Blockchain technology and internet of things: review, challenge and security concern

International Journal of Electrical and Computer Engineering (IJECE)
Vol. 13, No. 1, February 2023, pp. 718~735
ISSN: 2088-8708, DOI: 10.11591/ijece.v13i1.pp718-735  718
Journal homepage: http://ijece.iaescore.com
Blockchain technology and internet of things: review, challenge
and security concern
Mahmood Subhy Mahmood1
, Najla Badie Al Dabagh2
1
College of Science, University of Mosul, Mosul, Iraq
2
College of Computer Science and Mathematics, University of Mosul, Mosul, Iraq
Article Info ABSTRACT
Article history:
Received Dec 8, 2021
Revised Jul 14, 2022
Accepted Aug 19, 2022
Blockchain (BC) has received high attention from many researchers recently
because it has decentralization, trusted auditability, and transparency as its
main properties. BC has contributed fundamentally to the development of
applications like cryptocurrencies, health care, the internet of things (IoT),
and so on. The IoT is envisioned to include billions of pervasive and
mission-critical sensors and actuators connected to the internet. This network
of smart devices is expected to generate and have access to vast amounts of
information, creating unique opportunities for new applications, but
significant security and privacy issues emerge concurrently because it does
not contain robust security systems. BC provides many services like privacy,
security, and provenance to the systems that depends on. This research
includes analyzing and a comprehensive review of BC technologies.
Moreover, the proposed solutions in academia with the methodologies that
used to integrate blockchain with IoT are presented. Also, the types of
attacks on blockchain are collected and classified. Furthermore, the main
contributions and challenges that are included in the literature are explored,
then the relevant recommendations for solving the explored challenges are
proposed. In conclusion, the integration of BC with IoT could produce
promising results in enhancing the security and privacy of IoT environment.
Keywords:
Blockchain
Blockchain attacks
Blockchain with internet of
things
Consensus algorithms
Mining
This is an open access article under the CC BY-SA license.
Corresponding Author:
Mahmood Subhy Mahmood
College of Science, University of Mosul
Mosul, Iraq
Email: mahmoodsubhy1981@gmail.com
1. INTRODUCTION
Nowadays, cryptocurrency plays an important role in academia and industry domains. Bitcoin
considers one of the most common cryptocurrencies which has gained a substantial success with its
investment market represented by 220 billion dollars in 2020. It is estimated that it could reach 3 trillion
market caps by 2025 as shown in Figure 1 [1].
With a special structure of data storing, transactions in Bitcoin could occur in a decentralized
manner and the essential technology to create Bitcoin is the blockchain [2]. Blockchain technology is defined
as "a distributed ledger technology (DLT), which secures and records transactions in a peer to peer (P2P)
network instead of using single or numerous servers" [3]. When any malicious or hacker attempts to the
delete or alter the data stored in a blockchain, this action can be detected because there are multiple replicas
of the same ledger distributed over several locations [3]. Commonly, the blockchain has the following
characteristics (persistency, decentralization, auditability, anonymity, and redundancy). With these
characteristics, blockchain can really improve the efficiency and save the cost associated with developing the
applications based on it [2], [3]. Blockchain use-cases tried to explore in different applications such as

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Blockchain technology and internet of things: review, challenge … (Mahmood Subhy Mahmood)
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financial services, legal services, health care, manufacturing, supply chain, retail services, transport and
tourism, and governments [3], [4].
Figure 1. Sizing bitcoin’s opportunity [5]
The blockchain provides several services to systems that depend on it like security, privacy,
provenance and others. Currently, these services are provided to the internet of things (IoT) by reliable
brokers or by using incompetent distributed methods. So, security and privacy are the highest challenge for
existing IoT applications. Alternatively, the blockchain technology can offer security promises to resolve the
traditional challenges that mentioned above. Furthermore, it is offering a provably secure, fully distributed
and consensus solutions. Although, the blockchain has infinite potential for building the upcoming internet
systems such as IoT, it is facing several technical challenges such as scalability, mining in restricted resource,
blockchain operations which produce overhead traffic with limited-bandwidth in IoT devices [2].
Network of the IoT includes smart devices that produce, process, and interchange massive quantities
of sensitive information. It will become attractive targets of many cyber-attacks [5]. The security and privacy
hazards of IoT are exacerbated by the absence of fundamental security defenses in many of the first
generation of IoT products. Various security vulnerabilities have been recognized in connected devices such
as vehicles, smart locks [6] and smart home [7].
Several essential features of IoT increase its privacy and security challenges including: context-
aware and situational nature of risks, heterogeneity, attacks, and scale [8]. Traditional blockchain is
unsuitable for the IoT, because of IoT devices regularly want to operate with less bandwidth and
computational power consumption. In addition to that, blockchain (BC) is considered the initial phase of
development. Therefore, various researches in this domain are applied to enhance its efficiency [4].
Investigation of privacy and security for IoT by utilizing blockchain technology is getting a lot of attention in
the research community as we will discuss later.
In this paper, the essential contributions are demonstrated: i) all the blockchain's fundamentals and
architecture have been explained in details, ii) blockchain attacks have been listed and summarized, iii) the
role of blockchain in IoT is to encounter security and privacy challenges and solutions have been discussed.
The rest of this paper includes: section 2, which shows the blockchain's fundamentals that include
(architecture, "pros and cons," challenges, types, attacks and applications of blockchain technology).
Section 3 discusses the integration of blockchain with IoT. From the other hand, section 4 includes the
conclusion and future works.
2. BLOCKCHAIN TECHNOLOGY
In this section, the architecture of blockchain is explained. Consensus algorithms, platforms and
types are presented. Also, pros and cons, challenges and applications have been investigated.
2.1. Blockchain architecture
A blockchain technology consists of a network of nodes and a database, as shown in Figure 2. A
blockchain database is a distributed ledger, shared, append-only, fault-tolerant which involve certain numbers
of the transactions (i.e. transaction is a data unit on the BC) in blocks. While the blocks are shared by all
nodes in the blockchain, users cannot be altered or deleted. Blocks that are verified are linked together in a
chain by using cryptographic hash value and every block has a hash value of the previous block. Each block
holds some of confirmed transactions. Moreover, each block includes a timestamp that signifies the time of

