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Chapter 5
Jaswant Arya1, Arun Kumar2, *,
Akhilendra Pratap Singh1, Tapas Kumar Mishra2
and Peter H J Chong3
1Department of Computer Science and Engineering,
National Institute of Technology, Meghalaya, India
2Department of Computer Science and Engineering,
National Institute of Technology, Rourkela, India
3Department of Electrical and Electronic Engineering,
Auckland University of Technology, New Zealand
Today, centralized systems are more common, and the reversibility of the
process and transparency are kept hidden due to the ownership of a single
authority. The consistency is difficult to maintain as the network is
centralized, and the central server is responsible for managing overall
traffic. Blockchain brings out the new features like decentralization,
distribution, immutable, consistent and security by cryptographer hash
algorithms. Blockchain technology is based on the consensus which hires
distributed ledgers for enabling parties who do not fully trust each other
to maintain a set of global states. The intermediaries work to maintain
trust and the reduced number of intermediaries can make the process
easier, faster, transparent and cheaper. The consensus mechanism is used
to validate the transactions, and privacy is maintained by having multiple
account addresses. This chapter presents the basics of Blockchain
technology, applications, research challenges and opportunities in the
* Corresponding Author’s Email:
Jaswant Arya, Arun Kumar, Akhilendra Pratap Singh et al.
field. As a recent phenomenon, Blockchain has the potential to bring a
new perspective to security, resiliency and efficiency in the digital world.
This chapter discusses each issue in details.
Keywords: blockchain, cryptocurrencies, architecture, consensus
algorithms, smart contract, bitcoin, applications
Blockchain is a decentralized, distributed, and immutable ledger
secured by cryptographic hash algorithms. Blockchain ledger has chained
chronological and encrypted blocks of the synchronized data across the
peer to peer network. The data is formed into blocks, and each block is
appended to its previous block connected by hash. The blockchain first
stores the data into back linked blocks. It then verifies the blocks with a
distributed consensus process to keep the security, privacy, and
transparency in the whole blockchain network. The most prominent and
widespread application of blockchain technology is Bitcoin, the first
cryptocurrency [1].
Blockchain has become the most promising technology for
decentralized trust without any third party. The data-driven autonomous
industries adopted the blockchain technology as the mechanism to store the
data immutably in a distributed and secured manner. The digital tokens
used in blockchain for transferring and accessing the digital assets between
peer to peer users proves its prominent use to remove the need of any third
party [2]. Data security, as well as data integrity, has been achieved by
cryptocurrency and smart contract [3]. For executing the program on
different machines in a distributed manner with the consensus process, the
blockchain can be applied as an additional layer [4]. It is one of the reasons
why blockchain has given birth cryptocurrency and smart contracts that
impose the critical requirement on data security and integrity [5]. For
general bytecode execution, the orchestration for the global state can be
achieved with the distributed consensus and Blockchain technology is
expected to evolve the open access and transaction management on
decentralized autonomous systems [6]. It involves the decentralization of
the assets in the finance sector, IoT, in public services and decentralized
applications, etc.
Each new transaction is validated and recorded in a global ledger by a
miner. Each miner contributes his computational power to verify the data
blocks of distributed ledger and can compete in a consensus process to
create the next new block. Once the block is created, it can be appended to
the main chain. In the blockchain, data is of two types- on-chain data
which is to be kept in blockchain and off-chain, which can not be stored in
the block so that the verification process may be less complicated [7].
Blockchain uses a decentralized token system to authenticate the asset
transfer. Furthermore, a Blockchain maintains an arbitrary order of the
transactional records by cryptographically chaining the record subsets in
the form of data blocks to their chronic predecessors. Data tampering can
be prevented by the linked block using cryptography. Consensus protocols
maintain the consistency of the transaction in the weak synchronized
network. The authors [8] have reviewed the blockchain use cases and its
adaptability in IoT for integrity, anonymity, and the interoperability. The
authors in [9] have presented the blockchain as a solution for security,
privacy, and computational limitations of different IoT devices.
The details of the transfer are recorded on a public ledger that anyone
on the network can access. Blockchain is similar to the distributed public
ledger, in which all transactions are stored in a chain of blocks. So this
chain continuously grows when new blocks are appended to it. Blockchain
technology has essential characteristics, such as decentralization,
persistence, anonymity, and auditability. Blockchain can work in a
decentralized environment, which is enabled by integrating several core
technologies such as cryptographic hash [14], digital signature [15], and
distributed consensus mechanism. Blockchain has provided a much
powerful security alternative when compared to database systems [17]. In a
decentralized transaction, blockchain technology can reduce the cost and
improve efficiency without the existence of any third party.
Blockchain technology has shown its potential by its main
characteristics decentralization, distribution, and persistence. The
Jaswant Arya, Arun Kumar, Akhilendra Pratap Singh et al.
cryptographic hash algorithms contribute more in the security and digital
signature to add encryption between peer to peer user. Below, we list a few
of the properties of the blockchain.
1. Decentralization: In blockchain technology, the data is
decentralized so that each node, which is the part of the blockchain
network has consistent data. A central, trusted authority authorizes
centralized data. In most cases, this centralized authority works
well, but the increasing number of intermediaries trigger the cost
of the transaction. For example, in the banking system, the banks
are centralized trusted authorities, and in most cases, it works well.
However, this mediation limits the minimum practical transaction
cost and cuts its incentive. In a blockchain network, the transaction
can be completed without a centralized authority. This helps in
reducing the overall cost when multiple transactions are used for
the same product. Also, the workload of the central server can be
minimized significantly by partitioning the verification on multiple
2. Persistency: In the blockchain network, the transactions are
inherently stored in blocks for creating a chronologically
decentralized and distributed chain which is pursued by the entire
network. Each node contains the blockchain ledger to recover from
any data loss by a node. Persistence and consistency are easily
achievable by storing the data in different nodes and detecting the
missing data or any falsification.
3. Anonymity: In the banking system, the identity is stored by the
banks to verify the transactions, and the personal details of the
transaction can easily be traced. However, in the blockchain
network, a person interacts with the generated wallet address and
personal identity is kept hidden by using multiple addresses and
there is no central authority which stores the private data of a node.
