. They may have to manage
funds or authenticate external entities [8].

D. Consensus

A consensus is a secure fundamental trust mechanism. It

characterizes a general agreement of existence of the
members of a group. It allows you to make a decision
without the need of an intermediary or a trusted authority.

In the existing blockchain system, there are several
consensus mechanisms. We will quote the best known
below:

PoW (Proof of Work) :

Method used to validate Blockchain network blocks. This
method requires users to use their computing power to
validate a block. Minors compete against each other. As a
result, the higher the computational power (combining
several computers, to increase computing power), the more
likely they are to find the result of a “Hash” function and thus
validate the block. In the Bitcoin blockchain it is necessary to
count a validation every 10 minutes approximately [9].

PoS (Proof of Stake):

A chance to validate a block is based on how much of a
stake (or cryptocurrency) the miners have. For example, if
you had 5% of the cryptocurrency, you could extract 5% of
all its transactions. People with more currency are thought to
be less likely to attack the network. Its operating principle
based on the richest person has more power in the network is
unfair because the power here depends on the balance held in
the account [46]. The PoS save more energy (reduces the
amount of calculation) but increases the flow. Unfortunately,
if the operating cost is close to zero, attacks could result.

PBFT (Practical Byzantine Fault Tolerance) :

The problem of Byzantine generals is a metaphor that
deals with questioning the reliability of transmissions and the
integrity of the interlocutors. A Byzantine fault is therefore a
failure that consists of the presentation of erroneous or
inconsistent information. The consensus “Practical Bizantine
Fault Tolerance” (PBFT) is a state machine replication
protocol that tolerates arbitrary, or “Byzantine” faults. It is
fault-resistant, fast, long-lived and an attack does not impact
its performance too much. This protocol consists of three
phases: pre-preparation, preparation & validation, it requires
3f + 1 replicates to tolerate f simultaneous Byzantine faults.
When a message is sent on the platform, the nodes retransmit
the transaction to all peers. If at least 2/3 of the nodes confirm
the validity of the transaction it is confirmed. The platform
allows users to transfer peer-to-peer ownership regardless
[46].

Many other consensus mechanisms can be found , such as
DPoS (Delegated Proof of Stake), PoB (Proof of Bandwidth)
[10], PoEt (Proof of Elapsed Time) [11], PoA (Proof of
Authority) [12], Ripple [48], Tendermint[49] etc. that are
used in some blockchain systems.

A comparison between some of the most used consensus
algorithms is presented in table 2 [46].

With these advantages presented in this section from the
basic security techniques, smart contracts, shared ledger and
the consesus, blockchain technology has attracted attention in
several areas..

In the next section, we will introduce some areas of use of
blockchain technology as a solution concept.

Table 2: Comparison between some consensus algorithms

Property POW POS PBFT

Identity
management
of nodes

Without
permission

With
permission

Energy saving No partial yes

Power
tolerated

<25% computing power

<51% stake

<33,3% Defective replicas

Example Bitcoin Ethereum Hyperledger

IV. CASES USE OF BLOCKCHAIN AND APPLICATIONS

In our days, Blockchain technology is used in many

areas, not only in the financial application, but also in other
areas such as supply chain traceability, identity certification,
insurance, International payments, the Internet Of Things
and the protection of privacy etc [25, 26, 31, 32, 33,34].

We detail in this section some uses of blockchain
technology:

1) Digital Currency :

Several transaction systems have been built recently by

blockchain technology, which makes a revolution in digital
currency and online payment system. With these digital
currencies and the crypthography technique, transfers can be
made without the need of the central bank.

For example, we can send and receive bitcoins using
public keys, with all anonymity we can record transactions.

Several other cryptocurrency like ethereurm, ripple,
litecoin and etc [27].

2) Smart Contract:

Smart Contract is a digital contract that runs

automatically through a computer system. It controls the
digital assets of the user, by formulating a set of rules
containing the rights and obligations of the users. Smart
Contract is like an automatic trusting authority among
participants [28]. Ethereum is an open source blockchain
platform offering a decentralized virtual machine based on
the Smart Contract. To manage these contracts Ethereum
uses its digital currency called ETH, users can create many
applications, services or different contracts on this platform
[29].

3) Hyperledger :

Hyperledger is an open source blockchain platform,

launched by the Linux Foundation in December 2015 with
the aim of improving reliability and performance. It aims to
support global business transactions of large technology,
financial and supply chain companies etc [30].

4) Blockhcain To Ensure privacy, Access control and

Integrity

Protecting our personal information and our private life
is a challenge in our day. [35][36] uses blockchain
technology based POW in IOT applications to ensure
integrity and confidentiality. Blockchain can also be used for
access control. Just save the history daily in blockchain as a
signed transaction specifying public keys with access rights.
Only minors authetified with his private keys can include
this transactions in their blocks [37].

Based on blockchain technology, Ouaddah and al. [36]
presented the FaiAccess framework with its different parts to
allow users to control their data. Zyskind et al.

[34]

exploited the access control option provided by the
blockchain with storage in a distributed hash table of several

selected nodes. The Blockchain is used here for data location
management and their access.

Ali and al. [33] used blockchain to build “Blockstack ID”
which is an identity system and a decentralized PKI. This
system consists of a control plane that is a name registration
protocol and links and a data plan that is responsible for
storing the data that must be signed by the name owner’s
key.

5) Blockchain For Eletronic Transactions

The Blockchain can be used as a base that will support

the shared economy, based on machine-to-machine (M2M)
communication. Several propositions in the theme [38, 39,
40, 41, 42]. Blockchain technology allows agents to
autonomously perform a variety of transactions and to store
the history of each transaction with transaparance and no
deffiliation.

Sun et al. [41] specifies that Blockchain technology leads
to the Internet of decentralized and autonomous objects. The
blockchain supports all processing transactions between
devices and each device can manage its behaviors and roles
in an autonomous way.

Using the Bitcoin network, [40] described a model of
data exchange by electronic money, between a sensor and a
client. [38] described a Bitcoin-based e-commerce model for
IOT devices. This composite model consists of 4 layers (the
technical layer for the management of the Blockchain
module, the infrastructure layer containing the smart
contract platforms and IoT services, the content layer
containing the participants and the IOT products and the
layer exchange that contains the P2P transaction system).

We can find many other proposals that use Blockchain
technology for economic transactions for IoT like ADEPT
[43], Filament [39], Waston IoT platform [44], IOTA [42]
etc.

6) Blockchain To Secure Smart Home :

Dorri et al. [45, 50] proposed a lightweight blockchain

solution adopted for IoT without cryptocurrency to illustrate
a smart home containing a power computer that is
responsible to control and audit communications and provide
access control between devices. It maintains a private
blockchain and is considered minor without the need for the
proof of work concept because only this computer is
responsible for managing the blockchain. Other devices
receive a private key and a public key to perform
transactions. For example, if a sensor wants to open the
faucet, it will send a transaction to the faucet, which will
check in Blockchain if that sensor is allowed to open it.

A smart home is the best example for IoT Blockchain

combination. The services offered by blockchain technology
can be contribute to shared economies and to the smart
cities where objects connect seamlessly and anonymously to
exchange and share data.

V. SECURITY ISSUES OF A BLOCKCHAIN TECHNOLOGIE

We describe in this section some recently encountered

limitations that can affect the good functioning of
blockchain technology by presenting some models in the
form of the proposed improvement solutions to limit these
risks.

A. Risks to Blockchain

1) 51% Vulnerability:

The consensus mechanism has a vulnerability of 51%,
which can be exploited by attackers to manipulate the
blockchain.

In PoW, if the hash power of a minor> 50% of the
blockchain’s total hash power, the 51% attack can be
initiated. As a result, mining resources concentrated in a few
mining pools can cause fears, as a single pool controls more
than half of all computing power [13].

In the PoS, if the number of cryptocurrencies owned by a
single by a single miner is greater than 50% of the total
blockchain. A 51% attack can occur which an attacker can
arbitrarily manipulate information from the blockchain [47].

An attacker can exploit this vulnerability to carry out
attacks; we will mention some of each after following [15]:

Run a double spending by modifying the
transaction data (same coins are spent multiples
times).
Change the order of transactions.
Prevent normal mining operations of other miners
(Denial of service attack).

2) Double Spending attack:

A customer provides a seller with a signed transaction;
the seller verifies the validity with a peer who confirms the
transaction. If the client is malicious, it can create a
conflicting transaction by generating a double spend (the
same crypto currency spent twice) and having it validated by
another peer before the first transaction has spread across the
network. Both transactions are therefore proposed for
mining. Depending on which will be treated first, it is this
truth that will be imposed on the entire network by
registration in one block and invalidate the other. In this
case, if the seller had delivered before validation by the
minor, he was robbed … resulting in a double spending [14]
[47].

3) Smart Contracts Risks

Dependency of the transaction order:

In order to update the blockchain, in each era, each miner

will propose his own block. Since a block can contain
multiple transactions, the state of the blockchain can change
several times during an epoch.

This attack can be triggered if two successive

transactions of the same block invoke the same smart

contract. The order of execution of these two successive
transactions affects the final state because the execution of
the smart contract is associated with a single state [47].

The time stamp dependency:

Each block in the blockchain contains a timestamp field.

Some conditions for triggering smart contracts depend on
the timestamp, which is defined by the minor according to
the time of his local system. Smart contracts depend on time
stamp fields are vulnerable, if they can be changed by
attackers [47].

Under-Optimized Smart Contract :

The gas value corresponds to the computing resources

exploited by the bandwidth operation, memory occupancy
and many other parameters used in Ethereum as a function
of time.

We can find some resource-intensive operations such as
dead code operations and the use of loops by exchanging the
gas value according to the cryptocurrency. [47].

4) Denial Of Service Attack

An attacker can launch a DoS (Denial of Service) attack
by exploiting a set of operations executed in a single
transaction. This is because some heavy operations require
too low gas values. This can cause a waste of resources [16].

5) Selfish Mining Attack:

This attack is conducted by mining in order to obtain

undue rewards or to waste the computing power of honest
minors [18]. The attacker holds the blocks discovered in
private and then tries to forge a private channel. The authors
in [19] proposed a Selfish-Mining attack, which attract other
honest miners to dispel their computing resources
unnecessarily to keep working on blocks that lead to a
stalemate instead of attaching them to the longest chain.

6) Reentrancy Attack:

It is the fact of exploiting a recursive sending for

example the biggest flight about 60 million US dollar of the
contract CAO by this attack just after its deployment of 20
days [17].

7) Liveness Attack

In [20] the authors proposed this attack to exploit the

dilation of the confirmation duration in order to obtain a
target transaction.

8) The Balance Attack

Christopher and al. [21] proposed this attack based on
PoW blockchain, which consists of identifying subgroups of
miners with similar mining power and delaying messages
passed between them in order to mine blocks before them.

B. Security Improvements

1) Smart Pool

L. Luu et al [22] proposed a new Smart Poll mining pool
system, implemented as a smart contract. It is a
decentralized mining protocol that replaces the centralized
pool operator.

It retrieves client transactions that contain information
about mining tasks. Then the miner performs a hash
calculation and returns the completed shares to the
smartpool. A threshold sets an amount, if the shadow of
actions made reaches this threshold, the miners will be
committed to a smartpool contract that verifies the actions
and delivers rewards to the customer [47].