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the block creation and (nonce) a random value for cryptographic tasks. The blockchain network includes
nodes, which are distributed in a peer-to- peer manner [8].
Figure 2. Blockchain network, database, blocks, and transactions [9]
2.1.1. Building blocks of blockchain
Many methods are used to build the blocks of blockchain. Asymmetric-key cryptography, digital
signature, cryptographic hash function and others, are all explained. The following subsections include the
details of these methods.
a. Asymmetric-key cryptography
Asymmetric-key cryptography utilizes a couples of keys: private and public keys that are related to
each other mathematically. Asymmetric-key cryptography supports a trust links among users who do not
trust or know one another, by using a ‘digitally signed’ to confirm the authenticity and integrity of
transactions and to allow the transactions to remain public. A disadvantage of the asymmetric-key
cryptography is slow to find hash value [10]. Each transaction has a unique digital signature, which is based
on user's private key. Moreover, when a user receives the message, a digital signature and a public key of the
sender, it will be easy to confirm if the digital signature is valid [4]. The signature must have the ability to
offer non-repudiation, authenticity and integrity [11]. A process of applying a hash function to documents is
called Hashing, which provides a unique outcome (digest) for any size of input of (i.e., image, text, and file).
Many blockchain implementations are used an exact cryptographic hash function (secure hash algorithm
(SHA-256)) with a 256 bits output size [10].
b. Peer-to-peer network
In a peer-to-peer (P2P) network, control and accountability disseminated for many various peers,
which enhances the security of the network. BC uses a P2P distributed ledger to get rid of the hazards
associated with the centralized database by saving the data in network and to allow the access of everyone
(i.e. joining in an overlay network). When a node is connected to this network, it gets a full copy of the BC
that can be utilized later on to check that everything is in order. A node might be an electronic device such as
a computer, a smart printer, a smart phone, or even a TV, as long as it is connected to the internet. All the
nodes on the BC are equally important. Moreover, each node has various tasks in constructing a BC. Nodes
and the roles that play can be classified as the following [12]:
− Light node: It holds portion of the information, which is recorded on a BC.
− Full node: It stores an instance of the information recorded on a BC.
− Mining or forging node: It Processes the transactions, puts them in the current blocks, add blocks to a BC,
approve and broadcast the block that was joined to the network. These nodes collaborate to administrate,
secure and to expand the blockchain.
c. Block structure
Every block consists of a header and data. The header of block contains metadata. The data of the
block contains a group of authentic and validated transactions, which are brought when they are submitted to
the BC [10]. The block header includes many fields, which will be investigated in the following subsections
[11]:
− Height: Every new block is assigned an order number. The height is calculated by subtracting the first
block number from the last block number.

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− Header hash: It is a basic block identifier. Hash function operates by means of using the header of the
block as an input. It is not sent with the block but it is calculated upon receiving the new block.
− Previous block's hash: It allow the block to link with the preceding block, as we see in Figure 3.
− Nonce: A number is utilized to change the header hash value. By combining it with the difficulty field-
which will be explained later – it is utilized to verify whether a miner has achieved a work.
𝑑𝑖𝑔𝑒𝑠𝑡 = ℎ𝑎𝑠ℎ (𝑑𝑎𝑡𝑎 + 𝑛𝑜𝑛𝑐𝑒)
− Difficulty: It is a fractional hash value collision, and therefore, it based on the computational power of the
mining node to compute the hash value that fulfills this fractional collision. The miner will modify the
difficulty until reaching the fractional collision.
− Transactions: They mean the transmission of data (i.e. data unit of the blockchain).
− Merkle trees: It is a structure as binary tree that encapsulates the data and permits it to be tested securely
and efficiently within a huge dataset. The transactions in Merkle trees are packed as shown in Figure 4
[13].
Figure 3. Simplified structure of the block [11]
Figure 4. Blockchain structure example [13]
2.1.2. Mining
Mining is the process, which is responsible for updating the BC [11]. In a P2P network, the
mem_pool is a space that is assigned in the full node memory, which saves and relays the transactions to
other nodes. In order to update the status of the BC, certain nodes in the network, called the verifiers or
miners, which verify the transactions and compute (cryptographic calculations that are very complicated and
they need huge quantities of storage space and power) a block. The miners select certain transactions from

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the mem_pool that will be put in the blocks. Transactions pay a fee of mining, which can be regarded as an
impulse for the miners to so that the transaction can be mined by it. Normally, priority is given by the miner
to the transaction that pays a higher fee. Transactions that are not chosen by the miners remain in the
mem_pool till another one chooses them for a new block. Otherwise, transactions will be discarded [14].
2.1.3. Chaining of blocks
When the block is filled up, it is broadcasted across the network by the sender's node and then
mining must be achieved. After that, the block becomes attached concurrently to the former block to make a
ledger. This process goes on for a countless number of epochs to make a never-ending series of blocks [15],
as shown in Figure 5.
Figure 5. Block chain [15]
2.1.4. Addresses and address derivation
Each blockchain networks has an address, which is a short string of alphanumeric characters derived
from the user's public key by a cryptographic hash function, along with certain additional information (i.e.
checksums and version number) [10]. Most of the implementations of the BC make use of addresses as the
“from” and “to” endpoints in a transaction. In general, addresses are shorter than the public keys and they are
not secret. There is a method to generate an address, which is by creating a public key to which a
cryptographic hash function is applied to it and the hash is converted to a text [10].
𝑝𝑢𝑏𝑙𝑖𝑐 𝑘𝑒𝑦 → 𝑐𝑟𝑦𝑝𝑡𝑜𝑔𝑟𝑎𝑝ℎ𝑖𝑐 ℎ𝑎𝑠ℎ 𝑓𝑢𝑛𝑐𝑡𝑖𝑜𝑛 → 𝑎𝑑𝑑𝑟𝑒𝑠𝑠
Each blockchain may implement a different method to derive the address. For public BC networks,
which allow anonymous account creation, the user of the BC network can generate a considerable number of
asymmetric-key pairs, and therefore addresses as desired. This allows a varying degree of pseudo-anonymity.
Addresses could act as the public-facing identifier in the BC network for a user, and often an address will be
converted into a QR code for the purpose of easier use by mobile devices [10].
2.2. Consensus algorithms
One of the key aspects of BC technology is determining which node publishes the next block. This
is solved by implementing one algorithm amongst many possible consensus ones. For public BC networks,
generally there are many publishing nodes, which compete at the same time to publish the next block. They
usually do this to win cryptocurrency and/or transaction fees. They are generally distrusted nodes, who could
recognize each other by their addresses [10].
When a node joins a BC network it should agree on the initial state of the system. This is recorded
in the only pre-configured block, the genesis block (i.e. the first block in the blockchain). Every BC network
has a published genesis block and every block must be added to the BC after it, based on the consensus
algorithm agreed-upon. Regardless of the algorithm, however, each block must be valid and accordingly it
can be validated in an independent way by each node of the BC. By combining the initial state and to verify
every block, nodes can independently agree upon the current state of the BC. From the other hand, if there
were two valid chains provided to a full node, the common mechanism almost all BC networks is that the
‘longer’ chain is regarded as the correct chain, which is to be depended; due to having been worked on more
[10].
A major characteristic of the BC technology is that no third party is required to provide the status of
the system. Each node in the system can test the integrity of the system. All nodes must be in a common
agreement to add a new block to the BC. However, some temporary disagreement is sometimes permitted.
There are many consensus algorithms based on the BC implementations [3]. In Table 1, the basic
consensus algorithms are briefly discussed. Every BC consensus attempts to achieved three important
properties, which are (safety, liveness and fault tolerance) that can be implement efficiently [16].