Transactions are verified publicly, and the privacy is retained by
using the addresses for each transaction. This ensures that a miner
cannot know the identify the owner or the issuer of the wallet
address. Also, a central party does not ask for the private data of a
4. Transparency: Blockchain network provides transparency for the
whole network by enabling distributed and consistent transaction
records. In the supply chain and logistics industry, the requirement
of transparency cannot be ignored. Blockchain network reduces the
number of frauds by establishing a transparent system.
5. Auditability: The blockchain network includes the timestamp that
can be used to trace the transaction detail, and the user can verify
the transaction easily. The higher auditability is proved by the
transparency and traceability of the transactions.
6. Time reduction: Each software in the blockchain network has
consistent information and therefore removing the lengthy process
of verification. The only single version of agreed can reduce the
time by avoiding the need for a lengthy process of verification,
settlement, and clearance.
Earlier research work has discussed the blockchain in IoT, blockchain
in healthcare, blockchain in the logistics industry, and a few have
discussed blockchain consensus process and architecture. The authors [18]
presents a smart home application for blockchain in IoT. The authors [19]
describes the use of a smart contract to create a business model. The
authors [20] reviewed the integration of blockchain with IoT and the use
cases of this integration. The work [21] raised the security issues in the IoT
and brought the advantages of blockchain to resolve these issues. Many
surveys are focusing on the integration of blockchain with the specific area
or limited field. Still, there is no universal and general survey in the best of
our knowledge which combines the architecture of blockchain with the
applications and the challenges.
This chapter presents the underlying architecture, mechanism,
applications, challenges, and opportunities of the blockchain technology.
Moreover, the key characteristics of blockchain, such as decentralized,
immutable, distributed, and secured, are discussed. The comparison
between different versions and variants of the blockchain technology as
Jaswant Arya, Arun Kumar, Akhilendra Pratap Singh et al.
well as the fundamental difference between the various consensus
algorithms are presented in details. Also, the basic principle of
cryptocurrencies and their applications are presented.
Blockchain is the chain of sequentially connected blocks holding the
complete list of transaction records. Except for the first block, every block
has a parent block. The blocks can be reached by traversing back as every
block contains the hash of its parent block. The first block is known as the
genesis block. The block number identifies each block, and each block
contains valid transactions. The transactions are not stored directly, each
transaction is hashed, and Merkle tree of these hashes is formed. The
timestamp is always attached with a transaction for maintaining the
chronological order.
A. Block
A block is the data structure which contains a hash of the parent block,
transactions, Merkle root, timestamp, etc. The first block of a blockchain is
called Genesis Block. Block version indicates the set of block validation
rules to follow. The previous block of any block is called Parent Block,
and the Hash of the parent block is called parent Hash.
B. Hash of the Block
A hash can be understood as a fingerprint, which is unique for each
block. It identifies a block and all of its content, and it is always unique.
Therefore, once a block is created, any change inside the block will cause
the hash to change. In the following example, the value returned by a hash
is a fixed-length string which contains only two things -the upper case
alphabet and digits. Any complex function h(x) can be used to generate the
hash. For making cryptographic hash secured, the highly complex hash
algorithm like SHA-256 can be used [11]. For instance, MD2 hash
produces the following results-
Input: jaswant
Hash Output:d2ebbf629b8c2d492321eda2e6d3ef6b
Input: jaswanu
Hash Output: 964910d763b8ffbd35631e7b74a375f1
The hash function is created in such a way that a single change in data
leads to a drastic shift in output hash value. It is impossible to assume the
input data based on the hash value. Also, more than one input data cannot
give the same hash value.
C. Time Stamp
A timestamp server can prevent the double-spending problem. The
hash value of the block is published in blockchain network similar to news
publishing in a newspaper. Timestamp can prove the validation of data at
the right time to get into the hash. The chain of hash is formed by adding
the timestamp reinforcing the ones before it.
D. Difficulty
The difficulty is the value which decides the difficulty level to
calculate a hash threshold for a given target. If the number of transactions
increases, then it is challenging to create the new blocks in high speed and
of course there may be chances of evolving number due to attackers or
greedy miners. Therefore, to control the speed, the difficulty level is
increasedthe level of difficulty increases as the speed of block formation
Jaswant Arya, Arun Kumar, Akhilendra Pratap Singh et al.
E. Nonce
Nonce stands for a pseudo-random number used only once during the
process of mining. In Bitcoin blockchain, a nonce is a number proceeding a
certain number of zeros. During the mining process, the nonce is found
spending the energy to process the algorithms. Thus replay attack can be
exhibited in block creation.
F. Block Header
A block header is a string which contains block version, previous
block hash, Merkle tree root hash, timestamp, nBits difficulty, and Nonce.
A block header contains transactions as the information to be secured. A
block can have multiple transactions depending on its size. In Bitcoin
blockchain, each block size is 1 MB [1].
G. Blockchain Data Structure
The Blockchain data structure is an ordered back-linked list of blocks
of information/transaction/data. It can be created by forming a flat file or a
simple database. The identity of the block is kept by a hash which is
calculated from cryptographic hash algorithms such as SHA512, SHA256,
etc. Every block contains the parent hash. If an attacker tries to make
changes in the block, the hash of that block will be changed, and then the
proceeding blocks will not be further connected to this block due to
mismatched parent hash. If a new block is created, it is appended at the end
of the blockchain. The process of creating a genesis block is different as
there is no block on which it is appended. Therefore the initialization is
done to form a genesis block with zero parent hash.
Figure 1 illustrates the block structure of one block in the Blockchain
having one parent hash or previous block hash, one difficulty level, one
nonce value, multiple transactions and Merkle root formed by the hashes of
In the Merkle tree, the hash of the parent node is calculated by
appending the hash of the right node to the left node and then hashing to
generate the hash of the parent node. All the underlying transactions are
combined and hashed together to form the Merkle tree.
Figure 1. Blockchain Block Structure [1].
Jaswant Arya, Arun Kumar, Akhilendra Pratap Singh et al.
Figure 2. Blockchain Layers [45].
Figure 2 illustrates the six different layers of the blockchain network.
The blockchain network has different layers to perform specific tasks
similar to the ISO reference model of the Internet [45], [46]. Data layer
collects the data and stores it into a ledger using the asymmetric encryption
techniques, hashing, time-stamped Merkle trees. The network layer
provides the functionality to connect all the nodes in the network and
creates a decentralized peer to peer network environment. The consensus
layer maintains the consensus via the different consensus algorithms
among the network nodes and maintains the consistency of the transaction
records. The incentive layer distributes the reward to the miners on the
successfully mining a block. Contract layer contains the smart contract to
manage the data of money or assets records etc. Finally, the application
layer provides different applications and use cases of blockchain.