2) Quantitative Framework

In [23] the authors proposed a quantitative framework is

used to analyze the performance and security provisions of
the blockchain. it is a blockchain simulator and a security
model that mimics its execution to evaluate basic security
and performance.

This model specifically focuses on the attacks of selfish
and double-spending mining by taking into consideration the
consensus protocol used and network parameters such as
block propagation delays, block sizes, delays network, block
rate and the mechanism of propagation of information etc.

3) Oyente

Loi and al. [24] proposed a new program called Oyente

that tracks errors in smart contracts. This tool can also detect
bugs and injection attacks in smart contracts.

Oyente analyzes the bytecode of smart contracts and
follows the EVM execution model [47].

VI. CONCLUSION AND PERSPECTIVES

In this paper, we presented an overview of Blokchain

technology. We have described its different security
potentials by specifying a comparison between some of the
most widely used consensus algorithms in different
blockchain systems. We have also clarified the fields of use
of this technology because in recent years, it has shown its
potential in several applications and this is due to the
advantages of this technology and its decentralized nature.
These applications permeate everyday life, business and
society as a whole, transforming the world into a more
efficient world. And finally, we indicated that many
maneuvers of this technology, then specifying the
improvement solutions proposed to defend them.

Blockchain then presents many promising opportunities
that open up many paths for the future and for a connected
world in complete security. However, the challenges remain
in the resources and consensus models used.

That’s why, we aims in future work to leverage the
benefits, limitations of blockcahin technology, and
enhancement solutions to produce a new secure system

model that integrates this technology with the Internet Of
Things technology for a connected and secure world.

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Blockchain And The Future of the Internet:
A Comprehensive Review

Fakhar ul Hassan1, Anwaar Ali2, Siddique Latif3, Junaid Qadir4, Salil Kanhere5, Jatinder Singh6, and Jon
Crowcroft

1,4Information Technology University (ITU), Punjab, Pakistan
2,6,7Computer Laboratory, University of Cambridge, United Kingdom

3University of Southern Queensland, Australia
5University of New South Wales, Australia

Abstract—Blockchain is challenging the status quo of the
central trust infrastructure currently prevalent in the Internet
towards a design principle that is underscored by decentraliza-
tion and transparency. In ideal terms, blockchain advocates a
decentralized, transparent, and more democratic version of the
Internet. Essentially being a trusted and decentralized database,
blockchain finds its applications in fields as varied as the energy
sector, forestry, fisheries, mining, material recycling, air pollution
monitoring, supply chain management, and their associated
operations. In this paper, we present a survey of blockchain-
based network applications. Our goal is to cover the evolution of
blockchain-based systems that are trying to bring in a renaissance
in the existing, mostly centralized, space of network applications.
While reimagining the space with blockchain, we highlight
various common challenges, pitfalls, and shortcomings that can
occur. Our aim is to make this work as a guiding reference
manual for someone interested in shifting towards a blockchain-
based solution for one’s existing use case or automating one from
the ground up.

I. INTRODUCTION

The paradigm shift entailed by blockchain’s premise of
decentralization envisages an eventual migration from the end-
to-end principle to trust-to-trust principle [1]. According to
this new design principle, a user should ideally have complete
control over trust decisions particularly pertaining to user’s
data that powers a network application such as an online
social network. This decentralization aspect forms the basis
of the blockchain-based networks. This further paves the path
for an era of distributed trust and consensus. This implies
that large networks, in a peer-to-peer configuration, will guar-
antee the integrity of transactions (simply put interactions)
among their peers without the involvement of any centrally
trusted mediating third party. The provision of verifiable trust
guarantees further entails that such networks can be audited
in a trusted and transparent manner. This ability to audit is
useful to hold a peer of a networked system accountable
lest a dispute or malfunctioning of some other sort arises.
Moreover, any application that requires interaction among
various stakeholders for its operations in a mutually non-
trusting environment (where the stakeholders do not have to or
do not want to trust one another) can benefit from blockchain
as it creates transparency and trust in interactions among
the stakeholders without involving any third party. That is

the reason why industries such as transport, energy sector,
insurance, finance, and logistics have started to show their
interest in blockchain technology to automate their solutions
[2]–[5].

It can be observed that although the onset of the Internet
revolution heightened the societal collaboration among people,
communities, businesses [6], however, many of the Internet
applications (such as email and Domain Name System (DNS))
largely remain centralized as far as their management and
core development are concerned. The centralized governing
bodies are usually behind the trust guarantees associated
with such online applications. Similarly, the issue of trust in
cloud-hosted data storage is another contemporary challenge
predicated on the inherent centralized nature of the Internet
[1]. The clients of such online and cloud-based services, such
as cloud storage and computation, usually put their trust in
the claims put forward by the third party cloud providers. It
raises the pressing need for verifiability that the cloud is not
tampering with a client’s stored data and is always returning
correct results in response to the requested computation. A
single instance of a data breach in cloud storage or a faulty
execution of a requested set of computations can lead to
disastrous ramifications for such a business. As it has been
seen in a recent data breach that calls the trust in central
management of online services such as Facebook (an online
social network) into question [7]. Blockchain, on the other
hand, with its premise of immutability, transparency, and peer-
to-peer consensus can provide the means for a trusted audit
of networked systems while at the same time giving much of
the control back to the edges of a network.

A. Contribution of the survey

In this paper we provide a broad ranging survey of the
implications of blockchain on the future of the Internet with a
comprehensive take on their legal and regulatory ramifications
as well. Instead of limiting ourselves to one particular use case
or application (such as the Internet of things (IoT) [8], [9]),
we cover a wide range of use cases and try to observe the
common patterns, differences, technical limitations so that a
more informed decision can be made by someone interested in
deploying a use case from ground up or translating one’s use

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case to a blockchain-based solution. We provide a comparison
of our paper with other recent blockchain-based surveys in
Table I. Apart from encompassing most of the issues covered
by recent survey literature, a clear distinguishing feature of
this paper is that we also discuss a few of the most important
legal and regulatory challenges and ramifications of deploying
a blockchain-based solution. This is particularly important
given the development of new data protection regulations
(such as the advent of the General Data Protection Regulation
(GDPR) in Europe), and regular reports of data breaches and
government mass surveillance stories coming to light.

B. Structure of the survey

The rest of the paper is organized in three main sections.
In the section titled Background (Section II), we provide the
necessary background to understand the big picture of how
blockchain works by introducing distributed ledger technology,
distributed consensus, smart contracts, and public and private
blockchains. In the next section (Section III) titled Blockchain-
based Network Applications, we provide examples of how
blockchain can be used to evolve trust mechanisms for the
decentralized Internet, email, Internet of Things (IoT), content
distribution, distributed cloud storage, online social networks,
cybersecurity, public key infrastructure, and other miscella-
neous applications. Thereafter in the section titled Challenges
and The Road Ahead (Section IV), we discuss the current
challenges facing blockchain and their various technical, legal,
regulatory ramifications: in particular, we discuss governance,
operational, and regulatory issues; scalability issues; security
and privacy concerns; sustainability concerns; anonymity; the
use of artificial intelligence (AI) and machine learning (ML);
and issues related to usability and key management. Finally
the paper is concluded in Section V.

II. BACKGROUND

In this section, we provide the necessary background to un-
derstand what blockchain is and how it works. Our discussion
in this section follows an evolutionary approach which means
we start with Bitcoin [21] (the first incarnation of a blockchain-
based application) and discuss how the technology evolved
giving rise to other concepts and systems along the way.

A. Blockchain and distributed ledger technology (DLT)

The original premise of blockchain is to establish trust
in a peer-to-peer (P2P) network circumventing the need for
any sort of third managing parties. As an example, Bitcoin
introduced a P2P monetary value transfer system where no
bank or any other financial institution is required to make
a value-transfer transaction with anyone else on Bitcoin’s
blockchain network. Such a trust is in the form of verifiable
mathematical evidence (more details on it follow in Section
II-D). The provision of this trust mechanism allows peers of
a P2P network to transact with each other without necessarily
trusting one another. Sometimes this is referred to as the
trustless property of blockchain. This trustlessness further
implies that a party interested in transacting with another

entity on blockchain does not necessarily have to know the
real identity of it. This enables users of a public blockchain
system (see Section II-F for more details on public and private
blockchains), such as Bitcoin, to remain anonymous. Further,
a record of transactions among the peers are stored in a
chain of a series of a data structure called blocks, hence
the name blockchain. Each peer of a blockchain network
maintains a copy of this record. Additionally, a consensus,
taking into consideration the majority of the network peers,
is also established on the state of the blockchain that all the
peers of the network store. That is why, at times, blockchain is
also referred to as distributed ledger technology (DLT). Each
instance of such a DLT, stored at each peer of the network,
gets updated at the same time with no provision for retroactive
mutations in the records.

B. A clever use of hashing

Now we take a closer look at how hashing is used to chain
the blocks containing transaction records together and how
such records are rendered immutable. A hash is defined as a
unidirectional cryptographic function. A hash function usually
takes an arbitrary input of an arbitrary length and outputs a
seemingly random fixed-length string of characters. Each such
output is unique to the input given to this function and can be
considered the footprint for an input. If the input is even so
slightly changed then the output of the hash function almost
always completely changes in a random fashion (there are,
however, rare occasions where a collision occurs when two
distinct inputs to a hash function map to the same output)
[22]. This way hash of a piece of data can be used to check
the integrity of it. As an example Secure Hash Algorithm
256 (SHA256) is a member of the family of SHA2 hash
functions which is currently deployed by many blockchain-
based systems such as Bitcoin1.

A simplified version of a blockchain is shown in Figure 1.
It can be observed that there are four main fields shown in
the figure for each block. In the hash field the hash value of
all the contents (i.e., block number, previous hash, shown as
Prev in Figure 1, and data) is recorded. The most important
field is the Prev field. This field, in each block, contains the
hash value of the block that comes before it. This chains the
blocks together. Now, if the contents of a block are changed
then this change is reflected, in addition to the hash of the
block under consideration, in the portion of the blockchain that
comes after the block being mutated. This way, hashing and
the distribution of blockchain copies among the peers of a P2P
network makes the records stored in a blockchain immutable.
It can be noted in Figure 1 that the first block in a blockchain
is sometimes referred to as the genesis block indicated by its
Prev field initialized to contain all zeros.