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Table 1. The basic consensus algorithms of the blockchain
Consensus
algorithm
Description
Proof of work
(PoW)
In 1999, Markus Jakobsson proposed the PoW. Mining nodes that utilize this algorithm requires to resolve a
complex-mathematical processes that is altered repeatedly and should have been decided by all the miners.
The decision here depends on a common consensus. The problematic with this algorithm is that it wasted a
high power of computation [9]–[11], [14]. Furthermore, it is characterized with a great latency to confirm the
present transactions [3].
Proof of stake
(PoS)
PoS is essentially a generalized form of the PoW. The terms validators are used instead of miners, and they
(the minors) are called to the nodes to confirm the transactions [3]. Unlike the PoW, the PoS does not need
the mining to calculate the hash value. Instead, the next block creator is selected in a random manner. The
chance of a node actually selected to build the new block is based on the stake of node [9], [10], [14], [17].
The PoS keeps great computational resources compared to the PoW [3].
Practical
byzantine
fault
tolerance
(PBFT)
PBFT is used widely in the private Blockchains, when the network has a higher trust model different from
the PoS and PoW. In practical byzantine fault tolerance, the network is rearranged into a cluster of active and
passive replicas. A primary replica is specified from active replicas. PBFT process includes four phases: (the
pre- prepare, prepare, commit and the reply phases), as shown in Figure 6. Compared to PoW and PoS, the
PBFT has a greater message density [11], [14].
Moreover, there are another consensus algorithms, which can be used in a specific application in the
Blockchain such as proof of elapsed time (PoET) [11], proof of space [13], proof of importance [13],
measure of trust, minimum block hash [9], leased proof of stake (LPoS) [13], delegated proof of stake
(DPoS) [3], [13], proof of bandwidth [13], proof of authority (PoA) [13], [17], and round robin consensus
model [10].
Figure 6. Overview of PBFT protocol [11]
2.3. Blockchain platforms
There are many platforms related to the BC technology that could be used as primary approaches to
construct a wide variety of software such as [18]:
a. Bitcoin [19]: It was the leading BC that was theorized and applied and the cryptocurrency that works as a
digital financial asset. The Bitcoin utilizes a P2P networking, public key cryptography and a PoW for the
purpose of making transactions as well as verifying them. The system of the Bitcoin is programmed so
that a fresh block can be created one time each ten minutes [18].
b. Ethereum [20]: It was proposed in 2013 by the developer of Bitcoin Vitalik Buterin. Ethereum [5] is an
open-source, public, blockchain-based distributed computing platform featuring smart contract
functionality. it utilizes the PoW as its consensus algorithm and mechanism, but it quickly switches to the
PoS. The basic building of the proof of work algorithms is that the Ethereum currently used is a memory
hard hashing algorithm called Dagger-Hashimoto. The time for creating the block is considerably less
compared to the time consumed in several other systems and it is about 12 seconds. As the time of
creating the lower block results in a higher rate of stale blocks, therefore the system uses the GHOST
protocol to take the heaviest computational chain as the main blockchain. The heaviest chain in this case
includes the stale blocks too [18]. As the main platform is a network, which is free for all, a software can
be downloaded by anybody or use it in his/her computer. The incentive mechanism for running the
software is represented by to getting ether (i.e. a digital currency). While the main platform of Ethereum
is a free BC network, the software is an open-source and it permits the software developers to make the
network a private one, where nodes participating are only those which were given permission [20].