H. Digital Signature
The encryption techniques are used to authenticate, integrate a message
to keep its confidentiality and privacy. The digital signatures are more like
a stamped seal or handwritten signature, and thus, it solves the challenges
of tampering and fake authentication or impersonation in communication
[15]. The elliptic curve digital signature algorithm (ECDSA) [12] is widely
used for Blockchain network. In the blockchain network, the peer to peer
connection is established between all the nodes, and each node owns the
consistent ledger. This consistent ledger is the proof of a transaction. This
ledger is in the form of sequentially chained blocks connected with its
previous block by a hash.
Figure 3. Illustration of the peer to peer connected nodes in an open-access
Blockchain network.
Jaswant Arya, Arun Kumar, Akhilendra Pratap Singh et al.
Every block consists of the hash value of the previous block similar to
the singly linked list. The new transactions are kept in these blocks. Figure
3 illustrates the established peer to peer network between six nodes ranging
from N1 to N6 with each node having its ledger to verify the consistency of
transaction records. The higher security is achieved by sequential and
unidirectional traversing connectivity of the blocks.
The blockchain network must confirm the consistency and clarity of
the ledger in each software. This is completed by a few protocols which
proceed the transaction process having absolute standards. The verification
process is continued in a decentralized manner as performing the
transactions on the distributed database managed by peer to peer connected
nodes in the blockchain network. After validating and verifying the block,
the block of the transaction is published in the network. While it might be
possible that more than one miner is creating the block simultaneously, and
this is a challenge to maintain the blockchain network. The various
protocols are used to avoid inconsistency. The techniques or algorithms
which are used to reach the consensus are called the consensus algorithm.
The different strategies are proposed to reach the consensus like proof of
work, proof of stake, proof of authority, and Practical Byzantine Fault
Tolerance (PBFT). Each consensus algorithm has its strategy to pass the
agreement and may use different protocols. This paper describes in details
the backbone of the consensus algorithms.
The process of consensus is activated by requesting the first
transaction to trigger the creation of a new block. After the mining process
of the new block holding the transaction, this mined block is broadcasted
to every node. One can accept the valid block and also can reject the
invalid block. In Figure. 4, the flow chart illustrates the simplified process
of transaction requests, block creation, and its circulation within the
blockchain network.
Figure 4. Overview of the blockchain process.
A. Proof of Work (PoW)
In proof of work consensus process, every node which wants to
participate in the consensus process has to contribute its resources to solve
the mathematical puzzle. This puzzle has a different level of difficulties.
The transactions are broadcasted into the network to be added into the
memory pool by the active nodes. The target to find a value called Nonce.
Nonce has few sets of rules like the generated value must have a fixed
number of zeros as a prefix. When a miner finds the required Nonce,
Jaswant Arya, Arun Kumar, Akhilendra Pratap Singh et al.
he/she gets the right to create the new block and store the transaction into
the block according to the decreasing order of the transaction fees. Then
this block is broadcasted into the blockchain network. Other nodes can
accept or reject after verifying the transactions of the block. They can
accept the block by appending it to their chain. The consensus process
starts with the competition to take a right to pack the new transactions and
continues until the block acceptance.
Proof of work mechanism is applied to distribute the time stamp server
to the entire blockchain network. In this mechanism, a hash value is
calculated having n number of zero bits by scanning the Nonce. The Nonce
is incremented until its scanned value puts the required number of bits for
the block hash. The CPU effort is spent on every calculation of this work.
This calculation requires the work exponential in the number of bits
required. Each block is uniquely linked to its previous block except the
genesis block and hence redoing the work for any block directs to redo the
work for subsequent blocks. Therefore it is challenging to change any
block and its subsequent block. Proof of work consensus mechanism is
based on one CPU, one vote concept to verify the transaction and to accept
the longest chain. The exact and accurate chain can only be created by an
honest node and form the longest chain. If an attacker alters the transaction
in a single block by redoing the proof of work, he has to redo the proof of
work for each subsequently linked block. The hardware speed is controlled
by creating the difficulty as a target value to perform proof of work. If
blocks are generated fast, this difficulty target increases to compensate the
hardware speed and hence, a specific increasing time is consumed to
acquire the proof of work. An attacker can gain the 51% power of the
network, and this scenario ends up the blockchain use. The authors in [13]
proposed the hybrid consensus mechanism of proof of work and proof of
B. Proof of Stake (PoS)
Proof of work allows every node to participate in the mining of the
block, but among all those miners, only one miner receives the right to
create the new block and all other nodes waste the incentive and the energy
of the network. Moreover, if 51% of the miner attain the potential to mine
faster than others by having the higher energy to mining compared to other
miners, he may rule the whole blockchain network. Therefore the proof of
stake works better by allowing only all those nodes who prove themselves
the stakeholder. It saves the condition of one node owning the network
because only one node can not have 51% money of the whole network.
There is a probability of owning the network by one node in the beginning,
but later on, it becomes almost impossible to have money more than the
whole network. The proof of stake can efficiently reduce the energy
consumption by proof of work because the richer are not supposed to
attack the network. Thus the transaction speed can be boosted up by
reducing the number of miners. If the wealthiest person is dishonest, and
he/she tries to own the whole blockchain network, he has to own more than
half of the total currency of the network [63].
C. Practical Byzantine Fault Tolerance (PBFT)
The process to create consensus among byzantine soldiers is more
difficult if there are no nodes which can begin the process to reach on an
agreement. The possible byzantine fault tolerance (PBFT) is based on the
higher weight of the leader, so that final agreement to be without any
confusion. The byzantine faults are to be solved by pdf. In PBFT the
consensus is reached in multiple rounds.
In the first round, a new block is created, and in the subsequent rounds,
the primary node is chosen based on predefined protocols. These multiple
rounds can reorder the transactions. Proof of work requires the hash
process to reach the consensus, but in PBFT, a consensus is more likely to
be reached using the voting process.