C. A coin: Transaction chain

A transaction chain is shown in Figure 2. It should be
observed here that there is a difference between a transaction

1https://web.archive.org/web/20130526224224/http://csrc.nist.gov/groups/
STM/cavp/documents/shs/sha256-384-512

https://web.archive.org/web/20130526224224/http://csrc.nist.gov/groups/STM/cavp/documents/shs/sha256-384-512

Papers/Books
(Author) Year

Blockchain
Fundamentals

Challenges
Smart
Contracts

Blockchain
Applications

Future
Trends

IoT Blockchain
Types

Blockchain
Characteristics

Consensus
Algorithms

RegulatoryIssues

Zibin et al. [10] 2016 7 7 7 7

Ye et al. [11] 2016 7 7 7 7 7 7

Jesse et al. [12] 2016 7 7 7 7 7 7

Marc et al. [13] 2016 7 7 7 7 7

Michael et al. [14] 2017 7 7 7 7 7

Zibin et al. [15] 2017 7 7 7 7

Iuon-Chang et al. [16] 2017 7 7 7 7

Mahdi et al. [17] 2018 7 7 7 7 7

Yong et al. [18] 2018 7 7 7 7 7 7 7 7

Muhammad et al. [8] 2018 7 7 7

Karl et al. [19] 2018 7 7 7 7 7

Salah et al. [20] 2019 7 7 7

Our Survey 2019 (distinguishing feature)

TABLE I: Comparative analysis of our survey with the existing survey literature pool

Block# 1

Prev: 00000000000000 …

Hash: 0a2a55b65844af …

Data:

Block# 2

Prev: 0a2a55b65844af …

Hash: 72722cedc7f7d1 …

Data:

Block# 3

Prev: 72722cedc7f7d1 …

Hash: 6540ea9f539f54 …

Data:

Block header

Genesis block

Fig. 1: Hashing chains the blocks together and renders them immutable

chain and a blockchain. Each block in a blockchain can contain
multiple transaction chains. Each transaction chain in turn
shows the value transferred from one peer of the network
to another. Each such transaction chain is also sometimes
referred to as a digital coin or more generally as a token (as
an example Ethereum, discussed later, allows one to define a
custom token2).

D. Distributed consensus

Blockchain systems, such as Bitcoin and Ethereum3, make
use of different consensus engines. These engines enable peers
of the blockchain network to have a say about the overall state
of the records stored in the blocks of a blockchain network. In
this section, we discuss the most popular and widely adopted
consensus protocol called Proof-of-Work (PoW). We also
briefly discuss Proof-of-Stake (PoS) and Proof-of-Authority
(PoA) based consensus engines, which are mostly in devel-
opment phases but are poised to eventually replace the PoW-
based energy-intensive consensus mechanism.

1) Proof-of-Work (PoW): PoW-based consensus mechanism
was mainly popularized by Bitcoin [21]. PoW’s main goal is
to prevent double spending of a digital asset by providing
a verifiable trust guarantee to a payee. Such a guarantee is

2https://www.ethereum.org/token
3https://ethereum.org/

provided in the form of publishing an integer called a nonce.
Finding a nonce is a computationally intensive process and is
often referred to as mining. The peer of a blockchain network
that finds a nonce is called a miner. Specifically, a nonce is
an integer which, when hashed together with the contents
of a block, outputs a hash matching a predefined pattern.
Depending upon the underlying system, such a pattern is
usually defined to start with a predefined number of zeros. The
larger the number of leading zeros the harder (in computational
terms) it is to find a nonce that hashes to match such a
pattern. Any peer node of a blockchain network can perform
mining (i.e., collecting a set of transactions and finding the
relevant nonce). PoW is a lottery-based consensus mechanism,
which implies that in a given large network, the peer who
finds a nonce at a given time is decided randomly. Once a
miner finds a nonce (or mines a block), the network awards
such a node with a set number of cryptocurrency tokens
(such as bitcoins). This is how cryptocurrency is minted in
cryptocurrency networks and is put into circulation in such
networks.

As the mining process involves randomness, which implies
that it is computationally very hard for an attacker to tamper
with the stored data in blockchain if the majority of a network
(in terms of computational resources) is honest. However, if an
adversary (or a group of adversaries) gains more computational
power than the honest portion of the network then it can

https://www.ethereum.org/token

https://ethereum.org/

Owner 1’s SK

Hash

Owner 0’s signature

Hash of owner 2’s PK

Owner 1’s signature

Hash of owner 3’s PK

Owner 2’s signature

Hash

Verify

Transaction chain (a coin)

Hash

Owner 2’s SK Owner 3’s SK

Hash of owner 1’s PK

Sign Sign

Verify

Fig. 2: Transaction chain or a coin. Figure adapted from [21]

potentially alter the records stored in a blockchain. Such an
attack is sometimes referred to as a 51% attack. Figure 3 shows
a chain of blocks with an extra field labeled as nonce. It should
be noted in this figure that the hash of all the blocks (apart
from the genesis block) starts from a set number of zeros.

2) Proof-of-Stake: Blockchain-based systems, particularly
Ethereum4, are considering to eventually shift to PoS from
PoW. This is because of high computation, and in turn high
energy, costs associated with finding a nonce through mining.

In the PoS mechanism, the nodes with the largest stake (in
monetary terms) in the underlying network will have a greater
say when it comes to proposing a block to be appended to
a blockchain. The monetary worth owned by such nodes is
put at stake in order for them to behave honestly. Since PoS
is still in its development phase, it does come with its fair
share of issues. Most notable is the mismatch between the
actual interest of nodes with the same stake in the underlying
network5.

A variant of PoS is called Proof-of-Authority (PoA) which
is mostly being used by the test networks mainly for experi-
mentation (such as Rinkeby and Ropsten Ethereum networks).
The idea of PoA is quite similar to PoS: in PoA it is the
identity of nodes that is put at stake instead of the monetary
value owned by the nodes. This implies that PoA is mostly
used to establish permissioned blockchains (see Section II-F)
where the identities of the peer nodes are known and they are
given specific permissions to mine new blocks.

E. Smart contracts

One important aspect of blockchains is its use in enabling
smart contracts [23]. Smart contracts can simply be viewed as
algorithmic enforcement of an agreement among, often, mutu-
ally non-trusting entities. More technically, a smart contract is
a program that executes on blockchain in a distributed manner
and possesses unique identification. It contains functions and
state variables. These functions receive input parameters of
the contract and get invoked when transactions are made.

4https://github.com/ethereum/wiki/wiki/Proof-of-Stake-FAQs
5https://tinyurl.com/poa-network

The values of state variables are dependent on the logic
developed in the functions [24]. These functions are normally
written in high-level languages (such as Solidity or Python)
[25]. Compilers convert these programs into bytecode that is
then deployed on blockchain network. These programs are
invoked by receiving the user transactions for these smart
contracts, which are posted by any user on the blockchain
network [24]. Smart contracts help automate an arbitrary value
transfer system in an immutable manner where conditional
transactions are recorded, executed, and distributed across the
blockchain network. These contracts not only reduce the legal
and enforcement costs but also largely rule out the need for
central trusted or regulating authority [26]. Smart contracts can
create an environment of trust among the members of several
contrasting and diverse communities [3].

Ethereum6: is the signature project that introduced and
popularized the concept of smart contracts [27] [28]. It is
an open-source, blockchain-based platform that enables one
to develop and execute decentralized applications. One of
Ethereum’s goals is to ease the process of developing the
decentralized applications called dApps [29] [30]. Ethereum
can be considered as the next step, after Bitcoin, in the
evolution of blockchain-based systems. Before Ethereum, most
of the blockchain-based system, mainly cryptocurrency-based
projects, revolved around expanding on Bitcoin’s core protocol
and focusing on one specific application. Ethereum, however,
generalizes and allows multiple such projects to coexist on a
broader underlying blockchain-based compute resource.

Operations on Ethereum are performed by utilizing the
Ethereum Virtual Machine (EVM). EVM is the implemen-
tation of the Ethereum protocol responsible for handling state
transitions and carrying out computation tasks [31]. EVM
provides the runtime environment for the execution of smart
contracts [24]. The EVM generated binary comprises smart
contracts’ (usually written in high-level languages such as
Solidity7 or Serpent8) opcode that gets deployed on the
underlying blockchain.

6https://ethereum.org/
7https://solidity.readthedocs.io/en/develop/
8https://github.com/ethereum/wiki/wiki/Serpent

https://github.com/ethereum/wiki/wiki/Proof-of-Stake-FAQs

https://tinyurl.com/poa-network

https://solidity.readthedocs.io/en/develop/

https://github.com/ethereum/wiki/wiki/Serpent

Block# 1
Prev: 00000000000000 …

Hash: 000005b65844af …

Data:
Block# 2

Prev: 000005b65844af …

Hash: 00000cedc7f7d1 …

Data:
Block# 3

Prev: 00000cedc7f7d1 …

Hash: 00000a9f539f54 …

Data:

Nonce: 485
Block header

Mined blocks

Nonce: 32154 Nonce: 9875

Note: The fields of Nonce, Prev, and Hash contain arbitrary values

Fig. 3: Mined blocks in a blockchain. Hash in each block now starts with five zeros.

F. Public and private blockchains

The underlying blockchains of Bitcoin, Ethereum and, in
general, of most cryptocurrencies are open and public. This
implies that anyone can join the blockchain network and
transact with any other peer of the network. Moreover, such
networks also encourage peers to stay anonymous. As an
example in Bitcoin’s network, peers are assigned addresses
based on the hash of their public keys instead of based on
their actual identities.

On the other hand, there are permissioned and private
variants of blockchains as well. This concept was particularly
popularized by Linux Foundation’s Hyperledger Fabric (HLF)
platform 9. This platform is proposed for business use cases
where, in addition to data immutability and P2P consensus,
transaction confidentiality is also required. Permissioned and
private blockchain platforms such as HLF usually deploy a
cryptographic membership service on top of their blockchain’s
immutable record keeping. Each peer in such a network can be
uniquely identified based on its real-world identity. Proof-of-
Authority (as discussed earlier) functions on the same principle
of permissioned and private blockchains.

III. BLOCKCHAIN-BASED NETWORK APPLICATIONS

Other than cryptocurrencies, blockchain finds its applica-
tions in various other fields, particularly those that require
more transparency and trust in their record-keeping. Some
blockchain-based network applications with their platforms are
shown in Fig. 4.

A. The Decentralized Internet

The Internet has enabled the evolution of a number of
applications such as mobile health, education, e-commerce,
online social systems, and digital financial services. However
many parts of the world are still deprived of the Internet’s
boons due to the existence of a digital divide [52]–[55].
Moreover, the existing Internet infrastructure is predominantly
centralized creating monopolies in the provision of services to

9https://hyperledger-fabric.readthedocs.io/en/release-1.3/blockchain.html

TABLE II Timeline: Evolution of Blockchain

2018 • Blockchains potential got revamped by
more investments in wide range of use
cases [32]

2017 • Seven European banks, announced their
program to develop a blockchain-based
trade finance platform in collaboration
with IBM [33]

2016 • Ethereum DAO code was compromised
and hacked [34], Emergence of
permissioned blockchain solutions [10]

2015 • Blockchain trial was initiated by NASDAQ
[35], Hyperledger project was started [36]

2014 • With crowdfunding the Ethereum Project
was started [37], Ethereum genesis block
was created [38], [39]

2013 • Ethereum, a blockchain-based distributed
computing platform was proposed [40]

2012 • Coinbase, started as brokerage for Bitcoin
[41]

2011 • Silk Road launched with Bitcoin as
payment method [42], BitPay first
Blockchain-based wallet [43], Emergence
of other cryptocurrencies like Swiftcoin
[44]–[46], Litecoin [47]

2010 • First Bitcoin cryptocurrency exchange Mt.
Gox started working [48], [49]

2009 • First Bitcoin block was created [50], [51]
2008 • Bitcoin’s whitepaper was published by

Satoshi [21]

its users [56], [57]. Distributed denial of service (DDoS) at-
tacks on DNS servers10, certificate authority compromises (as
mentioned in Section III-H), cybersecurity-related incidents
[58]–[60] and similar other service disruptions are rife mainly
because of the largely centralized nature of the current Internet
and the services that it provides [61]. Whereas, the decentral-
ized approach to the online service provisioning gives more
control to the users (or the edges of the Internet) and ensures
fair participation and sharing of the resources. It is believed
that decentralization of the communication infrastructure may

10https://www.wired.com/2016/10/internet-outage-ddos-dns-dyn

https://hyperledger-fabric.readthedocs.io/en/release-1.3/blockchain.html

Fig. 4: Examples of blockchain-based network applications and their solutions

bridge the gap of the digital divide and make the Internet
services reachable to the remaining unconnected portion of
the planet [62].