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c. Rootstock: it is a fresh open-source platform, which is like the Ethereum in terms of making a smart
contract, except the fact that it uses the bitcoin. The benefits of this platform are that it occurs as a bitcoin
and it is compatible with the Ethereum. So, all Ethereum contracts can simply run-on rootstock. The most
important advantage, however, is the fact that they can be merged-mined with bitcoin and thereby making
rootstock as secure as possible [18].
d. Hyperledger fabric [6]: it is the implementation of the private (permissioned) blockchain technology that
is employed as a foundation for developing the BC applications that are hosted via the Linux for a wide
variety of industries. Therefore, its architecture is modular, which allows components such as consensus
and membership services to be plug-and-play. It leverages the container technology (docker) to enable
smart contracts called “chaincode”, which comprises the system application logic. The Hyperledger fabric
is an open-source distributed ledger software, which is built and maintained by the Hyperledger
community. It is a collaborative effort aimed at developing the cross-industry blockchain technologies
[20]. Moreover, it is uses the PBFT as a consensus algorithm instead of the PoW algorithm. PBFT can
process thousands of requests every second with a latency of increase of less than a millisecond [18].
2.4. Types
Blockchain could be classify to three main types. They are public blockchain, private blockchain
and consortium blockchain. The details of these types are listed in Table 2 [11].
Table 2. Types of blockchain
Blockchain type Description
Public Blockchain
(Permissionless)
Decentralized ledger platforms can be used by anyone to broadcasting blocks in it without the need to
obtaining the approval from the authority site. As permissionless blockchain is exposed to any participant,
a hacker may try to broadcast blocks in a fashion that disrupts the system. To inhibit this, the
permissionless blockchain repeatedly use a consensus algorithm [10]
Private Blockchain
(Permissioned)
In the private blockchain, users who broadcast the blocks need be certified through a certain authority site
[10]. Private organizations could use this type of Blockchains [4]
Consortium
Blockchain
The greatest consortium blockchains are semi-decentralized. More than one party can access the
blockchain instead of only one part by means of regulating protocol [4]
2.5. Pros and cons of the blockchain
Although blockchain technologies have many important characteristics such as decentralization [3],
validity [16], transparency [3], anonymity and identity, redundancy [3], auditability [16] and immutability
[3], it has many pros and cons, which are explained as Through the nature of the blockchain design, the pros
that are contracted by implementing a blockchain solution are [3]:
a. Distributed: The BC has many nodes that are distributed over the world (data availability). e.g.,
Ethereum.
b. Transparency: Data are distributed on a public manner and other concerned node and manager can easily
get it.
c. Security: It is a main subject of the digital world of nowadays. The certified documents and the
transactions are executed and constantly stored in the blocks, which cannot be altered or deleted by
anyone (data integrity).
d. Trust: Participants in this blockchain are the ones who decide the transactions to be added before inserting
them in the blockchain. So, trust becomes higher in terms of altering, writing or even reading the
information.
e. Efficiency: In the blockchain technology, efficiency of a network can be improved when the financial
groups collaborate.
f. Resilience: If a huge number of nodes, the strength of information is improved with extended life.
On the other hand, the blockchain has some cons which are [3]:
a. Block size: In the blockchain, each block that is inserted in to the blockchain increases the size of the
database.
b. Speed and cost of the network: It is hard to manage all the nodes in a BC once the node numbers become
higher.
c. Wasteful: Every node has to continue the consensus alg., which provides the mistake tolerance and
ensures zero interruption. Then, they are totally wasteful since every node tracks the relevant task to grasp
consensus. So, the chance of calculating a valid nonce rise according to the computing power that the
faster computers have.
d. Standards: Because blockchain is at its initial age, there are no definite standards.

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e. Performance: Compared to the centralized database, the blockchain is slower, because the blockchain
performs all the operations of a traditional database system in addition to various additional loads like.
Consensus algorithm, signature confirmation and others when it executes the transaction. In order to mitigate
the drawbacks mentioned above, efforts were made to improve the protocol speed and the efficiency with a
special care to the algorithms associated with the limited access and consensus algorithm [21].
2.6. Challenges
Based on the literature, the blockchain technology encountered many challenges. Some of these
challenges are related to security, attacks, power consumption and so on [3]. The most common challenges of
blockchain with their details are presented in Table 3.
Table 3. Challenges of blockchain
Challenge Description
Private key security A secret and single key is given to each participant in the blockchain (private key). Now, a malicious
can catch the user's private key. It is hard to detect the hacker's behavior since the blockchain is
decentralized.
Privacy leakage An assured degree is reserved to keep the privacy of the users contained in the transactions.
Criminal activities Currently, criminal activities are growing in BCs. As for bitcoin, customers are provided with
addresses, which are not related to users' identities. So, the events that are made via bitcoin is greatly
hard to be tracked.
Attacks Many malicious users and attackers attempt to attack a node or a network by conducting several hacks,
as we discussed in the sub section 2.7.
Standardization Increasing the number of nodes from dissimilar networks, in the blockchain no standard to permit the
customers to cooperate. The absence of standardization permits developers or coders to prepare
everything as they hope and this makes issues for the IT.
Environmental cost Implementation the blockchain technology in any company, needs definite software and applications
that requires to be established via a software organization. Thus, it is acquisition is costly. Also, the
company may have not the fixed hardware for the usage of the software.
Energy consumption Allows the users to operate in the blockchain as some difficult algorithms need to be achieved and their
outcome is a large depletion of power.
Slow and cumbersome The blockchain technology is not fast to perform a transaction related to the traditional fee system (e.g.,
cash or debit card), because the blockchain achieves more complexity and encryption operations.
Public perception People must identify the variances amongst blockchain technologies before adopting the blockchain
technology as it can be so useful to eliminate the bad idea about blockchain technology in its own.
2.7. Blockchain's attacks
Blockchain is subjected to many attacks. Because the decentralized nature of its operational
environment, hackers have conducted numerous multipurpose attacks against blockchain technology by
exploiting the vulnerabilities of (structure of the blockchain, peer-to-peer system and applications) [4], [14],
as it is shown in Figure 7. In the following, we summarized some attacks against to the structure of the
blockchain, peer to peer system, and the applications of blockchain [14].
a. Forks [22]: It exploits the blockchain structure by chain splitting and revenue loss and this in turn affects
the blockchain.
b. Stale and orphaned blocks [23]: It exploits the blockchain structure by revenue loss that has an effect on
the blockchain, miners and mining pools.
c. Selfish mining [24]: It exploits the blockchain's peer-to-peer system by revenue loss and malicious
mining that have an effect on the blockchain, miners and mining pools.
d. Majority attacks (51% attack) [25]: It exploits the blockchain's peer-to-peer system by chain dividing,
malicious mining and revenue loss that impact the blockchain, miners and application.
e. Domain name system (DNS) hijacks [14]: It exploits the blockchain's peer-to-peer system by revenue
loss, partitioning and information theft that have an effect on the miners, exchanges, mining pools and
users.
f. Border gateway protocol (BGP) hijacks [26]: It exploits the blockchain's peer-to-peer system by revenue
loss, partitioning and theft information that effect on the miners, mining pools and users.
g. Eclipse attack [27]: It exploits the blockchain's peer-to-peer system by partitioning that effect on the
miners and users. A set of hacker nodes separates its bordering nodes utilize internet protocol (IP)
addresses, thus compromising their received and leaving traffic.
h. Distributed denial-of-service (DDoS) attacks [28]: It exploits the blockchain's peer-to peer system by
malicious mining and information theft that have an effect on the blockchain, miners and mining pools. In
bitcoin network, 51% attack could lead to denial-of-service (DoS).
i. Block withholding [29]: It exploits the blockchain's peer-to-peer system by revenue loss and malicious
mining that impact the miners and mining pools.