A node can reach to the preparation phase from preprepared phase if it
receives 2/3 rd nodes favor of all the nodes of the network. Each node has
Jaswant Arya, Arun Kumar, Akhilendra Pratap Singh et al.
to receive 2/3 rd favor nodes to enter in the commit phase from the
preparation phase again. Thus the consensus process is completed in three
subprocesses. The node should be active while voting to receive the
agreement from each node. The nodes can ask their neighbor nodes for any
information for such as voting process or consensus process. The weight of
votes for a few nodes might be higher or lower depending on the network
node types. Hyperledger Fabric uses PBFT consensus for creating its
product based on blockchain technology [64].
D. Delegated Proof-of-Stake (DPoS)
For energy-efficient process, the number of nodes which participate in
the mining process can be decreased by allowing the nodes which get
priority according to their stake. Unlike to PoS, DPoS reduce the number
of verifiers by awarding the priority to the top stakeholder and make the
consensus process faster and energy-efficient. This priority rule names the
DPoS as the representative democratic process. In Bitshares, the DPoS is
used as the consensus process. EOS[27] is based on delegated proof of
E. Proof of Space (PoS)
Disk space can be another choice to select the miners because of the
more disk space, the more probability to mine a block successfully. By
initializing the dedicated disk space to mine, the block can be energy
efficient. The unused disk space can be reused by the miners to make the
miner process cheaper.
If a miner node mine the block unsuccessfully then he would not lose
the much higher energy like proof of work. Thus, Proof of space has a very
high economic advantage, and every node always can invest unused disk
space [16].
F. Proof of Authority
If a node creates the block, this block would be verified by the
authenticated nodes in multiple and different scheduled rounds. In a
different schedule, the authenticated nodes are asked to propose the next
block [10].
G. Ripple
If one of the subnetworks is trusted, its nodes can be listed separately
and be asked to verify the transactions. Unlike PBFT, Ripple uses the
nodes of intersecting groups with the most of non-Byzantine nodes. Server
node can verify the transactions and create the block. Client nodes can
transfer the funds. There is a different node list to be queried by the server
when the transactions are stored into the ledger [30].
H. Tendermint
In this consensus process, the block is created and confirmed in three
steps: prevote, pre-commit, and commit. In the first step, a prevote is
broadcasted for the newly created block as a proposal. In the second step, a
pre-commit is broadcasted if the prevote is favored by 2/3 of all the nodes.
At last, the commit is broadcasted if pre-commit is favored by 2/3 of all the
nodes and block is validated. A node can accept the block if the commit is
favored by 2/3 of all the blocks [34].
Table I presents the comparison between different consensus
algorithms. This comparison is derived from [63]. The blockchain network
can follow any consensus algorithm to validate the block and to maintain
the network established with honest nodes. If an attacker tries to disrupt the
network, he should be identified and punished by the honest nodes, and
Jaswant Arya, Arun Kumar, Akhilendra Pratap Singh et al.
therefore, the private industry can create its network of nodes to be secured
and be more auditable.
Table I. Comparison between different consensus algorithms
Proof of
Sybil attack [54]
Single task
Bitcoin and
Litcoin [55]
Proof of
No interaction
using Fiat
and random
Merkle proofs
Can be
Proof of
space [16]
Storage market
Redoing the
PoR over time
May be
used or
may not
ASIC resistance
trade-off in
Memory hard
Ethash [60]
ASIC resistance
Calculation of
and memory-
Proof of
participation in
Merkle proof
process and
of proof
To save the disk space, the transactions contained by the blocks are
discarded, but the block hash remains unchanged. If the root hash is
achieved, then the branch hashes are stubbed off from the old blocks.
Initially, there is no issue to store the blockchain of a smaller number of
transaction. But, in future, the increasing number of transactions may
require a higher speed of block-creation and the massive disk space. The
complexity of block creation deduces the wastage of time and may trigger
the situation of insufficient disk space. The older transactions might not be
required, and then they must be discarded to save the disk space as well as
the complexity of blockchain [1]. Figure 5 shows the Blockchain Block
Structure consists of Merkle tree hashes.
Figure 5. Blockchain Block Structure consists of Merkle tree hashes [1].
Figure 6. Blockchain Block Structure after stubbing off the branches to diminish
transaction 0, 1 and 2 [1].
Jaswant Arya, Arun Kumar, Akhilendra Pratap Singh et al.
Figure 6 illustrates the block having discarded branches of the Merkle
tree for the older transaction. Forking is possible in blockchain when a
node accepts two or more blocks simultaneously. Each miner is
independent of other miners to create and validate the block, and it may be
possible that more than two miners generate the block simultaneously. In
this case, a node can accept both the blocks and the chain can grow for
both the blocks until one of them proves to be authentic and to be retained.
The more massive chain is retained. It takes a specific time to generate and
broadcast the newly mined block. We observe a case when two miners
generate the blocks simultaneously and broadcast it. In that case, a node
can accept both the blocks to create the fork with accepting further blocks.
Figure 7 illustrates the forking condition when two blocks (x+3) and
(x+3)’ are mined simultaneously. In this scenario, the nodes can accept
block either one of them x+3 or x+3’ or both. This chain grows until one
longest chain is not perceived. Both the chains grow until the one chain
becomes longer than other branches, and then the longer one can be
accepted by every node. Figure 8 shows an example of the acceptance of
the longest chain.
Figure 7. Blockchain Forking.
Figure 8. Acceptance of longest chain.
The participation of nodes in blockchain network may vary depending
on the permission to take part in the consensus process. The set of nodes or
a single node should be permitted. The read permission may differ
depending on the individual nodes. There are three variants of the
blockchain network.
A. Public Blockchain
In a public blockchain network, everyone can verify, broadcast, and
add the block to the blockchain ledger; it means that everyone can take part
in the consensus process. A computer having internet can join the network
as a node. The record of transactions is secured in multiple nodes of the
blockchain network. Anonymity is maintained by the complicated
generating address for each user.
Jaswant Arya, Arun Kumar, Akhilendra Pratap Singh et al.
Redundancy of the blockchain makes it secured against the massive
attacks, but it also increases the latency of the process and causes the
additional wastage of the electricity. For creating a transparent, secured
decentralized network, the public blockchain is preferred most.
B. Private Blockchain
The blockchain platform can be used either as a general-purpose
software or a specific purpose software. Ethereum has provided the global
platform to create any decentralized blockchain application. Ethereum
tools are used to create smart contracts and decentralized applications [35].
Hyperledger has also provided its tools to create more reliable blockchain
decentralized application. Not limited to a smart contract, it provides more
functionality for a vast area of applications such as the finance, supply
chain, and logistics, health-care, and traffic signal systems, etc. [36]. It
allows the private and the consortium blockchain tools which can be used
by various individual organizations.