In this section, we try to re-imagine different components of
the Internet through the perspective of Blockchain’s premise
of decentralization and distributed trust.

1) Decentralized naming systems: Domain name system
(DNS) is an example of online namespace system. Its primary
goal is to resolve each unique hostname to an IP address(es)
and vice versa. Presently, the largely centralized nature of
DNS raises the odds for single-point failures and makes such
systems prone to malpractice and malicious activities by the
main stakeholders and governments. In the past, the seizure of
hundreds of domain names by governments or the regulatory
institutions have turned scientists, activists, and enthusiasts
to think about possible alternatives to this largely centralized
namespace system [63]–[67].

Most applications place a demand for a namespace system
that can ensure security during the provision of such identi-
fiers. Blockchain can enable a namespace system by making
use of global, tamper-resistant, and append-only ledgers and
thereby guarantee the integrity, availability, uniqueness, and
security of name-value pairs. While some challenges remain to
be solved, the blockchain technology can successfully provide
the essential basis for the construction and governance of
secure and distributed naming services [68]. Such blockchain-
based networks further encourage the inclusion of honest net-
work peers since for a sufficiently large blockchain network, it
becomes very difficult and costly for the adversarial elements
to tinker with the blockchain records [69].

In 2011, an experimental open-source startup called Name-
coin came into being providing distributed DNS services based
on blockchain technology with the aim of improved security

mechanism, decentralization, confidentiality, and agility [70],
[71]. Namecoin is designed to work on top of a blockchain
and as an alternative to the existing conventional DNS root
servers for the storage of registered domain names [69]. Being
a blockchain-based system (with secretly held private keys
corresponding to the registered domain names) it is immune
to censorship or seizure of the registered domain name ac-
counts. Similarly, any change in domain names, recorded on
a blockchain, requires proof-of-work by the longest chain of
honest network peers (see Section II-D1 for details), which in
turn is in control of the highest computing pool [67], [72].

Another blockchain-based namespace system called Block-
stack, inspired by the Namecoin network, improves upon
various performance limitations of Namecoin (for a detailed
analysis of Namecoin, please see [67]) most importantly
security and scalability [69]. The aspect of security was partic-
ularly improved by Blockstack by migrating from Namecoin’s
blockchain to Bitcoin’s larger blockchain. The reason being
the bigger size of Bitcoin’s network, which makes it harder
(as compared to Namecoin’s relatively smaller network) for a
51% attack [73] (see Section II-D1). One of the distinguishing
features of Blockstack system is the introduction of a virtu-
alchain [74]. Virtualchain is a logical overlay layer that sits on
top of a production blockchain such as Bitcoin. Virtualchain
eases the process of modifying the underlying blockchain
without requiring actual consensus-breaking changes to it.
Blockstack system facilitates users to register unique human-
readable usernames and employs the distributed PKI system
to bind user identities with arbitrary data values. This new
registration system thus functions without the requirement of
any centrally trusted third party [61], [69]. Blockstack enables
users to own and control their data and access to this data at
all times.

2) Routing in the decentralized Internet: The interoper-
ability of many still distinct (and largely isolated and self
contained) blockhain networks will pose a problem in future
if they are to come together to enable a wide-spread adoption
of blockchain-powered decentralized web. There is a need
for a routing mechanism that can take into account different
characteristics of different blockchain networks and route a
transaction from one network to a potentially different one
and back. The main problem in inter-blockchain network
routing is of verification of blockchain records among different
blockchain networks and the provision of communication be-
tween any two peers belonging to any two distinct blockchain
networks. In a single network this problem gets trivial with
all the peers agreeing to follow the same consensus protocol
(for example PoW). The motivation to enable interoperability
among different blockchain networks can be taken from the
concept of a lightweight client of a blockchain network. Such
clients are able to verify the existence of a record of a
transaction in a blockchain network without downloading the
entire bulk of blockchain data. The lightweight clients do so by
making use of a technique called Simple Payment Verficiation
(SPV)11 [21] which allows a client to verify the existence of
a transaction record only by downloading the, comparatively
lightweight, block headers, in the form of a Merkle branch, in
comparison to the entire blockchain data. Following a similar
principle, Blocknet12 proposes a solution for inter-blockchain
routing infrastructure [75]. Blocknet achieves interoperability
by making use of two main components namely XBridge and
XRouter. XBridge is responsible for implementing the ex-
change functionality which implies enabling of atomic swaps
of tokens between two blockchains. XRouter on the other
hand implements communication functionality and in unison
with XBridge and making use of SPV a transaction can
then be performed between two peers belonging to different
blockchain networks.

Another project that proposes a solution to enable cross-
ledger payments is called Interledger13 [76]. Interledger
presents the concept of connectors that act as decentralized
exchanges between two distinct blockchain ledgers and route
transactions (or packets of money as per Interledger’s vernac-
ular). Interledger takes its inspiration from IP routing and
instead of IP addresses it makes use of an ILP (Interledger
packet) address. ILP packets differ from the best-effort IP
routing in the way that ILP packets can not be lost or stolen
since in the case of ILP, funds with real monetary value are
transferred instead of data. This is achieved by making use
of Hashed Timelock Agreements (HTLA)14 in combination
with SPV to settle cross ledger payment claims. HTLAs work
across the ledgers and enable conditional transfers. Conditional
transfers involve a preparation step whereby a transfer is first
prepared which implies that a sender’s funds are put on hold
by a ledger’s contract until a condition is met which manifests
itself in the form of a digest of a cryptographic hash function.
Its incumbent on a recipient to present this digest in the form

11http://docs.electrum.org/en/latest/spv.html
12https://blocknet.co/
13https://interledger.org/
14https://interledger.org/rfcs/0022-hashed-timelock-agreements/

of a preimage within a certain time window. If the time expires
the funds are automatically released to the sender. This way,
by making use of HTLAs the funds can not be lost in transit.

In conclusion, we see the problem of blockchain interop-
erability as akin to the Border Gateway Protocol’s (BGP)
routing problem where different Autonomous Systems (ASes)
interoperate with each other with a mutually agreed upon
control plane information. In our opinion these two problems
seem to fit well together. Both domains (i.e., BGP routing
and blockchain interoperability) can motivate solutions in each
other. As an example, in our opinion, it would be beneficial if
BGP attributes such as AS prefixes with corresponding control
plane information (such as peering agreements) are stored
in an immutable manner in a blocckhain-based database for
routing checks. There will, however, be scalability and latency
concerns as a blockchain’s transaction rate must keep up with
the dynamic nature of the changing network topologies in
different ASes. Still, storage of network topological graphs
with peering agreements will create an opportunity for a more
trusted, transparent, and auditable routing decisions with a
lesser chance for censorship and collusion.

B. Decentralized Email

Today, electronic mail (email) is a common form of com-
munication among many that usually consists of a mail client
and an associated server. There are various protocols such
as SMTP, ESMTP, POP, and IMAP for formatting, process-
ing, delivering, and displaying email messages by ensuring
interoperability among different mail clients and servers. The
security of an email system relies on a continuous process of
planning and management. Email messages pass through the
non-trusted external networks that are often beyond the control
of an email provider’s security system. These email messages,
without appropriate security safeguards, can potentially be
read, modified, and copied at any point along their path [77].
Melissa, Sasser worm and other embedded hyperlinks and
viruses have damaged millions of computers and their data
[78]. Email solutions (such as Yahoo) have suffered from data
breaches in the past and have resultantly urged their users to
change their password keys [79]. In order to improve on these
centralized email systems to better safeguard the users’ private
and sensitive information, a radical change in the underlying
technology seems imperative.

One of the solutions to address the vulnerabilities of the
email system described above can be in the form of a
blockchain-powered decentralized and distributed email sys-
tem. Email addresses, in a similar way to DNS address
assignment as discussed in the last section, can be assigned
to the users over blockchain technology. In this system,
there is no centralized controlling server in order to gain
access to personal data and records. Most importantly, email
communication using blockchain technology is not under the
influence of government authorities that could exploit the
centralized email providers such as ISPs and technology giants
such as Google, Amazon, and Facebook, etc. John McAfee
Swiftmail15 is a blockchain-based email solution with 256-bit

15http://johnmcafeeswiftmail.com/

http://docs.electrum.org/en/latest/spv.html

https://blocknet.co/

https://interledger.org/

https://interledger.org/rfcs/0022-hashed-timelock-agreements/

end-to-end encryption for the protection of data. CryptaMail

is another blockchain-based email service that claims 100%
security based on the decentralized system without third party
involvement. Gmelius blockchain architecture is a hybrid
system that offers a scalable and cost-effective framework that
anchors email associated data into the Ethereum [80].

C. Blockchain for the Internet-of-Things (IoT)

The Internet of Things (IoT) broadly speaking is a network
of everyday objects in which the IoT devices capture or
generate enormous amounts of data and send it over the
network [81]. This interconnection of a large number of
IoT devices is known to cause many privacy and security
issues [82]–[85]. The IoT-based social, such as health-related,
applications often end up monitoring and collecting sensitive
personal information. When such information is exposed to
third parties, such as health-care providers, the prospects of
inadvertent or malicious privacy compromises become highly
probable [86]. Compliance with the privacy and security rules
and policies for a particular application is a significant chal-
lenge in IoT-based systems [87]. In such systems, blockchain-
based solutions can help in addressing the issues related to
security and privacy. Besides the by-design existence of some
implementation constraints of energy, delay, and computation
overhead in IoT devices, businesses have started initiatives
to use blockchain into their various domains such as in
production and supply chain management [88], [89]. For
example, the IBM Watson IoT platform17 empowers the users
to put their data on blockchain ledgers, which can later be
used in shared transactions among different members of an
IoT-related business consortium. This way members of such
consortium can take part in verifying transactions against IoT
data, dispute resolution, and accountability mechanism in a
trusted, transparent, and mutually agreed upon manner. The
data collected from devices in an IoT network is formatted
into such API formats that are understandable to blockchain
smart contracts. The IBM Watson IoT platform enables a
business solution to manage, analyze, and customize IoT
data, according to a pre-agreed policy, to be shared among
permissioned clients, members, and smart contracts [88].

The importance of IoT can be gauged by observing the man-
ufacturing industry, which is increasingly adopting IoT-based
solutions for machine diagnostics, manufacturing automation,
and health management of industrial machines [24]. Cloud-
powered manufacturing systems along with IoT technology
help in the provisioning of manufacturing resources to the
clients as per the existing demand. This usually requires the
involvement of a centrally trusted third party. A blockchain-
based platform called Blockchain Platform for Industrial In-
ternet of Things (BPIIoT) is a trustless P2P network where the
exchange of services may take place without the need for a
central trusted third party [24]. BPIIoT provides a platform for
the development of dApps pertaining to P2P manufacturing
applications. BPIIoT improves on a similar project called

16http://www.cryptamail.com/
17ibm.co/2rJWCPC

Slock.it18, according to the authors of [24], being generic in
terms of dApp development. BPIIoT’s platform consists of a
single-board computer that provides a bridge to both cloud and
blockchain services. BPIIoT enables customer-to-machine and
machine-to-machine transactions without the involvement of
third parties. For more details on the applications of blockchain
for the Internet of things (IoT), the interested readers are
referred to a comprehensive survey on this topic [8].