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j. Finney attacks [14]: It exploits the blockchain's peer-to-peer system by revenue loss, which affects the
miners, mining pools and users, by creating an identical to the preceding transaction and drives it to a
receiver. After the receiver gets the transaction and brings the result, the miner publishes the preceding
block with the basic transaction in it.
k. Consensus delay [29]: It exploits the blockchain's peer-to-peer system by the delay and info loss that have
an effect on the miners, mining pools and users. A hacker might insert false blocks to increase the latency
or to prevent peers from achieving the consensus success around the state of the blockchain.
l. Timejacking attacks [30]: It exploits the blockchain's peer-to-peer system by delaying the malicious
mining, chain splitting and revenue loss that affect the miners, mining pools and application. A hacker
can calculate a new block and put its timestamp forward of network’s timestamp with the values of
50 minutes.
m. Blockchain ingestion [14]: It exploits the blockchain's applications by info loss that affects the
blockchain. The examination of the public blockchain could expose beneficial information to an
opponent.
n. Double-spending [31]: It exploits the BC's applications that impacts the blockchain and the users, by
using a one-time transactions several times.
o. Cryptojacking [32]: It exploits the blockchain's applications by chain splitting and malicious mining that
effect on the application and users.
p. Wallet theft [33]: It exploits the blockchain's applications by revenue loss and theft that impact the
exchanges, application and users. Associated keys with peers in the network are saved in a digital wallet,
the “wallet theft” attack get up with sure associations on the applications.
q. Smart contract DoS [14]: It exploits the blockchain's applications by revenue loss, delay and theft that
affect the blockchain, application and users.
r. Reentrancy attacks [14]: It exploits the blockchain's applications by revenue loss and theft and this in turn
affects the application and users.
s. Replay attacks [14]: It exploits the blockchain's applications by revenue loss and info loss, which impact
the blockchain, mining pools, application and users, including the creation of one transaction and send it
to dissimilar two blockchains.
t. Overflow attacks [14]: It exploits the blockchain's applications by theft that affects the application and
users, when the type adaptable value exceeds (2^256).
u. Short address attacks [14]: It exploits the blockchain's applications by revenue loss and theft that impact
the application. Abuses a bug in Ethereum’s virtual machine to create additional tokens on boundless
consumptions.
v. Balance attacks [14]: It exploits the blockchain's applications by revenue loss and theft, which impact the
application and users.
Figure 7. Attacks on blockchain based on vulnerabilities
2.8. Blockchain applications
Nowadays, a great number of corporations earn their profits means of employing BC, as it could be
the most suitable solution, if the following conditions are met [3]: i) when the shared common database is
needed, ii) when trust is not reciprocated by participants, iii) when the database is common for various
writers or parties, iv) when the system or the network is subjected to hackers and malicious, v) when the

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same rules are applied to all the participants in the system, vi) when there is transparency in the result of the
decision making for all the participants, and vii) when the transactions are no more than 10,000
transactions/second. There are many sectors used a BC, some of them are:
− Business: Recently, organizations are in no need to a third party or a host to safeguard their assets [3]. In
particular, financial and healthcare are accustomed to encounter security problems because of the
malicious users. By using the BC this issue can be settled.
− Supply chain: Almost all the organizations possess enterprise resource planning (ERP) and supply chain
administration software to ensure that the operations move smoothly [34]. But the restricted details and
the visibility related to products are two essential elements, which become vital with the increase of the
product number. Therefore, BC is a good solution that can follow every item in the organization by means
of the process of the supply chain and can also fulfill a powerful security means. To enhance the product
safety, several records should also be updated, like the ambient conditions at each stage that minimizes
the loss or harm inflicted on products when shipping. The BC is also used to achieve an update and
replacement can be performed during the lifetime of any device or product.
− Copy rights: Because of the insufficient transparency, the information of multimedia like photos and
music. Encounter the copyright problem when trying to specify the valid owners to use them properly.
Authorized owners are not capable of controlling their documents on the internet. A great number of
hackers merely by copying the contents of the documents in an unauthorized way and distribute them via
the internet. The BC mitigate the issue mentioned earlier by enhancing the information availability
concerning the ownership of copyrights. This sort of information is provided as “trusted timestamping”.
Hence, any timestamp will be identified as an encoded information, which show the time and date of
occurrence. So, a trusted timestamping "is a process that takes place to track the modification securely in
addition to the creation time of any document".
− Electricity management: In the developed states, management of electricity is considered as an essential
concern as the user 's information of electricity is often leaked. Since the number of electricity users is
tremendous, it is hard to manage the system entirely. So, the BC is utilized to resolve the above problem
by adopting private BC and smart contracts [35].
− Distributed storage: Currently, cloud data storage service is one of the popular services that is used by
numerous users. However, one of the significant disadvantages of cloud-based service is that it is
centralized and the cloud service provider (CSP) controls all the processes [36]. Occasionally, the CSP
uses the users’ confidential data illegally to obtain revenue even without notifying the user, and therefore,
users’ data may be at hazard. The BC data storage service CSP minimizes the above problem through its
decentralized characteristic [37]. Hence, the users store their unmodifiable data.
− Digital identity: In the present time, every state is considering the digital identity. so, the digital identity is
used in national security, banking industry, healthcare services, citizenship documentation and online
retailing. Several states spend considerable amounts of cash in the digital identity field. Occasionally, a
digital identity is misused or hacked by malicious users. Therefore, BC can resolve this problem by
managing and tracking the digital identity in a secure and efficient way. In this case, the identity is
authenticated in a secure and an immutable way. Instead of using system that is based on the password, in
BC, identity will be verified by using the digital signature that depends on the cryptography (public key).
− Autonomous organizations: BC is utilized to create decentralized companies through making several
smart contracts. These contracts adhere to an interaction in a specific protocol.
2.9. Integration blockchain with IoT
Cryptocurrency and financial transactions have firstly used blockchain where all nodes in the
blockchain execute and store all the transactions. Also, the blockchain provides many benefits because it
could be adapted with many domains and one of the common domains is IoT [38]. There are many networks
smart devices, which construct the IoT such as Raspberry, ESP and so on. IoT interconnects heterogeneous
objects and smart devices seamlessly to create a network, which is used for sensing, processing and
communication processes. IoT smart devices are managed and controlled automatically without the need to
human interventions and they consume low energy and have a lightweight process. According to Statista
Com [39], the number of IoT objects in 2020 is estimated to be 31 billion devices worldwide. By the end of
2025, this number is predicted to increase to be 75 billion devices [40], as shown in Figure 8.
In IoT, the smart devices have to spend the largest portion of their energy and execute the process to
achieve vital application tasks, which makes achieving the privacy and security tasks fairly challenging.
Malignant attacks can prevent IoT services in addition to threatening the users’ privacy, data security and the
confidentiality of the entire network [13]. There are four main categories of attacks in IoT-based system,
which are: physical attack, network attack, software attack and data attack [41]. In The first category, which
is called the physical attack, the attacker will be physically near to the network and attempt to conduct the