There are other platforms like Hijro [37], Tierion [38], Gem [39] and
Provenance [40] which can be used for a specific purpose. The company
can use the public or restricted platform according to its requirement as
Provenance or Hijro for logistic, Tierion or Gem for healthcare, etc.
There are many applications based on blockchain technology. All these
applications come under three categories Blockchain 1.0 or
cryptocurrency, Blockchain 2.0 or smart contract and Blockchain 3.0 or
decentralized autonomous organizations (Dapp). The first blockchain
application, Bitcoin, started the concept of cryptocurrency in the financial
sector. After a few years, the new version of the blockchain application
came as a smart contract. The cryptocurrency and smart contract are
successful, but the concept of Dapp has generated multiple number of
applications in the public sector [42]. Figure 9 illustrates the different
versions of Blockchain and their applications/Implementations.
A. Blockchain 1.0
Blockchain 1.0 version can be understood as a database that has the
record of transactions in the group of linked blocks. Each block is
connected with its previous block hash in the chronological order. There is
a distributed consensus mechanism to verify, validate, and append the
block in the transaction ledger. The cryptographic hash functions and peer
to peer connection are used to obtain higher security. The transaction
ledger is stored in each node. There is a public key and a private key. The
sender can access its account by the public and private key and can send
the message to other peers by knowing his public key. The message can
not be modified in anyway.
B. Blockchain 2.0
After the success of cryptocurrencies, another version of the
blockchain smart contract brought the next phase, which offers the digital
contracts based on the blockchain technology. This type of blockchain
provides the decentralized self-executable programs which are executed
when certain predefined conditions are satisfied.
This type of blockchain is used to create smart contracts for different
purpose like ownership of the vehicle, home or the things, etc. The scripts
can be written in the solidity, Go or any other suitable language for
defining the conditions and rules of the contract. These conditions are
stored in the blockchain ledger [41].
Jaswant Arya, Arun Kumar, Akhilendra Pratap Singh et al.
Figure 9. Different Version of Blockchain.
C. Blockchain 3.0
Blockchain 3.0 or decentralized autonomous organization offers the
implementation of the smart contract. In the public sector, decentralized
applications can promise a vital role. Dapp is an abbreviation of the
blockchain. Blockchain is trusted more than any other technology, and
therefore, Dapp codes are made as an open-source to get the audits from
third parties. The tokens are too used to value the support of participants of
the system. All the components of the Dapp are hosted and executed in a
decentralized manner [43].
Table II. Different cryptocurrencies
Bitcoin Cash [50]
Time reduction by increasing the block size
Zilliqa [51]
Higher security by using the more powerful cryptographic
techniques zero-knowledge proof
Enhance the programmability of the Blockchain network,
smart contracts and Dapps
maintain the stability between the national currency and
innovative consensus mechanism
innovative consensus mechanism
created for the Internet of things
created for global financial settlement
prediction of market application
Cryptocurrency is highly secured, distributed, decentralized, and peer
to peer digital currency, which is based on Blockchain technology. It
requires distributed verification of transactions without a central authority.
Verification or validating of the transaction includes confirming
transaction amounts, storing transaction history and preventing from
double-spending [48]. Cryptocurrency system provides the user with a
wallet with a generated address as a public key with a private key to sign
transaction as an owner. In some cases, a proof-of-work or proof-of-stake
or any other consensus scheme is used to create and manage the currency.
The maximum size of the block is fixed. A block of cryptocurrency
blockchain contains block header, block version, timestamp, a hash of the
previous block, nbits, nonce, Merkle root, hash, difficulty, and transaction
counter for storing the transactions. The computer or node which validates
and creates a new block is called validator or miner. Generally, for
validating and creating the block, one computer has to spend energy,
electricity, time, and more space in the existing memory to contain a full
copy of all the connected blocks. Till 8th July 2019, there is more than
2200 type of cryptocurrencies in the market with the current market cap of
Jaswant Arya, Arun Kumar, Akhilendra Pratap Singh et al.
more than $330 billion. Among all the cryptocurrencies, Bitcoin has the
highest market cap around $210 billion. Few of them are listed in table II
in the decreasing order of their market cap [47]. Centralized service
providers or government-controlled the earlier digital currencies, but
Cryptocurrencies are entirely decentralized and controlled by distributed
consensus algorithms among the different nodes of the peer to peer
network. The identity of a person is kept hidden by using the address
instead of a user name or any other data. In the centralized system, the user
information is always preserved by the service providers.
A. Bitcoin
The traditional electronic cash systems do not use the address, but as
an email, Bitcoin uses the address to hide the identity. Bitcoin network can
circulate a maximum of 21 million Bitcoin to avoid inflation or deflation.
Unlike traditional digital currency, Bitcoin source codes are open, and
anyone can see and understand how Bitcoins are generated.
The value of Bitcoin is directly proportional to its active users in the
network. Unlike the issuance of the currency by the centralized authority,
the Bitcoin is created by the mining process named Proof of work. After
the successful creation of the block, the miner is rewarded with few
Bitcoin. This incentive maintains the encouragement among the minors to
join the network, contribute their CPU energy, and get Bitcoin as a reward.
Transactions are hashed and encrypted using the secured hash algorithm or
SHA-256 if all the minor validate the transactions.
The first genesis block of Bitcoin generated when Nakamoto sent ten
Bitcoins to the noted programmer Finney and completed the first
transaction. First time Florida bought $25 worth two pizzas in the
exchange of 10k Bitcoin [49]. Now Bitcoin market cap is more than $210
billion [47]. Bitcoin is the digital currency generated in the distributed
nodes. Bitcoin transactions are stored in public shared ledger in a
distributed peer to peer network. The consensus process and the incentive
mechanism makes it the most popular cryptocurrency among investors.
The minors can participate in the competition individually, or they can join
the mining pool to get more probability to win. The investor can also buy
the goods from a merchant with an exchange of Bitcoin. Both the seller
and buyer can use the Bitcoin software to record the exchange of Bitcoin
and goods. The transactions are initiated, and their hash value is calculated.
The transactions are broadcasted throughout the network. The block is
created, and the transactions are combined into that valid block. The
timestamp is used to verify the existence of transactions to prevent double-
spending. Now the current block is hashed. The target is to find hash such
that hash must have a fixed number of zeros as the prefix. To achieve this,
the Nonce is used. This task is the difficulty to create the new hash of the
block [65]. This mechanism consumes the energy but also verifies the
block easily [66].