Another IoT project, managed by IBM in collaboration with
Samsung, is the blockchain-powered and Ethereum-based Au-
tonomous Decentralized Peer-to-Peer Telemetry (ADEPT) sys-
tem. Ethereum is a blockchain-based generalized technology
that can be considered as the compute framework for trustful
messaging. Contracts authored under this framework endorse
the rules designed for interaction between network nodes and
thus are considered more secure. It also provides developers
with a platform for building applications integrated with the
Ethereum message passing framework [27]. ADEPT realizes a
decentralized IoT solution by following the three principles: i)
P2P messaging, ii) distributed file sharing, and iii) autonomous
coordination among the devices of IoT network. ADEPT
makes use of Telehash (an encrypted mesh networking pro-
tocol)19, BitTorrent, and Ethereum respectively to realize the
three principles just described. Ethereum’s blockchain enables
device owners of ADEPT’s IoT network to automate rules
of engagement, the registration and authentication processes,
and interactions among themselves in a decentralized and
trusted manner. This can be achieved in one of two ways
namely: i) proximity-based: taking into consideration physical,
temporal or social distance and ii) consensus-based: taking into
consideration selection, validation, or blacklisting criterion
[128]–[130].

Among other works is Filament, a blockchain-based tech-
nology stack that enables IoT devices to discover, register,
manage, and communicate in a decentralized manner [131],
[132]. In [133], a system named modum.io20 has been pre-
sented, which utilizes blockchain-based IoT devices to ensure
the immutability of the transactions related to physical prod-
ucts and facilitates in the regularization of the supply-chain
management process in the various fields.

D. Blockchain-based Content Distribution

Content distribution networks (CDNs) are an effective ap-
proach to improve Internet service quality by replicating the
content at different strategic geographic locations in the form
of data centers. Users can request and access data from
the closest replica server instead of always fetching it from
the data-originating server. Generally, large companies such
as Netflix and Google’s YouTube service, have their own
dedicated CDNs, while smaller organizations can rent CDN
space from other companies like Akamai. BitTorrent is a
P2P content distribution protocol that enables the propagation
of data using networks of computers for downloading and

18https://slock.it/landing.html
19http://telehash.org
20https://modum.io

ibm.co/2rJWCPC

https://slock.it/landing.html

http://telehash.org

https://modum.io

Scope Example(s) Description

Cryptocurrency
Bitcoin, Bcash, Iota, OmiseGO,
Litecoin, Ripple, Dash, Zcash, Monero

Decentralized peer-to-peer electronic cash system for online payments.

Smart Contract Ethereum [27], Ripple [23]
Occurrence of certain events triggers transfers of different things, i.e., security deposit payment,
saving wallets, decentralized gambling, wills etc.

Cloud Services Abuse Prevention [90] Defence to stop attacks and service abuses in cloud computing applications.
Message Exchange Bitmessage [91] Secure system to send and receive messages.
Identity and Privacy ChainAnchor [92] Trusted, privacy-preserving, identity management system.
Voting System Electronic Vote [93] Electronic vote transaction system for a voter to spend the vote in favor of one or more candidate recipients.
Digital Content Content Distribution [94] Decentralized and peer-to-peer digital content management system with rights management mechanism.
Health Patient Data [95] Patient data sharing system based on blockchain technology.
Transportation Vehicle Communication [96] Secure vehicle to vehicle communication system.
Agriculture ICT E-Agriculture [97] Distributed ledger system to safeguarded transparent data management.
Software Software Connector [98] Software components states sharing system without trusting a central integration point.
Micro Finance Stellar [99] Creates services and financial products using blockchain architecture.
E-Commerce OpenBazaar [100] Provides trading platform for users where they can make free transactions among themselves.
Mobile Banking Atlas [101] Atlas provides platform for mobile banking and connects world communities through it.
Storage Sia [102] A cloud storage platforms, enables anyone to make money.
DNS Namecoin [103] A blockchain-based domain name system.
Document Management Blockcerts [104] Issue and verify certificates for academic, professional, workforce and civic records.

Storage
BigchainDB, MaidSafe,
Filecoin [105] [106] [107]

Scalable storage which supports diverse applications, platforms, industries and use cases.

Business and Economy IBM Blockchain Platform [108] Integrated platform designed for creation and acceleration of blockchain based businesses.
Internet of Things (IoT) IBM Watson IoT [109] Accountability and security in blockchain-based internet of things.

TABLE III: Examples of blockchain-based applications

Scope Startups Description
IoT and Economics Chronicled [110] Provides trusted data, ensures data provenence of IoT devices and helps in business process automation
Security and Intelligence Elliptic [111] Necessary intelligence information to security agencies and financial departments.
Data Security LuxTrust [112] Provides security to customer’s electronic data and digital identity.
Regulatory Compliance GuardTime [113] Data protection regulatory compliance software.
Financial Augur [114] A market forecasting tool to increase profitability.
Transportation Lazooz [115] Real-time ridesharing services.
Property Records Ubiquity [116] Provide service for secure ownership record of property.
Process Compliance Startumn [117] Ensures process integrity and improves regulatory compliance.
Music Mycelia [118] Music industry online services.
Asset Management Gem [119] Secure identification of assets.
Data Security Tieriom [120] Data protection service.
Tracking and Ownership Provenance [121] Maintain digital history of things.
Music Ujo Music [122] An online music store.
Smart Contracts SkuChain [123] Offers services like: Smart contracts, provenance of things, Inventory Management.
Storage Storj [124] A distributed storage platform.
E-commerce Gyft [125] An online gift transfer platform.
Firearms BlockSafe [126] A secure and privacy enabled firearm solution.
Health and Environment BitGive [127] By using blockchain technology it works for the improvement of public health and environment worldwide.

TABLE IV: Examples of blockchain-based startups

uploading simultaneously without a central server [134]. Bit-
Torrent’s network consists of a large number of peers, which
complicates the task of traffic management. The other major
issue with the current CDNs is that the content creators receive
an inadequate share of the revenue, especially in digital content
distribution sector [135]. Similarly, the media sector is also
significantly suffering because the content can be easily copied
and distributed.

Blockchain technology can be the solution with the neces-
sary ingredients to significantly resolve the challenges related
to content distribution. It can stabilize the rights management
related issues for studios and artists by providing a better way
of content control. This can enable a more agile method for
content delivery with a more trusted, autonomous, and intel-
ligent network. In a blockchain-based CDN, the participants
can independently verify a record and its origin without the
need for a centralized authority for verification. Blockchain
can store all the record related to the content (e.g., its origin),
and share over the network in an immutable form along with
the provision of enabling a monetization system to empower
the content creators.

DECENT21, as an example, is a blockchain-based CDN that
provides secure content distribution and maintains the reputa-
tions of the content creator with a mechanism for the payment
between authors and client nodes also in place. Content (e.g.,
ebooks, videos, and audio) is released cryptographically over
the global DECENT network and other nodes can then pur-
chase them with DECENT tokens. SingularDTV22 is a media
industry initiative in which an Ethereum-based entertainment
studio is developed that can enable rights management as well
as P2P distribution to empower artists and creators.

E. Distributed Cloud Storage

Today, consumers and enterprises face the storage and
management problems caused by an ever-increasing volume
of data on non-volatile data storage systems. Despite the
popularity of cloud storage solutions (such as Dropbox and
Google Drive), the control, security, and privacy of data remain
major concerns [136]. It is largely due to the current model
being adopted by the cloud storage systems that often puts

21https://decent.ch/
22https://singulardtv.com/

https://singulardtv.com/

them under a centralized institutional authority. In this model,
data is transferred over TCP/IP from a client to the host
servers in the legacy client-server model [137]. The infor-
mation thieves, censorship agencies and spies can potentially
tamper with or copy the stored confidential files from hosting
servers through technological means, legal tactics and political
strategies [138]–[142].

Such problems, mostly caused by central and identifiable
points in the current cloud storage systems23, can potentially
be solved using decentralization and (transparent and trusted
execution in the form of) automation based on a trust agree-
ment between a client and a host service provider. There
exist some storage solutions such as MaidSafe24 and Tornet25

that outline possible alternatives for a decentralized cloud, but
security, scalability, and cost efficiency of these solutions still
remain in question. Therefore, a cloud storage system with
trusted and verifiable security guarantees, high redundancy,
and scalability, is required that should be economically viable
while being practical at the same time. Blockchain-based cloud
storage solutions inherit characteristics such as decentraliza-
tion, anonymity, and trusted execution of transactions among
the members of a trust agreement and can pave the way for a
verifiable and trusted cloud computing era.

Storj26 is a blockchain-based P2P distributed data storage
platform that enables users to tailor their data sharing and
storage as per individual agreements with other network
peers and the third party service providers. Entities can earn
cryptocurrency-based micro-payments by sharing the unused
disk space and Internet bandwidth of their computing devices.
In the context of distributed cloud, Dong et al. [143] pro-
posed a game-theoretic, smart-contract-based verifiable cloud-
computing framework. This enables the clients to analyze
collusion between two different clouds by making them per-
form the same computing task. In this framework, the users
use smart contracts to simulate distrust, tension, and betrayal
between the clouds to detect, and in turn, avoid cheating
and collusion. Similarly, Sia27 is another blockchain-based
cloud storage platform. Sia platform automates trusted service
level agreements (SLAs) between a user and storage provider
using smart contracts. It is an open source platform that splits
users’ data into encrypted fragments and distributes them
across a P2P network that increases network resilience and
reduces downtime. Unlike the traditional storage solutions, the
data in this scheme becomes more secure in the sense that
one can only access this data if in possession of associated
cryptographic keys. Another important work is Filecoin [144].
Filecoin realizes the concept of distributed storage network in
terms of an algorithmic marketplace for storage. Filecoin is
built as an incentive layer on top of another distributed file
system called Inter-Planetary File System (IPFS). The miners
in Filecoin host the storage space with the mining capability
determined by the storage capacity a miner possesses. Filecoin
enables verifiable markets, which dictates how and where data

23https://newsroom.fb.com/news/2018/09/security-update/
24https://maidsafe.net
25https://github.com/bytemaster/tornet
26https://storj.io
27https://sia.tech

is written to and read from. Each read/write transaction is
powered by the underlying cryptocurrency called Filecoin.

F. Applications in Online Social Networks

The engagement of people with online social networks
(OSNs) has increased greatly in recent years [145]. Users often
put trust in these OSNs and share their personal details with
their online social community. Privacy and security concerns
however still remain an issue with many OSNs. Any breach of
trust has the potential to detriment a user’s virtual and, often
in turn, real-world identities [146]. As an example, in one of
the biggest data breaches28, a data firm named Cambridge An-
alytica got the access to personal information of more than 50
million Facebook (an online social network) users during 2016
US presidential campaign. The firm provided software tools
to analyze/predict American voters’ behavior/personalities and
influenced their choices of the ballot29.