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malicious processes in the entire system through many forms such as manipulating the IoT device, blocking
the RF signals, injection of malicious code and performing the side-channel attack. The researcher used the
physical unclonable function (PUF) to provide the authentication of IoT devices [42], and thus physical
attacks are prevented. The PUF has a characteristic that is impossible to copy the accurate microstructure of
the IoT device. In the second category; (network attack), the attackers attempt to manipulate the network of
the IoT using many ways such as RFID spoofing, man-in-the-middle, Traffic analysis and Sybil attacks. To
prevent this kind of attacks, authentication technique and the secure hash function are used [43]. From the
other hand, in the third category, which is called software attacks, the attacker utilizes the current software
advantages in the IoT system. The last category is called the data attack and it could be achieved by the
unauthorized access to the data and data inconsistency. To prevent these type of attacks, the blockchain could
be used by providing privacy-preserving tech. efficiently [44].
Figure 8. The predicted growth of IoT devices from 2015 to 2025 [40]
The traditional security approaches tend to be costly for IoT in each of processing overhead and
consuming of energy [45]. In addition to that, a lot of the state-of-threat security contexts are extremely
centralized and they are not suitable for IoT because of the many-to-one nature of the traffic, difficulty of
scale [46] and the single point of failure [47].
Integrating the blockchain with IoT achieves many advantages, which are:
− The distributed and decentralize attributes of the blockchain technology do not need a central server that
provides a scalable method to manipulate the increasing number of IoT devices.
− The blockchain provides more security and privacy because it uses complex cryptography algorithms
such as timestamp and hash functions to ensure a secure environment [46].
− The blockchain provides an immutable ledger and tamper-proof to protect the data from malicious
attacks. Consequently, the trusted system will be produced (i.e. only the trusted participants of the IoT
devices could accept or reject transactions depending on their consent).
− The blockchain has an important property, which is called Anonymity [8].
− The blockchain supplies a 160-bit address space, which offers 4.3 billion addresses that enable it to assign
addresses for multiple objects.
− The monitoring and tracking of ownership, trustworthiness, authorized identity registration could be
provided by the blockchain.
The capability of applying blockchain in IoT relies on many assumptions [48]. Firstly, the IoT
application requires a decentralized peer-to-peer ecosystem. Secondly, the IoT application needs to keep
payment operation for the available services between the two parties only. Finally, If the logs and traceability
of the ordered transactions are required by IoT applications. However, Implementing the blockchain in IoT
will require addressing the following challenges [8], [13]:
− The mining process of blocks takes a great deal of time, while most of IoT applications require little
latency.

729
− The mining process exhausts energy because of its high computation ability, while the common devices
of IoT are resource-constrained.
− The basic Blockchain protocols generate a lot of overhead traffic; therefore, it may be unwanted for a
certain bandwidth-restricted IoT devices.
− The blockchain scales is unwanted when the IoT networks are predictable to cover a huge number of
nodes.
− IoT sensors produce a huge amount of data, therefore, processing the transactions in blockchain will be
very slow or will have a high latency.
− The anonymity of transaction history cannot be ensured on public blockchain. So, the hackers can
determine the identities of users or devices by examining the transaction style.
The researching in the field of integrating the blockchain with IoT have seen a major renaissance
after the interest rose in cryptocurrencies and mining process. Also, different research publications
have provided an advanced solution for constructing decentralized social networking systems,
telecommunications, voting, smart homes, and smart city [15] which were all suggested. Recently, there were
many studies that investigated the integration of blockchain technology with IoT to solve the privacy and
security challenges in the IoT domain. For instance, Dorri et al. [8] presented a lightweight architecture for
IoT, which depends on the blockchain to produce IoT system with high privacy and omits the overhead of
blockchain. Uddin et al. [13] suggested an efficient lightweight integrated blockchain (ELIB) model, which
was established to meet the constraint of IoT. Polyzos and Fotiou [49] demonstrated the importance of
blockchain technologies in examining the requirements of the IoT security and how the security challenges
can be solved by combining the IoT with the blockchain technologies. Thakore et al. [50] provided a
complete survey on the fundamentals of both technologies and the blockchain-based IoT Architecture.
Karthikeyan et al. [51] presented a summary of IoT security problems and suggested the blockchain
technologies to resolve these problems. They also explain the probability of integrating the blockchain with
the IoT. Ramesh et al. [15] discovered the way to keep the IoT data on a mixture of a Blockchain with
Ethereum swarm and inter planetary file system (IPFS) in an encrypted style. Fotiou et al. [52] proposed a
smart contract-based solution to solve the privacy and security problems in the IoT system. Uddin et al. [13]
investigated the latest state-of-the-arts improvements in Blockchain for cloud and IoT, Blockchain and IoT
and B.C. and fog of IoT in various applications are analyzed. Tandon [53] presented a review of blockchain
technology and the way in which it provides the suitable solution to resolve privacy and security challenges
of the IoT system. In addition to that, he discussed the pros and cons of integrating IoT with blockchain.
Minoli and Occhiogrosso [54] provided the places of interest in some of IoT environments where Blockchain
Technology play an important role. Khan and Salah [55] presented a review of security challenges for IoT
and then they presented blockchain technology to resolve the security problems of the IoT system. Sengupta
et al. [41] presented a review about the security attacks and the problems associated with each of the IoT and
industrial IoT (IIoT) and that organizing it depends on the vulnerability. Also, authors showed the
methodology of using blockchain technology for detecting these attacks. Banerjee et al. [56] proposed a new
method represented by using the blockchain to provide IoT dataset for solving the sharing of the IoT dataset
problems.
Moreover, the importance of integrating the blockchain with IoT it has been applied on various
domains. Tables 4 and 5 shows the most common domains and the recent studies, which depend on the
integration of blockchain with IoT applications. Most of these studies were concerned with providing
security and privacy services for the IoT environment by using blockchain. As a result, blockchain is
considered an effective and active tool to provide these services. We note in Tables 4 and 5 that the articles
within this review suffers from some challenges such as (privacy and scale). Thus, we introduce in Table 6
some recommendations for those challenges.
In addition to that, the blockchain can adapt to other technologies such as the following:
a. Software-defined networks (SDN): In this technology, the resources of this network are managed via a
centralized controller, which acts as the networking operating system (NOS) [19]. Yet, scalability is a big
constraint in the single networking environments that are enabled by SDN and thus the adaptability of BC
with SDN can facilitate multi-domain SDNs interconnection and communication [57].
b. Decentralized email: Nowadays, the security service of an email is dependent on an ongoing process of
planning in addition to management. One of the solutions to address the vulnerabilities of the email can
be in the form of a blockchain-powered decentralized and distributed email system. Email addresses can
be allocated to the clients over BC. Most vitally, the communication of email by BC is not influenced by
the authorities of government that might invest the centralized email providers such as ISPs and
technology giants like Google, Facebook, and Amazon [17].
c. Blockchain-based content distribution: Content distribution networks (CDNs) are regarded as effective
approach that enhances the quality of the service of Internet through the content replication at various