Bitcoin software can be downloaded on the computer and request a
wallet. In a blockchain network, nodes can validate transactions by solving
the complex algorithms, add them into their ledger, and broadcast the new
block to the bitcoin network. After every 10 minutes, a block which
contains the accepted transactions is created and added to the bitcoin
blockchain network. This block is published to all nodes, without requiring
any central oversight. Miners keep consistency in the network. The size of
the bitcoin network block is 1 MB. This bitcoin protocol includes a higher
level of security from the attacks like unauthorized spending, double
spending, race attack, history modification, deanonymisation of clients.
Bitcoin’s implementation of public-private key cryptography removes
unauthorized spending. For example; when A sends a bitcoin to B, B
becomes the new owner of the bitcoin. C observing this transaction want to
spend the bitcoin B just received, but C cannot sign the transaction without
the knowledge of B’s private key. Double spending is the case when a user
sends the same currency to more than one user. This situation is solved by
using a timestamp and tracking the transaction in the ledger in the
blockchain network. Race attack is when two transactions are created with
the same funds at the same time, intending to spend those funds twice.
Memory and computational power are required for proof construction. This
requirement of proof restricts the number of transactions to be validated in
Jaswant Arya, Arun Kumar, Akhilendra Pratap Singh et al.
a given period. Each mined block produces new currency units, the total of
which is finite. It is necessary to slow down the rate of production to
prevent untimely exhaustion. In the case of Bitcoin, the number of newly
produced bitcoins is halved every 210,000 blocks. Therefore, at most, there
can be 21 million Bitcoins [1]. The success of Bitcoin has inspired the
industry to create many other cryptocurrencies for a specific purpose or for
B. Bitcoin Cash
Bitcoin Cash is created as a solution for scalability. It has a block size
of 8 MB. The greater size of the block can have a higher number of
transactions. Bitcoin permits only one block to be created in 10 minutes
with a size of 1 MB. However, in Bitcoin Cash, the block is verified with a
size of 8 MB, i.e., a higher number of transactions, and thus it boosts up
the speed of transaction processing and saves the time to confirm the
transactions. Due to the constant block size and fixed average confirmation
time per block, Bitcoin has had very lower transaction throughput as less
than ten transaction per second [50]. Bitcoin Cash and Ethereum both have
higher throughput than Bitcoin, but still, their throughput is very less than
the traditional payment systems like Paypal and Visa [50].
C. Zilliqa
Zilliqa has the mechanism to divide the task of mining to the sharded
blockchain network. The blockchain network is divided into subnetworks
called a shard. The mining process is performed on this shard in parallel.
Thus the validation and verification can be done in parallel in the
effectively lesser time. It can get a higher efficiency if there are a very high
number of transaction to be mined [51].
D. ZCash
ZCash has higher security and privacy rather than Bitcoin. It has more
powerful cryptographic techniques zero-knowledge proof [53], [54].
USDT is designed to maintain the stability between the cryptocurrency
and the real currency of the country. This cryptocurrency is the best
alternative to the real national currency.
F. Litecoin
There are only a few differences between Bitcoin and Litecoin like
Litecoin uses the script and its average block creation time is 2.5 minute
and allows the faster block confirmation. Litecoin is more complicated
than Bitcoin.
G. Peercoin
Peercoin is based on proof of stake. Proof of stake includes additional
proof of work in its consensus algorithm and uses the double SHA-256
algorithm. Peercoin is the first cryptocurrency that used PoS. The coinage
is reflected as output, and therefore, the mining probability is higher for
those nodes which keep the older coin.
H. Permacoin
Permacoin uses the consensus algorithm proof of retrievability. It
requires clients to invest in large space, not just computational resources
Jaswant Arya, Arun Kumar, Akhilendra Pratap Singh et al.
like Bitcoin. Permacoin system involves an alternative scratch-off puzzle
based on Proofs-of retrievability (PORs). It requires large memory and
therefore, the designer of Permacoin proposed storage instead of CPU
cycles to secure the network for a new method to back up the data. Miners
have to solve the mathematical problem, and this mathematical problem is
called a scratch-off puzzle. It uses the considerable storage for
decentralization of files.
I. Ethereum
Ethereum is based on EtHash. EtHash is the hashing algorithm
developed by Ethereum developer. EtHash is ASIC resistant, and it has the
property of memory hardness, or it is more follower of the fastness of data
transferring in the memory. Ethereum uses proof of work. However, it does
not use the previous hash algorithm. It mitigates the problem of mining
centralization, in which a small mining operation can acquire a
disproportionately large amount of power to manipulate the network.
The current cryptocurrencies, according to their market cap, are listed in
Table III.
Blockchain technology has most successful application in the finance
sector as cryptocurrencies. However, there are many other sectors where
blockchain can be utilised. There are many research projects ongoing to
explore the adoption of blockchain for different applications. The critical
characteristics of blockchain show the use of blockchain in the Internet of
things, cloud and edge computing, big data, public and social services,
electricity and in many more fields. The attracted feature of blockchain is
distributed immutable information, and data is verified with the
decentralized consensus mechanism. The broad spectrum of blockchain
applications can be classified into three major sector- IoT, Big data, Cloud
and Edge computing. The other sectors are economics and markets,
business solution, smart contract and automation, traceability in supply
chains, medical informatics, communications and networking, and others.
A. Internet of Things (IoT)
The smart devices using IoT have shown their potential in the different
sectors. These devices are capable of working for a different purpose. Still,
the system is not capable of handling huge data and the problematic
complex computing mechanism such as described in [67-74]. The
electronic devices such as sensors are increasing in the number, but their
capability to collect and transmit the data is not efficient. The blockchain
can be applied to the distributed devices to increase the security and
privacy in IoT. Figure 10 shows the various sectors where the Blockchain
technology can be applied.
Figure 10. Different Sectors of Blockchain.
Jaswant Arya, Arun Kumar, Akhilendra Pratap Singh et al.