Decentralization, transparency, and P2P consensus gives
blockchain the potential to address most of these aforemen-
tioned security and privacy concerns prevalent in OSNs [147].
As an example, a blockchain-based social media platform
named “Steem”30 gives online community an opportunity to
have a say on the nature of the content that gets popular on
a social network. Steem enables users to earn rewards on
the basis of votes received by the community against their
contributions [148]. This encourages an honest participation
of community peers in maintaining the quality of the overall
network. Such OSN systems can further be made self-healing
by a blockchain-based “reputation system”, such as the one
proposed by Dennis et al. [149]. This system keeps records
of users’ reputation based on their transaction history. In our
opinion, such techniques, while not being free of some ethical
concerns, greatly reduce the snooping and policing by the
centralized authorities such as governments31.

G. Cybersecurity

A study on cybercrime [150] conducted on some organi-
zations, says that information loss remained the major cost
component and increased from 35% in 2015 to 43% in 2017.
Blockchains in particular can be a costly target for cyberattacks
[151], [152]. As an example, DDoS attacks on a blockchain
system can take the form of flooding the network with small
transactions. Still such transactions must be paid for (in the
units of gas) in order for them to be confirmed by the network
[151]. The operations that require very (disproportionately)
low gas costs are vulnerable to exploitation by attacker32.
However, when it comes to the execution of smart contracts
then there is a large attack surface area. This is because, often,
a set of smart contracts is deployed to automate an application
with all of its members working in unison. If one member of

28https://www.nytimes.com/2018/03/19/technology/facebook-cambridge-
analytica-explained.html

29https://www.theguardian.com/news/2018/mar/17/cambridge-analytica-
facebook-influence-us-election

30https://steem.io
31https://www.theguardian.com/world/2013/jun/06/us-tech-giants-nsa-data
32https://www.coindesk.com/so-ethereums-blockchain-is-still-under-attack/

https://newsroom.fb.com/news/2018/09/security-update/

https://maidsafe.net

https://github.com/bytemaster/tornet

https://storj.io

https://sia.tech

https://www.theguardian.com/world/2013/jun/06/us-tech-giants-nsa-data

https://www.coindesk.com/so-ethereums-blockchain-is-still-under-attack/

such a set malfunctions it can then trigger a domino effect
rendering the whole set malfunction [151]. As an example an
ambitious ethereum-based project implemented called decen-
tralized autonomous organization (DAO) got hacked resulting
in the theft of about 60M Ether33. Such attacks can further
be avoided by providing further trust guarantees for the code
and logic of the smart contract itself. For instance, Tezos
proposes the concept of a self amending ledger and to make
the deployment of a smart contract more trusted it provides
formal proofs of the code of a smart contract in order to secure
the trust of all the parties interested in the execution of this
smart contact [153].

Based on blockchain technology, REMME34 is a password
authentication system for safeguarding the confidential cre-
dential information from cyberattacks and at the same time
disregarding the need to remember passwords [154].

Estonian cryptographer Ahto Buldas co-founded an infor-
mation security company named Guardtime35 in 2007. This
company has been working to secure sensitive records using
blockchain technology. The company has designed a Keyless
Signature Infrastructure (KSI) [155] against the commonly
used Public Key Infrastructure (PKI). In this new infrastruc-
ture, centralized Certificate Authority (CA) uses asymmetric
encryption and manages public keys. Thus helping in reducing
the risk of informational asset loss from cybersecurity-related
incidents.

Obsidian36 is also blockchain technology based platform
for secure message exchange without any provisioning of
centralized management mechanism. In this system, the meta-
data about the undergoing communications is spread out in
distributed ledgers and cannot be collected at centralized lo-
cations. Hence, in the context of cybersecurity, it decreases the
chance of surveillance or tracking and in this way addresses
privacy issues [156].

H. Public Key Infrastructure (PKI): Certificate Authority (CA)

Public Key Infrastructure (PKI) establishes a link between
identities like domain names to a cryptographic public key
with help of certificates [157]. Among traditional approaches
to PKIs, the most common choice is the use of Certificate
Authority (CA) that serves as a trusted third party and manages
the distribution digital certificates over the network. This
creates a single point of failure in such PKIs in practice
[158]. There have been many incidents when these centralized
CA’s have been compromised—e.g., the DigiNotar attack:
531 fraudulent certificates issued [159] [160]; Trustwave’s
issuance of digital “skeleton key” for surveillance [161]; De-
bian’s predictable random number generator in the OpenSSL
package [162]; Stuxnet malware: compromise on code-signing
certificates [163] [164]; Duqu malware: stealing of digital
certificates along with the private keys [165]–[168]; and the
console hacking of Playstation 3 with compromised private
keys [169].

33https://tinyurl.com/DAOattack
34https://www.remme.io
35https://guardtime.com
36https://obsidianplatform.com

Developing a blockchain-based PKI is a feasible alternative
to the existing PKIs, which can provide the required security
properties [170]. In a blockchain-based implementation of
the PKI system, the user identities are bound to public-
keys using distributed public ledgers [69]. A blockchain-
based decentralized PKI system called “CertCoin” for secure
identity management and retention has been in use. This
system trusts the majority of peer network users instead of
any central trusted party. It has two different mechanisms for
verification of the known public key and the lookup for a new
public key, which are supported by decentralized efficient data
structures [158]. In [171], another blockchain-based distributed
PKI scheme has been proposed that resolves the single point
failure issue. This scheme ensures validity and ownership
consistency of public-key certificates by miner’s proof-of-
work. It uses Merkle Patricia tree (see for details [172]) for
efficient accessibility of certificates without relying on any
central trusted third party. Similarly other blockchain-based
PKIs have been discussed in [173]–[176].

I. Other Applications

Using the blockchain technology, a company named Factom
has started a land registration project with the Government of
Honduras to ensure integrity and correctness of the informa-
tion. Using the same technology, they have engaged in projects
related to smart cities, document verification, and the finance
industry [177].

In another application, a blockchain-based startup Ev-
erledger is working on bringing transparency to the sup-
ply chain of diamonds, which was previously perceived as
complex, risky and prone to carrying false and incomplete
information. Everledger has been designed to reduce fraudu-
lent modifications in the records to help financial institutions,
businesses, and insurance companies with actual details of
information [178].

A bitcoin-based startup Abra for transferring money to
anyone with minimal charges of transaction. No intermediate
party gets involved in this transaction [179]. Blockchain is
being considered as a novel software connector, which can
provide a decentralized alternative to existing centralized sys-
tems resulting in quality attributes. For example, Xu et al. [98]
found that blockchain can improve information transparency
and traceability as a software connector.

Openchain37 is a distributed ledger based system, which
helps in the management of digital assets while ensuring
their robustness, security, and scalability. AKASHA38 provides
people with a platform to publish and share their content
online. Participants of this system get rewarded for their
content based on the votes against their entries.

OpenBazaar39 is a blockchain-based platform, which facil-
itates people to make transactions freely among themselves.
Users of this system cannot censor the transactions or freeze
the payments. Users also enjoy the flexibility of sharing
information as much as they want. However, the buyers and

37https://www.openchain.org
38https://akasha.world
39https://www.openbazaar.org

https://tinyurl.com/DAOattack

sellers can engage intermediate moderators to resolve any
dispute that may arise between the involved parties [99].

IV. CHALLENGES AND THE ROAD AHEAD

The blockchain is expected to drive economic changes on
a global scale by revolutionizing industry and commerce by
redefining how digital trust mechanisms through distributed
consensus mechanisms and transparent tamper-evident record-
keeping. The disruption of blockchain is evident, and people
are beginning to adopt this distributed ledger technology.
There are, however, various hurdles that are slowing down the
rate of blockchain’s adoption. Some of these challenges are
discussed below and with pointers to how these challenges
might find a solution in the future.

A. Governance, Operational & Regulatory Issues

Blockchain has great potential to enable efficient and se-
cure real-time transactions across a large number of indus-
tries by providing financial services visibility along a supply
chain and streamlining government authorities and consumers.
Blockchain technology is still far from being adopted en
masse due to some unsolved challenges of standards and
regulation. Although its hard to regulate the development of
the blockchain technology itself, blockchain-based activities
(such as financial services, smart contract, etc.) should be
regulated [180]. To support its emergence and commercial
implementation, the development of standards and regulations
are required to establish market confidence and trust. These
regulations can also be used for law enforcement to monitor
fraudulent activities e.g., money laundering.

In May 2016, a complex set of smart contracts named
Decentralized Autonomous Organization (DAO) was built on
top of Ethereum blockchain. It was a crowd-funding platform
for defining organization rules40. After this smart contract’s
creation, there was a period of funding during which users
could earn its restrictive ownership by purchasing Ether (i.e.,
the underlying cryptocurrecy). After the completion of that
funding period, the DAO started its operation in which the
restrictive owners (also called members) casted their votes
for the usage of collected funds. Initially, this operation was
very successful and raised over $150M from 11,000 members
within a one month duration [181]. In June 2016, almost
$70M were drained after a hack making use of a recursive call
exploit. The hackers used this exploit to get Ether back from
DAO repeatedly before its actual balance update41. Another
such incident happened in May 2017, when the WannaCry
ransomware cyberattack targeted computers, encrypted their
data and demanded the ransom money in cryptocurrency. In
total, an amount higher than £108,000 was paid in Bitcoin
cryptocurrency by the victims. The impact of this cyberattack
was reportedly seen in 150 countries worldwide42.

40https://www.coindesk.com/understanding-dao-hack-journalists
41https://www.cryptocompare.com/coins/guides/the-dao-the-hack-the-soft-

fork-and-the-hard-fork
42https://www.theguardian.com/technology/2017/may/12/global-cyber-

attack-ransomware-nsa-uk-nhs

If blockchain is to get widely adopted, centralized regulatory
agencies, such as governmental agencies and multinational
corporations, may be unable to control and shape the activities
based on blockchain technology [182]. Because blockchain has
no specific location and each node may subject to a different
geographic jurisdiction and therefore different applicable laws
and legal requirements. There is no central administration
for each distributed ledger, therefore, territorial regulations
constitute a problem [183]. As a result, there is an increased
need to focus on the regulation of this cross-border nature of
technology.

In the Roadmap for Blockchain Standards Report [184],
it has been emphasized that there is a need to establish
international standards regarding blockchain terminology, in-
teroperability (between blockchain systems), user privacy,
security, user identity, governance and risk related issues so
that people’s confidence in blockchain-based businesses may
be developed. The report has further highlighted the need for
collaboration among committees and experts in order to further
strengthen the regulated use of the blockchain technology.

In [185], it has been described that there are many in-
terpretations of the blockchain technology in literature and
formal blockchain terminologies are yet to be defined, i.e.,
permissioned blockchains vs. private distributed ledgers are
few of those used interchangeably. In this [186] literature, the
importance of standards in paving the way for interoperability
between multiple blockchain platforms and applications, have
been discussed. The author is of the view that developing such
standards for ensuring interoperability can help in minimizing
the risk of fragmented blockchain systems.