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geographic locations represented by data-centers. Blockchain technology can be the solution with the
necessary 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
[17].
d. Distributed cloud storage: Users and organizations encounter and storage management of data problems
resulting from the huge growth of data on non-volatile storage systems. the security, privacy and control
of data are still important concerns [58]. The solutions of cloud storage that are based on BC inherit
characteristics such as anonymity, decentralization and the trusted execution of transactions for the
trusted members and can level the path for a cloud computing era characterized with verification and
trust.
e. Smart cities: There are inclusive major ingredients related to the smart cities, such as smart healthcare
(BC is the well-known method that delivers a major level of democratization in the sector of healthcare
and thus enhance their status), supply chain management (SCM), smart transportation (BC can improve
information exchange, support the performance of vehicle and enhance the dependability of the network
lifetime. Furthermore, BC invigorates the transportation industry by making less turnaround times, faster
security detection, swifter data management and inspections), smart grid and financial system [17].
Table 4. Recent studies of integration blockchain with IoT (part 1)
Citation Survey/
Review
IoT IoT
security
IoT
privacy
Supply
chain
Health
care
Agriculture Contributions Weakness
Huh et al.
[59]
* - * - - - - BC to control and
configure IoT devices
Did not focus on scale
and privacy
Ruta et al.
[60]
* - - - * - - BC framework for SWoT
as Service
Did not focus on
Security and privacy
Zhang and
Wen [61]
* * - - - - - IoT E-business model-
based BC
Did not focus on
and scale
Atlam et al.
[47]
* * - - - - - Integration IoT with each
of BC and AI
Did not focus on
and scale
Novo [18] - - * - - - - Access control sys. for IoT
Based on BC
and privacy
Khan and
Salah [55]
* - * - - - - Present major Security
problems for IoT
Did not focus on
performance and privacy
Badr et al.
[62]
- - * * - * - BC to secure and privacy
for e-health
Did not focus on
performance and scale
Uddin et al.
[13]
* * - - - - - BC for Cloud IoT and BC
for Fog IoT
Did not focus on
and scale
Dorri et al.
[8]
- - * * - - - secure, privacy and
lightweight BC-IoT
Did not focus on
Salman
et al. [9]
* - * * - - - Security, privacy, access
control, provenance
Did not focus on
Lo et al.
[63]
* - - - - - - Analyzing academia,
methodology BC-IoT
Did not focus on
and scale
Rejeb et al.
[64]
* - - - * - - Enhance supply chain
transparency, trust
Did not focus on
performance
Dai et al.
[65]
* - - - * - - Investigate the integration
BC-IOT
Did not focus on
and scale
Zhang et al.
[66]
- - * - - - - Investigates an access
control issue in IoT
Did not focus on
privacy, scale,
performance
Lao et al.
[67]
* * - - - - - Components of IoT-BC,
BC applications
Did not focus on
and scale
Huckle et al.
[68]
* - - - * - - Shared economy
applications
Did not focus on
privacy, scale,
performance
Patil et al.
[69]
- - * * - - * Lightweight BC for
greenhouse farms
Did not focus on
Polyzos
and Fotiou
[49]
* - * - - - - Potential of a BC-IoT Did not focus on
performance, privacy,
scale
Zhu and
Badr [70]
* - * * - - - Digital identification
management for IoT
Did not focus on
Mishra and
Tyagi [71]
- - * - - * - Secure E-healthcare based
on BC
Did not focus on
performance, privacy,
scale