Table III. Cryptocurrency according to market cap [107]
List of Cryptocurrencies in Market
S. No.
Market Cap
$212.08 B
$24.14 B
$13.19 B
$5.68 B
Bitcoin Cash
$5.99 B
$3.88 B
Binance coin
$4.39 B
Bitcoin SV
$2.58 B
$4.04 B
$1.32 B
$1.51 B
$1.47 B
$1.62 B
$1.25 B
$949.28 M
$657.13 M
$757.59 M
$768.34 M
Ethereum Classic
$688.90 M
$559.87 M
$901.02 M
$485.88 M
$451.59 M
$545.49 M
$808.43 M
Bitcoin Gold
$282.64 M
27 Chain
$409.45 M
BasicAtt. coin
$277.42 M
$268.67 M
$352.57 M
$277.35 M
USD Coin
$433.62 M
$290.92 M
$203.36 M
$156.50 M
Bitcoin Diamond
$144.27 M
$151.33 M
$123.56 M
$135.06 M
$162.17 M
True USD
$201.09 M
S. No.
Market Cap
Pundi X
$125.73 M
$117.12 M
$143.72 M
$120.97 M
$106.96 M
$91.57 M
Huobi Token
$253.53 M
RIF Token
$66.47 M
$49.51 M
$47.91 M
$45.59 M
$43.99 M
$43.93 M
Ecoreal Estate
$43.61 M
$44.68 M
$43.44 M
$76.20 M
$122.43 M
$120.00 M
$119.03 M
$123.14 M
KuCoin Shares
$162.37 M
$108.51 M
$98.30 M
$97.58 M
Insight Chain
$102.89 M
Paxos Standard
$202.03 M
$104.78 M
$83.42 M
$265.24 M
$125.07 M
$78.85 M
$104.75 M
$68.14 M
$63.59 M
$108.54 M
$92.40 M
$178.72 M
$102.00 M
Nash Exchange
$108.25 M
Enjin Coin
$52.63 M
$64.70 M
Jaswant Arya, Arun Kumar, Akhilendra Pratap Singh et al.
Table III. (Continued)
S. No.
Market Cap
$48.99 M
$70.05 M
$66.82 M
$62.26 M
$80.60 M
$54.48 M
$136.63 M
$59.54 M
$77.12 M
$53.37 M
Project Pai
$41.57 M
Maximine Coin
$52.90 M
$64.42 M
$51.34 M
Clipper Coin
$52.15 M
$69.45 M
The blockchain technology can also be used to monitor the energy
resources and consumption using the blockchain-based energy resource
management application for cloud data centers. The authors [75], [97] has
investigated the security and privacy issue in IoT and proposed the solution
by creating the integrated framework of blockchain and IoT. The authors
[22] came up with the possible features of the blockchain and IoT. There
are many applications of blockchain in IoT. The authors [23], [24] focused
on the application of blockchain in IoT. The research work [25] has
categorized the applications and use cases of Blockchain for IoT.
B. Blockchain in Big Data
Initially, it was difficult to collect, store, and operate a large amount of
data, but the advancement of machine learning and data mining techniques
have paved the way for big data analysis [76],[77]. The rise of Big data
with IoT still suffers from several issues of security, privacy, and
centralized trust. The blockchain technology can be used to manage the
distributed data and decentralization of the data processing. Unlike other
techniques, blockchain provides the decentralized mechanism to store,
manage, and process big data. It solves the privacy issues and empowers
data security in Big data analysis [78]. The work [79] focused on how to
anonymize dataset trading using blockchain technology. They designed a
blockchain technology-based application using Hyperledger Fabric in
which the nodes, being data brokers, collects and verifies the transactions
in each data transfer. The blockchain-based platform achieves higher
security in data distribution and data trading market. The authors [81] has
negotiated the security issues in fog computing or centralized server-based
cloud system. The fog computers are located such that only secured, and
reliable data is sent to the centralized cloud systems. Still, drawbacks like
the shutdown and reprocessing in the fog computers create significant
issues to the cloud system. Blockchain technology brings out the security
features to share and control the authenticated transactions by digital
signature and consensus among fog computers. The research work [81]
designed blockchain-based fog computing platform to solve the security
issues of the centralized fog computing and cloud system. When a fog
computing is shutdown, the distributed consensus process of blockchain
successfully restores all the transactions to the fog computing.
Centralized database system suffers from security vulnerabilities like a
single point of failure, IP spoofing, and Sybil attack [54]. The work [80]
has proposed blockchain-based data management and searching method to
ensure security issues. The digital signature to authenticate the information
and check the user identity. The signature validation can prevent IP
spoofing. The authors [80] developed such a system that can locate the IoT
devices by having all the combined UUID that is resulted from gateway
and IP address assigned IoT devices. The transaction includes the name
with IP address and port number with the signature, and therefore the
transactions can be traced by the name and authenticated by the signature.
Jaswant Arya, Arun Kumar, Akhilendra Pratap Singh et al.
C. Blockchain in Cloud Computing and Edge Computing
Edge computing, the processing of the data at the network edge, has
shown its potential to improve the response time, battery life, bandwidth,
data safety, and privacy. The services of the cloud is pushed into the edge
network. The research work [82], [77] has listed the edge computing for
smart home, smart city, and the data sharing and collaboration between
networks of long-distance. However, it suffers from the challenges of
system integration, resource management, the programmability of edge
computing, naming mechanism, security and privacy issues in
transmission, storage and computation, etc. The authors [82-85] proposed
the cloud computing-based system for enhancing the cybersecurity for
large enterprise networks with reduced operational delays and can detect
threats by parallel cloud computation for both the signature and anomaly-
based detection. Cloud and edge computing both have a significant
application, but with blockchain the distribution mechanism and the
consensus process, the challenges of cloud and edge computing are
resolved. The service contract management of blockchain for cloud and
edge computing allows the programmability for the users. Blockchain for
the distributed centralized large datacenters brought the consensus and
decentralized layer.
The authors [86] proposed the cloud architecture based on blockchain
technology with fog computing and software-defined networking for the
efficient management of the data produced by the cloud and edge
computing. This proposed blockchain-based architecture provides high
security. Scalability and resiliency with the very low latency between the
computing resources and IoT devices. The cloud computing cost and the
number of trusted third parties can be reduced significantly by this
The authors in [87] have shown the data unavailability, higher
operational cost, and data security issues in the traditional cloud storage
and brought a blockchain-based model with keyword search services to
solve all these issues. The encrypted client data is distributed to the cloud
nodes to ensure the data availability using cryptographic algorithms with
the private keyword search feature for the owner. They built a system to
enable the outsourcing of data storage to a distributed network by using the
proof of retrievability consenses. Blockchain is used to process and prove
the anonymous credential with the private keyword search feature. The
work [88] has enlighted the edge devices less efficient for computational
resources and the available bandwidth leading to fog or cloud. By
measuring the network latency, they have shown that the use of fog and
cloud to make the IoT application with the blockchain technology.