At first, the organizations who have been governing the
Internet, considered blockchain technologies as beyond their
scope but this opinion changed later [187]. The World Wide
Web Consortium (W3C) has been discussing online payments
by utilizing the blockchain’s potential43. The Internet Gover-
nance Forum (IGF) has been arranging sessions on blockchain
technology to devise a distributed governance framework44.
The Her Majesty’s Revenue and Customs (HMRC) issued
a policy paper describing the tax treatment for the income
earned from Bitcoin (blockchain-based cryptocurrency) and
other cryptocurrencies-related activities [188]. The Financial
Crimes Enforcement Network (FinCEN) has recommended
that decentralized currencies should follow the money laun-
dering regulations [189] [190].

The European Securities Market Authority (ESMA) has
issued a paper [191] in which the benefits and risks of
the blockchain technology in securities markets have been
discussed. The UK Treasury has issued a report [192], which
has emphasized the need for Government to make efforts
for the necessary regulatory framework in parallel to new
blockchain-based developments. Moreover, other US regula-
tory authorities and agencies like Securities and Exchange
Commission (SEC), Commodity Futures Trading Commission
(CFTC), Internal Revenue Service (IRS) and Federal Trade
Commission (FTC) have been working to make regulations

43https://goo.gl/NjVLri
44https://goo.gl/9pPeiQ

pertaining to blockchain-based businesses and applications
[30].

1) Blockchain and GDPR: The European General Data
Protection Regulation (GDPR) was adopted in 2016 by the Eu-
ropean Parliament and the European Council [193]. Since then,
two years were given to the businesses to prepare themselves
to comply with the regulation. In this section, we discuss
where does the compliance with GDPR put the blockchain
technology? Will the original premise of decentralization and
immutability be able to sustain under the GDPR ramifications
particularly when we consider the right to be forgotten clause
of GDPR? In what follows we first provide a brief overview
of the GDPR, the duties it puts on businesses, the rights it
gives to the users’, and finally what are its ramifications on
the blockchain technology in general?

After its legislation, the GDPR came into effect on May 25,
2018, and is applicable to any kind of information that can
be associated with an either identified or identifiable living
person45. Some of the examples of identifiable information
include names, unique code number, IP address, single or
multiple identifying characteristics. Further, GDPR applies
throughout the lifecycle of personal data i.e., from data col-
lection, to data processing through to the ultimate disposal
of this data. GDPR-compliant businesses are bound to collect
only data for the clearly stated purposes and process it with
the users’ consent. After the use of personal data for the said
purpose, according to GDPR, the businesses are incumbent
to delete the personal information from their local storage.
However, this excludes data pertaining to a deceased person
and processing of such data is at the disposal of local policies
in place at a particular geographic region [193].

GDPR gives users certain rights when they interact with
businesses which provide a service based on their personal
data collection and processing. These rights include:

1) Awareness: This entails that the users’ must be informed
about how their personal data will be used;

2) Access: The users must be able to access copies of their
data collected by a business or a service provider free
of charge;

3) Correction: If a user finds some inaccuracies in her data
held by a company then she must be able to flag it as
disputed;

4) Deletion: A user must be able to make a company
delete all the information pertaining to her whenever
she chooses. (This right is sometimes referred to as the
right to be forgotten);

5) Restriction: If a user is in a process of assessing the
accurateness of her data use she must be able to restrict
the access to her data during the process;

6) Objection: A user must be able to object to the uses of
her data if she disagrees with some of the automated de-
cisions involving her (such as marketing ads or shopping
recommendations).

It can be observed that many of these rights seem to fit
quite well with the blockchain’s premise of decentralization,

45https://gdpr-info.eu/art-4-gdpr/

tamper-evident record keeping, transparency, and auditability.
There are however a few nuances which we discuss next.

There are two important terms that GDPR defines namely
data controller and data processor which require special
attention when dealing with blockchain-based projects. Both
of these entities take part in users’ personal data processing
with their specific consent. There is, however, a nuance in the
way these two entities function. Controller is an entity which
sets the purposes and means for data processing. Controllers
can take the shape of a natural or legal person, authority, or
an agency. Data processors, on the other hand, is similarly a
natural or legal person, authority, or an agency that processes
personal data on a controller’s behalf strictly following the
rules specified by the corresponding controller. There should
also be an agreement between a controller and a processor
clearly defining their roles and functions [194]. Given the
users’ rights, as mentioned above, one of the underlying prin-
ciples of GDPR is auditability which provides the provision
to hold the process and the entities involved in personal data
processing accountable for their responsibilities, functions, and
actions46. In the decentralized environment of blockchain the
important issue is related to specifying who gets to be a data
controller and who a processor [194], [195].

In terms of blockchain, we consider a number of scenarios
(self open, self private, open, private, consensus protocols) to
answer the questions related to deciding the roles of controllers
and processors. First, an entity (a business for instance) can
choose to make use of the open and permissionless blockchain.
In this scenario, such an entity can potentially write the core
blockchain protocol and make it open source. Further, such
entities can also deploy a set of smart contracts defining data
processing rules and interactions among nodes of the network.
We conjecture that this way such an entity can assume the
role of data controller. Further, anyone can download the
client software and become a node in the overall blockchain’s
P2P network. It has been a common practice that the open
and public blockchains make use of PoW-based consensus
mechanism. This implies that any node in the network can
process transactions and validate them by including them in
a mined block and ultimately appending that block to the
overall blockchain. As we discussed earlier that PoW-based
mining is a lottery-based process which means that it is a
random event that a node in a network finds a nonce hence
mining and ultimately appending this block to the blockchain.
In this scenario, it is not a trivial task to decide who is the
processor. Since potentially all the nodes in such a PoW-based
network process data at the same time. We conjecture, either
the whole network should be considered as a processor or the
responsibility of being a processor should be weighted as per
the processing power of either individual nodes or pool of such
nodes (which are sometimes referred to as mining pools).

The second scenario is of private and permissioned
blockchain. In this scenario, a number of entities can come
together to form a consortium and then automate the dynam-
ics of such a consortium using a permissioned version of

46https://thenextweb.com/syndication/2018/07/26/
gdpr-blockchain-cryptocurrency/

https://gdpr-info.eu/art-4-gdpr/

https://thenextweb.com/syndication/2018/07/26/gdpr-blockchain-cryptocurrency/

blockchain. In this setup the entities can make use of a PoA-
based consensus mechanism or Hyperledger’s channel-based
permissioned blockchain 47. Further, such entities can rent
storage and computational resources from a third party cloud
provider and hence rendering them as data processors. On the
other hand, the consortium as a whole can assume the role
of a data controller. Again, we conjecture, if the consortium
makes use of a consensus mechanism such as PoS then as
far as accountability is concerned then each node can be held
accountable according to the stake value that such node holds
in the overall network.

2) Right to be forgotten: Although many of the principles
outlined in GDPR such as data auditability fit quite well with
blockchain’s premise, the main bone of contention, however,
in the way of making blockchain-based decentralized solutions
in compliance with GDPR is the so called right to be forgotten.
This right dictates that a user must be able to instruct a
business at any time to remove personal data pertaining to
her. Further, as discussed above, the businesses are incumbent
to delete personal data after a set duration of time. This right
seems at odds with the blockchain’s data integrity guarantees.
The situation gets worse as blockchain-based solutions are
usually distributed with multiple copies of the records stored
at different nodes of the network.

As paradoxical as deletion and integrity seem at first glance
there are, however, proposals to reconcile these two seemingly
opposing principles48. One of the solutions could be to encrypt
each data entry in blockchain with a key pair and only store the
ciphertext on a blockchain. This way deletion can be achieved
by simply deleting the corresponding private key while still
preserving the ciphertext on blockchain. In some geographical
jurisdictions (such as in Britain49) the interpretation of GDPR
does recognize such methods of digital deletion. However,
such techniques do not provide a future-proof guarantee since
with the advent of new and faster technologies and techniques
such as quantum computing, such encryption methods pose a
risk for future data breaches50.

Another proposal is to only store hashes of data on
blockchain while storing the actual data in an off-chain stor-
age. This way the deletion can be achieved by deleting the
off-chain stored record while keeping the hash of it intact on
blockchain. An argument against this technique is that the hash
of a blob of data can still qualify as personal data since if an
entity possesses this blob then she can easily reconstruct the
hash and decipher what was stored on blockchain in the first
place. To get around this problem, we can use hash peppering
whereby a random and secretly kept nonce is appended to the
blob of data before taking its hash and storing it on blockchain.
This, however, implies keeping the nonces well protected and
secret and does imply some level of trust on third parties that
are responsible for peppered hashing of data.

47https://hyperledger-fabric.readthedocs.io/en/release-1.3/channels.html
48https://thenextweb.com/syndication/2018/07/26/

gdpr-blockchain-cryptocurrency/
49https://bit.ly/1VBf6Y8
50https://www.bundesblock.de/wp-content/uploads/2018/05/GDPR

Position Paper v1.0

Another way can be to make use of a technique similar to
the way channels are implemented in Hyperledger Fabric51.
Channels can be understood as confidential and permissioned
islands of smaller blockchain instances on top of a larger
blockchain infrastructure. A blockchain instance pertaining
to a channel can be audited in the same way a public and
open blockchain opens itself to auditing. However, the actual
contents of transactional records are encrypted and one can
not decipher the nature of the business being automated in
a channel’s instance of blockchain. This way by deleting the
cryptographic information related to such a channel the whole
instance of the corresponding blockchain can be rendered
redundant.

B. Scalability Issues

Scalability is one of the major concerns in the way of wide
spread adoption of blockchain-based technological solutions.
We discuss this concern with following three different per-
spectives.

1) Transaction throughput: Although the Bitcoin is a pop-
ular blockchain-based global cryptocurrency, scaling it to
handle the large transaction volumes worldwide raises some
concerns. Among other things, the transaction processing rate
of Bitcoin is affected by (1) the available network bandwidth,
and (2) the network delay affects. Miners with high bandwidth
and with less network delay can broadcast their blocks among
peer nodes with ease and speed, while on the other hand low
bandwidth miners with limited computational resources pos-
sess less probability of getting their fair share in a successful
execution of proof-of-work [196].

Bitcoin has seen an increasing interest, which has raised
questions about its scalability. Scalability was one of the
reasons that led to the creation of Bitcoin Cash52; a forked
version of Bitcoin but with a larger block size to allow more
transactions per block.

The blockchain-based systems are usually self-managed
and accept transaction blocks after approximate intervals of
time. The throughputs of these transactions are mainly based
on block interval and maximum block size [197]. It has
been predicted that if the blocks size were to continue to
grow at the same rate then it might attain a value close to
its maximum capacity level by 2017 and this could be a
significant scalability concern [198].

Increasing the block size does imply a higher transaction
throughput, however, this will also mean that the larger blocks
would require more time to reach to the peer nodes of the
network resulting in higher latency when it comes to proposing
new blocks or reaching consensus on the state of a blockchain.
On the other hand, the latency would decrease with decreased
block interval but at the cost of potential disagreement in
the system [199]. Similarly, other consensus protocols such
as PoS-based consensus (as meniotned in Section II-D2) are
in development phase which are aimed to addressed the
scalability and energy concerns.