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Table 5. Recent studies of integration blockchain with IoT (part 2)
Table 6. Some recommendations for security, privacy and scale challenges
Literatures challenges Recommendation
Maintaining privacy
in BC
Homomorphic encryption and proxy re-encryption technique have been investigated
by several studies of BC and IoT to resolve the issue of user’s privacy on the BC network. In addition,
Federated learning can be integrated with Blockchain technology to ensure the privacy-preserving
computation on users’ data [13].
Maintaining security
in BC
Federated learning allows a machine-learning algorithm to be trained by the participants of the Blockchain
without exchanging their data where the Blockchain can guarantee the security of the trained algorithm in
the form of a smart contract [13].
Resource and power
constraints
Energy effective consensus algorithms are introduced to save the transactions conducted recently only
(e.g., mini-BC [84], PoS and delegated proof-of-space). Xu et al. [85] suggested the management of smart
resource for cloud data-centers by utilizing the BC technology.
Scalability and
availability
Sharma et al. [57] also submitted an architecture of cloud that depends on integrating the BC with
software-defined networks (SDN) and fog-computing.
Citation Survey/
Review
IoT IoT
security
IoT
privacy
Supply
chain
Health
care
Agriculture Contributions Weakness
Banerjee
et al. [56]
* - * - - - - Datasets IoT security
and integrity
Did not focus on
performance, scale
Wang
et al. [72]
* - - - - - - BC to support IoT
application
Did not focus on
performance, privacy, scale
Sengupta
et al. [41]
* - * - - - - Security of Industrial
IoT (IIoT)
Did not focus on
Jesus et al.
[11]
* - * * - - - Secure and privacy of
IoT
Did not focus on
Dwivedi
et al. [73]
* * - - - - - Integration BC-IoT and
BC-IIoT
Did not focus on security,
privacy and scale
Kamilaris
et al. [74]
- - - - - - * Impact of BC in food
supply chain, Agric.
privacy and scale
Ferrag et al.
[75]
* - * - - - - Enhance the security of
communication
Did not focus on
Zheng et al.
[2]
* - - - - - - BC architecture Did not focus on security,
privacy and scale
Thakore
et al. [50]
* * - - - - - Combine BC with IoT Did not focus on security,
privacy and scale
Hassan et al.
[17]
* - - - - - - BC – based network
App.
privacy and scale
Lin et al.
[76]
- * - - * - - Traceability food
system based on BC
privacy and scale
Dogo et al.
[77]
- * * - - - * Water management
based on BC-IoT
Did not focus on privacy
and scale
Yaga et al.
[10]
* - - - - - - Overview of BC Did not focus on security,
privacy and scale
Kadam and
John [78]
- - * * - - - IoT architecture, BC-
IoT
Did not focus on privacy,
scale, performance
Maroufi
et al. [12]
* - - - - - - BC-IoT challenges and
solutions
privacy and scale
Alamri et al.
[79]
* - * * - - - Integrate BC-IoT for
security and privacy
Conoscenti
et al. [80]
* - - - - - - Review on BC Did not focus on scale and
performance
Atlam and
Wills [81]
* - * * - - - Combine Distributed
Ledger Tec. With IoT
Did not focus on scale and
performance
Saad et al.
[14]
* - - - - - - BC attacks Did not focus on
Atlam et al.
[46]
* - * - - - - Integrate fuzzy with
expert judgment risk
Did not focus on
Abadi et al.
[82]
* - - - - - - BC-IoT recent works -
Tandon [53] * - * * - - - BC-IoT for security and
privacy problems
Did not focus on
performance, scale
Vokerla
et al. [4]
* - - - - - - Blockchain overview Did not focus on security,
privacy and scale
Karthikeyan
et al. [51]
* - * - - - - BC-IoT security issues
and applications
Did not focus on scale and
performance
Fotiou et al.
[52]
- - * * - - - Access control for IoT -
Hang and
Kim [83]
- - * * - - - Integrate BC-IoT Did not focus on
performance
Mahmood
et al. [42]
* * - - - - - Multi_server auth. By
(PUF)
Did not focus on

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3. RESULTS AND DISCUSSION
In this paper, after examining 49 review literatures, it was found that 50% of those literatures are
related to the security of internet of things, 25% related to the privacy of IoT, 25% of them to both security
and privacy of IoT. The rest of the literatures were related to other fields such as health, agriculture and
supply chain, as shown in Table 7. However, the objective of this paper is to present a general reference
guide for researchers and practitioners in the fields which are mentioned.
Table 7. Numbers of related literature categorized by scope contribution
Literature scope Number of literatures
IoT Security 25
IoT Privacy 13
BC and IoT Security 17
BC and IoT Privacy 8
BC-IoT security and BC -IoT Privacy 13
BC-IoT eHealth 2
BC-IoT Supply Chain 5
BC-IoT Agriculture 3
IoT Scale 5
4. CONCLUSION AND FUTURE WORKS
Blockchain Technology is a fresh tool for many applications in various organizations, which allows
to secure transactions in a decentralized authority. In this paper, an overview on blockchain technology is
presented. Generally, fundamentals of blockchain are discussed. Also, some types of attacks on blockchain
are demonstrated and summarize. Based on the literatures, the integration of Blockchain with another
technology such as IoT could provide a better result in some possible domain. This integration shows the
features of Blockchain that makes it an attractive technology to solve some IoT challenges such as privacy
and security issues.
In future works, concern should be focused on investigating new tendencies for security and privacy
services by using blockchain technology in particular to design intrusion detection systems (IDS) which work
in the IoT environment. The main goal includes the reduction of the numbers of the fields in the blocks and
also developing a lightweight mining and consensus algorithms.
ACKNOWLEDGEMENTS
The authors would like to thank the University of Mosul/College of Computer Science and
Mathematics for their facilities, which have helps to enhance the quality of this work.
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BIOGRAPHIES OF AUTHORS
Mahmood Subhy Mahmood received the B.Sc. degree in Computer Science
from Mosul University, Mosul, Iraq, in 2003 and the M.S. degree in Computer
Science/Network Security from Mosul University, Mosul, Iraq, in 2011. Currently, he is
Lecturer at the Department of Biology, Mosul University and he is a Ph.D. student in
Computer Science Department in College of Computer Science and Math., Mosul University.
His research interests include network security, internet of thing security, computer network,
cyber security, programming, and artificial intelligence (AI). He can be contacted at email:
mahmoodsubhy1981@uomosul.edu.iq.
Najla Badie Al Dabagh received her B.Sc. degree in computer science from
Mosul University, Iraq in 1988, M.Sc. degree in Computer ScienceData Security from
Technology University, Iraq in 1995 and Ph.D. degree in Network security from Mosul
University, Iraq in 2006. Currently she is an Associate Professor at the department of
Computer Science, College of Computer Science and Mathematics, Mosul University. Her
research interests include cryptography, honeypot and intrusion detection, IoT security,
network security, and AI. She can be connected at email: najlabadie@uomosul.edu.iq.

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