D. Blockchain in Supply Chain and Logistics
The blockchain technology can distribute information throughout the
whole supply chain network with product tracking, traceability facility, and
improved quality, sustainability, and flexibility. The blockchain-based
supply chain platform can improve cost, time, and risk management. The
authors in [89] have presented the use of blockchain for increased
transparency and accountability for different phases of product
development. The blockchain technology brings out the powerful features
to trace the product with transparent, secure, and accurate information. The
smart contract can be used to automate the payment when a product is
returned to the issuer or seller. The supply chain can have real-time
tracking verified by consensus process and connect all the members on the
same platform.
In [91], the authors designed a blockchain-based system using proof of
concept consensus to prove the ownership of products along with radio
frequency identification or RFID to diminish the counterfeits in the supply
chain. The authors in [92] proposed a blockchain-based agri-food supply
chain traceability system using RFID. In work, they proved the traceability
and trust in the entire agri-food supply chain by transferring, processing
and distributing the authentic data [93]. The digital records created with the
blockchain, RFID, and IoT can deduce the conflicts and solve the
judgment in the supply chain. All the critical information about materials,
Jaswant Arya, Arun Kumar, Akhilendra Pratap Singh et al.
transaction processes, product quality, location, and quality is maintained
on the blockchain-based supply chain process.
The authors [89] have described the problems in product deletion
management in the current supply chain operations like origination,
operation, manufacturing, logistics, and reverse logistics. They built
blockchain-based product management to empower product deletion and
E. Industry Perspective
The traditional industry uses the intermediaries for registering the
transactions, confirming and removing the duplicate transactions. The
blockchain-based platform can perform these task of registering,
confirming, and storing the transactions with higher efficiency in minimum
time. Blockchain can be used for the insurance policies, verifying the
claims via smart contract to reduce the overall cost.
Figure 11. Challenges of Blockchain technology.
Blockchain technology can be used in a broad spectrum of applications
to secure the data with increased reliability without requiring any central
trusted authority. It has attracted the researchers due to its key
characteristics decentralization, immutability, consistency, and security.
Though, it still suffers from a few challenges like scalability, storage
management, consensus performance and resources management, etc.
Figure 11 shows the significant challenges of Blockchain technology.
A. Scalability
The prefixed block size and block creation time are efficient for a fixed
number of transactions processing, but a high number of transactions can
cause slower transaction processing. Several blockchain applications are
suffering from the scalability issue such as Bitcoin block size is 1MB and
average block confirmation time is 10 minutes, whereas Ethereum has the
average block confirmation time 16 seconds [94-95]. For the high number
of transaction processing, the block confirmation time must be low, but to
have the security from being attacked by the attacker, this average time
should be high. To solve this problem, there are a few solutions proposed
by the researchers. The authors [7] proposed the solutions like on-chain to
change on main, off the chain for change on the main chain after
transaction processing. Also, side chain to change the assets of the
different side chain, child chain to record the result in parent chain and
interchain for the communication between the chains. In [97], the authors
have proposed solutions like lighting protocols, sharding, etc. This work
[100] has analyzed the better performance of Ethereum by managing the
scalability issues.
Jaswant Arya, Arun Kumar, Akhilendra Pratap Singh et al.
B. Security and Privacy Issues
There is an urgent need to research for tor offers and beyond tor such
that the privacy issues can be tackled in blockchain [97]. To create and
manage the digital identities, the rules and policies are to be restricted to
keep the privacy for controls and ownership while accepting the
blockchain model.
C. Storage Management
Blockchain ledger contains all the transaction starting from geniuses
block to the latest mined blocks with all the side and interchains. The
blockchain provides higher security by storing all transactions in a ledger.
This ledger is distributed to each node of the blockchain network. This
redundancy spends nonpredictable storage. The storage management
depends on the activity of the nodes of the network.
D. Consensus Performance
There are several consensus algorithms proposed for different
applications. The consensus for a specific purpose can create scalability,
efficiency, and privacy issues. The efficiency of the consensus process
decreases when the number of nodes in the network increases.
E. Resource Management
The resources used to mine the block are less in the beginning as there
are very fewer transactions, but for the vast number of transactions, the
spent resources are very high. If miners spend the resources, they must get
a satisfactory reward. Therefore it becomes a challenge to maintain
compatibility between the miners and the spent resources. Resources
consumption should be managed effectively.
F. Conditions Inflexibility
The smart contract is designed in a specific programming script
languages that do not provide comfortable functions to the programmer.
The programmers have shown the indeed requirement of the function to
get the flexibility to create the programs so that the contract evaluation
may be easier. While creating a smart contract, the complex understanding
and compilation process may create unpredictable worst results.
Inflexibility restricts to test the contract against the bugs and prevent the
attacks. The authors [101] has shown the hacking event of the smart
contract built on Ethereum [102-103], and the work [104] have shown the
vulnerabilities and the possible improvement while creating the smart
G. Lack of Governance, Standards, and Regulations
It is indeed a requirement to standardize the blockchain for its
integration, interoperability, governance, and sustainability, etc. The
blockchain development must follow the rules, laws, policies, and
regulations of the government. It is challenging to manage the governance
of the blockchain platform among different participants. The authors [105]
has raised the questions on the blockchain standards and standardization
importance to maintain sustainability and trust. This work [106] has
reviewed the blockchain regulation and has measured the performance
Jaswant Arya, Arun Kumar, Akhilendra Pratap Singh et al.
This chapter has presented the underlying architecture, mechanism,
applications, challenges, and opportunities of the blockchain technology.
The chapter has described the key characteristics of blockchain, such as
decentralized, immutable, distributed, and secured. The comparison
between different versions and variants of the blockchain technology as
well as the fundamental difference between the various consensus
algorithms has been described in details in this chapter. The basic principle
of cryptocurrencies and their applications are presented for naive readers.
Moreover, the chapter has depicted the current research and industrial
challenges to adopt the blockchain for different application in terms of
scalability, privacy and security, storage and resource management, and
lack of governance and standardization.
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Full-text available
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Full-text available
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Conference Paper
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