51https://hyperledger-fabric.readthedocs.io/en/release-1.3/channels.html
52https://www.investopedia.com/tech/bitcoin-vs-bitcoin-cash-whats-difference/

https://hyperledger-fabric.readthedocs.io/en/release-1.3/channels.html

https://thenextweb.com/syndication/2018/07/26/gdpr-blockchain-cryptocurrency/

https://bit.ly/1VBf6Y8

https://www.bundesblock.de/wp-content/uploads/2018/05/GDPR_Position_Paper_v1.0

https://hyperledger-fabric.readthedocs.io/en/release-1.3/channels.html

https://www.investopedia.com/tech/bitcoin-vs-bitcoin-cash-whats-difference/

2) Storage: In addition to the block size scalability concern,
the storage capacity of peer nodes is another issue. The trans-
action rate has a direct relation with the storage capacity of
the participating nodes. With more nodes joining the network,
the transaction rate would likely be higher and will require
more storage space on the peer nodes, which might be seen
as a limitation from the perspective of the consumers [200].

It has been identified that blockchain technology is not
limited to cryptocurrencies, but there are various blockchain-
based prototype applications that are being used in domains
such as IoT, Botnet, P2P broadcast protocols, smart property,
and others. This shows the potential of blockchain technology
for various other industries. Currently, the size of blockchain-
based applications, in terms of their user base, is relatively
small. Bitcoin is the largest solution based on the blockchain,
but the transaction rate in bitcoin’s network in comparison
to the traditional digital payment solutions is considerably
lower. However, in future, blockchain-based solutions could
be used by millions or trillions of individuals and the number
of transactions would increase drastically. Because of the
distributed storage characteristic inherent in blockchains, it
will put increasing pressure on storage nodes, which could
result in increased synchronization delay, power consumption,
and server costs. We believe that more research is required in
order to address these these scalability issues.

3) The Lightning Network and Sharding: The scalability
issue can, up to some extent, be addressed by distributing the
transaction execution process into multiple steps. To ensure
scalability, the execution of transactions can be performed
outside the blockchain, whereas the validation should take
place within the blockchain network. This would decrease
the transaction confirmation time. For example, the Lightning
Network is able to perform 45000 transactions per second by
executing the transactions outside the blockchain [201].

Another possible solution could be a decentralized database
that can be used by both public and private blockchain and
deploying sharding (which implies horizontal partitioning of
records given large databases) and then merging the shardes
at regular intervals53 [202], [203]. A decentralized database
would be able to process millions of writes per second with
the storage capacity of petabytes and latency in sub-seconds.
This will also allow more nodes to be added to the platform,
which would increase the performance and make the capacity
scalabile.

C. Security and Privacy Concerns

Besides security being in the system by design of the
blockchain-based transactions, privacy remains a concern in
applications and platforms [204]. The blockchain technology
has been considered as privacy-preserver and rated well in this
context [205] [206] [207]. However, third-party web trackers
have been observed deanonymizing users of cryptocurrencies.
These trackers fetch user’s identity and purchase information
from shopping websites to be used for advertisement and
analysis purpose. Normally, these trackers have sufficient

53https://medium.com/edchain/what-is-sharding-in-blockchain-8afd9ed4cff0

information required to uniquely identify the blockchain-based
transaction along with user’s identity [208].

It has been widely believed that blockchain is safe as its
transactions are executed with generated addresses instead of
real identities [10]. Besides this, in [209] [28], it has been
shown that the blockchain transactions do not ensure privacy
since the transaction balances and values against public key(s)
remain available for all.

In addition to the privacy-related issues, there are some
security concerns related to blockchain technology. There are
certain scenarios that may affect the expected behavior of
the blockchain system. Consider the case where a miner-A
successfully generates two blocks but does not disclose it to
the peer honest network nodes, instead withholds these. We
may call these as secret/hidden or private blocks. The miner-A
releases these secret blocks when some honest nodes complete
mining of a new block (say grey block). After the release of
secret blocks, the miner-A successfully adds his two secret
blocks in the blockchain network (since the miner-A holds the
the longest chain of honest network nodes), whereas the newly
added grey block does not remain a part of honest blockchain
because the grey block does not hold the longest chain of
honest network nodes [210] [211]. This type of attack is called
selfish mining attack (see Figure 5 ) and this results in the
undermining of the fair share of the block mining rewards

51% attack [12], [212], [213] is another type of attack on
blockchain systems. In this attack, a miner having more than
half (i.e, 51%) of network node’s computational resources
dominates the blockchain system in terms of transaction
generation, approval, and verification and thus paves the way
for fraudulent transactions generation [214].

D. Sustainability Issues

Blockchain has attained an extraordinary amount of interest
and attention and a large number of industries are adopting
this virtual digital ledger. However, it is still unclear that any
particular solution of blockchain can attain a certain level
of adoption for their sustainability. As a new technology,
blockchain still facing operational, technical and its adoption-
related issues. Similarly, there are also some aspects of
blockchain technology that may need further modification or
development to attain its anticipated potential. For example,
although blockchain does provide a reliable cryptocurrency
mechanism, it also adds latency to the network since the ver-
ification of the transaction requires consensus, which requires
a certain amount of computation and a certain amount of time.

The sustainability of blockchain is still uncertain for in-
ternational development projects, especially in developing
countries. These projects require a very large infrastructure
and involve various stakeholders, cross-border organizations,
governments, and public or private parties. In these scenarios,
the practicality of blockchain is unclear and it is the time
to explore how blockchain will facilitate and sustain in such
projects. Therefore, sustainability scientists and blockchain
developers must discuss problems and solutions. More re-
search is needed to find energy efficient approaches for Bitcoin
mining. Behavioral and psychological research is required to

https://medium.com/edchain/what-is-sharding-in-blockchain-8afd9ed4cff0

Fig. 5: Workflow of selfish mining attack

attain people’s trust in technology for cryptography. Most
importantly, lawyers and programmers must collaborate to
formulate smart contracts and dictionaries will be necessary
that connect computer codes and legal languages.

E. Anonymity

In a blockchain system, the users utilize generated ad-
dresses, which are mostly in the form of public keys, for
their unique identification over the blockchain network. The
blockchain users can generate their multiple addresses in order
to avoid the revelation of their real identities. These addresses
are generated in the form of cryptographic keys. The said keys
are then used to send and receive blockchain based transactions
[215].

Moreover, there is no central storage system for preserv-
ing the user’s private identification details in the blockchain
network. By this way, the privacy in blockchain system is
maintained up-to certain extent, however, the user’s privacy
protection is not guaranteed since the transaction amount
details and the blockchain-based cryptographic keys (i.e., used
for user identification) along with their respective balances, are
publicly visible [10].

The blockchain-based applications still do not completely
guarantee the preservation of transactional anonymity. The
transactional transparency is impacted due to the lack of strong
anonymity support for the end users [206]. In [209], the author
showed that the movements of blockchain-based transactions
are traceable and thus do not possess enough anonymity [216].
Few other anonymity tracing techniques are discussed in [217]
[48].

F. Use of Artificial Intelligence and Machine Learning

Recent advancements in blockchain technology are making
new ways for the involvement of AI and machine learning
(ML) that can help to solve many challenges of blockchain
with several important future applications. Blockchains is a
technology that is being used to verify, execute and record
the transaction. AI can help in understanding, recognizing,
assessment decision making in the blockchain. Whereas ML
techniques could help to find ways to improve decision making
and smart contracts. For instance, AI can help to build an
intelligent oracle without the control of the third party. This
would learn and train itself to make the smart contract smarter
[10]. The integration of AI and ML with blockchain will
potentially create a new paradigm by accelerating the analysis
enormous amount of data. Examples include automation of
tokens creation, recommender systems, security enhancement,
etc.

1) Use of Big Data Analytics: Recently, many companies
are focusing to adopt the blockchain technology in their
frameworks. This is creating new types of data for analysis
by the powerful tools of big data. There are a huge number
of blocks—increasing rapidly and constantly throughout the
globe. Each block is full of information (i.e., details of
every financial transaction) that can be used for analysis to
explore thousands of patterns and trends. The blockchain is
a technology that provides integrity, but not analysis. By
using big data, it will be possible to detect nefarious users
with whom business would be dangerous. Big data can also
provide real-time fraud detection based on the users’ records
and history. The risky transactions or malicious users can be
detected quickly by using big data analytics. This will result
in cost reduction for real-time transaction [218]. Further, user

trading patterns can also be used to predict trading behaviors
and potential partners for trade with the help of big data
analytics [10]. A good resource to conduct big data analysis
on (real-time updated) data related to Ethereum and Bitcoin’s
blockchain is by using Google’s BigQuery54,55. For more
details on the applications of blockchain for enabling AI, the
interested readers are referred to a comprehensive survey on
this topic [20].

G. Usability and Key Management

One of the primary challenges that any new technology
faces is the usability. This issue is more acute in blockchain
because of new architecture and high stakes. The transaction
flow should be visible to users to analyze the whole transaction
flows. This will improve the usability and help the individuals
to understand and analyze the whole blockchain network
[12]. There are some systems such as Bitconeview [219]
and Bitiodine [220] that proved to be very effective for the
detection and analysis of blockchain-related patterns. These
systems also help to improve security and privacy-related
concerns.

It has also been reported in the challenges and limitations
of blockchain that the bitcoin API is difficult to use for the
developments [3]. Bitcoin users have to deal with public key
cryptography that differs from the password-based authentica-
tion system. The usability of bitcoin key management also
presents fundamental challenges for end users [221]. This
requires more research in the future to provide more ease to
the end users and the developers.

V. CONCLUSION

In this paper, we provide a study on blockchain-based
network applications, discuss their applicability, sustainability
and scalability challenges. We also discuss some of the most
prevalent and important legal ramifications of working with
blockchain-based solutions. Additionally, this paper suggests
some future directions that will be helpful to support sustain-
able blockchain-based solutions. At the time of writing, we
believe that, blockchain is still in its infancy implying there
will be sometime spent before it gets ubiquitous and widely
adopted. However, the aim of this study is to provide a guiding
reference manual in a generic form to both the researches and
practitioners of the filed so that a more informed decision can
be made either for conducting similar research or designing a
blockchain-based solution.

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I Introduction

I-A Contribution of the survey
I-B Structure of the survey

II Background

II-A Blockchain and distributed ledger technology (DLT)
II-B A clever use of hashing
II-C A coin: Transaction chain
II-D Distributed consensus
II-D1 Proof-of-Work (PoW)
II-D2 Proof-of-Stake
II-E Smart contracts
II-F Public and private blockchains

III Blockchain-based Network Applications

III-A The Decentralized Internet
III-A1 Decentralized naming systems
III-A2 Routing in the decentralized Internet
III-B Decentralized Email
III-C Blockchain for the Internet-of-Things (IoT)
III-D Blockchain-based Content Distribution
III-E Distributed Cloud Storage
III-F Applications in Online Social Networks
III-G Cybersecurity
III-H Public Key Infrastructure (PKI): Certificate Authority (CA)
III-I Other Applications

IV Challenges and the Road Ahead

IV-A Governance, Operational & Regulatory Issues
IV-A1 Blockchain and GDPR
IV-A2 Right to be forgotten
IV-B Scalability Issues
IV-B1 Transaction throughput
IV-B2 Storage
IV-B3 The Lightning Network and Sharding
IV-C Security and Privacy Concerns
IV-D Sustainability Issues
IV-E Anonymity
IV-F Use of Artificial Intelligence and Machine Learning
IV-F1 Use of Big Data Analytics
IV-G Usability and Key Management

V Conclusion
References

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