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317
Jordanian Journal of Computers and Information Technology (JJCIT), Vol. 06, No. 04, December 2020.
1. Hazem W. Marar is with Department of Electrical Engineering, Princess Sumaya University for Technology, Amman, Jordan. Email:
h.marar@psut.edu.jo
2. Rosana W. Marar is with Department of Computer Graphics and Animation, Princess Sumaya University for Technology, Amman, Jordan.
Email: r.marar@psut.edu.jo
HYBRID BLOCKCHAIN
Hazem W. Marar1 and Rosana W. Marar2
(Received: 10-May-2020, Revised: 28-Jun.-2020 and 20-Jul.-2020, Accepted: 8-Aug.-2020)
ABSTRACT
Blockchain is a revolutionary technology that gained widespread popularity since the emergence of crypto-currencies.
The potential uses of Blockchain surpassed digital currency into a wider space that includes the Internet of Things
(IoT), security applications and smart embedded systems, among others. As the number of Blockchain users increases,
several drawbacks start to emerge, since Blockchains consume excessive amounts of energy to store and manipulate
data. Furthermore, the limited scalability nature of Blockchains due to their massive storage requirements might
become an issue. To improve the overall performance, several challenges in the current Blockchain structure should
be tackled. This paper presents a hybrid system architecture that combines the distributed nature of Blockchains with
the centralized feature of servers. Users will connect to servers via personal Blockchains, while servers will share a
chain of Blockchains to ensure integrity and security. This will significantly decrease the storage requirements of end-
users and enhance the scalability of networks. Businesses will highly benefit from this proposed structure, since it
creates a reliable scalable business model.
KEYWORDS
Blockchain, Bitcoin, Crypto-currency, Distributed, E-commerce, Hybrid.
1. INTRODUCTION
Since the emergence of Bitcoin in 2008 [1], crypto-currencies and hence Blockchain technology have gained
widespread popularity. The technology features enable it to become the infrastructure for a new generation
of internet interactions that include secure online payments [2]-[3], data exchange and transaction of digital
assets [2]-[4]. Blockchain provides a decentralized, open, Byzantine fault-tolerant transaction mechanism
[5]-[7]. Users can consider Blockchain as a sort of data structure that consists of an ever increasing number
of blocks linked together through cryptography. Each block includes a cryptographic hash of the previous
block, a timestamp and data that users wish to exchange throughout the network [8]-[9]. Data blocks are
shared among users and not saved on a centralized server. The chain of blocks continuously grows when
new blocks are appended into it and this change will be reflected to all users within the chain. Hence, by
design, Blockchain is relatively resistant to data modification. However, in order to append a new block to
the chain, computers, called miners, compete and run a complex hashing algorithm trying to produce a valid
block hash. This will dissipate huge amounts of disproportionate power and time. Furthermore, the ever
increasing chain of blocks will require massive storage capabilities from all users, limiting the scalability of
such technology. Blockchains are governed by a consensus algorithm used as a mechanism to achieve the
necessary agreement on the validity of data among distributed processes. With the recent advances in
Blockchain technology, numerous consensus algorithms were proposed to make endpoints reach an
agreement on the order and state of blocks of transactions and update the distributed ledger accordingly
[10]-[11]. In this paper, a hybrid Blockchain architecture that is suitable for online banking and e-commerce
platforms is proposed. The proposed solution addresses the storage and power consumption requirements
while improving the Blockchain's integrity and security. The paper is organized as follows: Section 2
introduces the Blockchain technology and its methodology, whereas Section 3 illustrates the architecture of
the proposed hybrid solution and Section 4 concludes the paper.
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Hybrid Blockchain, H. W. Marar and R. W. Marar.
2. BLOCKCHAIN TECHNOLOGY
Blockchain is a distributed transactional database that is governed by network consensus and secured by
advanced cryptography. As illustrated in Figure 1, a Blockchain consists of a series of datasets that are
composed of chains of data blocks. Each block holds several transactions (TXn). Once a set of transactions
is complete, an additional block is appended to the chain, hence representing a complete ledger of
transactions history. The chain of blocks continuously grows when new blocks are appended into it and this
change will be reflected to all users within the chain.
Figure 1. Basic blockchain structure.
In addition to the transactions, each block holds a Unix time timestamp which indicates the time of each
transaction. Moreover, each block has a 256-bit hash value of the previous parent block and a nonce, which
is a random number that verifies the hash. Several hashing algorithms can be used, but the SHA-256
cryptographic hash is the most common [7]. Hash values are unique and therefore, any alteration in the data
would immediately change the respective hash value. As a result, this structure ensures the security and
integrity of the entire Blockchain down to the first block known as the Genesis Block. In order to alter the
contents of any block, network consensus must be achieved. This implies that a minimum of half the
endpoints connected throughout the network reach a common agreement about the present state of the
current distributed ledger or transaction. Hence, in large networks, Blockchain is relatively resistant to data
modification due to the huge number of endpoints. On the other hand, the computational power and storage
power required are immense. Various research studies around Blockchains have been published in recent
years to analyze and tackle the issues and limitations in Blockchains and evaluate consensus algorithms.
The practical byzantine fault tolerance algorithm (PBFT), the proof-of-work algorithm (PoW), the proof-
of-stake algorithm (PoS) and the delegated proof-of-stake algorithm (DPoS) are the four main methods of
reaching consensus in the current Blockchain technology. In PBFT, each node in the network maintains an
ongoing internal state. When a message is received, nodes use the message in conjunction with their internal
state to run a computation algorithm to validate the message. After reaching a decision about the new
message, the node shares that decision with other nodes in the system. A consensus decision is determined
based on the total decisions submitted by all participating nodes. This method of establishing consensus
requires less effort than other methods. However, anonymity can be a great risk on the system.
An alternative method of reaching consensus within a Blockchain is the proof-of-work (PoW) algorithm,
which is used by Bitcoin. In contrast to PBFT, PoW does not require all nodes on the network to participate
and submit their individual conclusions in order for a consensus to be achieved. Rather, PoW is an algorithm
that uses a hash function. Transactions can then be independently verified by all other system participants.
The scheme allows for easy, broad participation while maintaining anonymity.
Proof-of-stake (PoS) algorithm is similar to PoW algorithm, but the participation in the consensus building
process is restricted to a predefined set of nodes known for having a legitimate stake in the Blockchain. The
hash function calculation is replaced with a simple digital signature which proves ownership of the stake. A
more centralized way of achieving consensus is using the delegated proof-of-stake (DPoS) system. The
algorithm works in a similar manner as in the PoS system, except that individuals choose an entity to
represent their portions of stake in the system.
In [12], a specially designed attack scenario is presented, in which collaborating miners’ revenue can be
larger than their fair share. Such attacks can have significant consequences on the Blockchain structure.
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Jordanian Journal of Computers and Information Technology (JJCIT), Vol. 06, No. 04, December 2020.
Miners might prefer to join the attackers and the colluding group will expand in size until it becomes a
majority. At this point, the Blockchain system ceases to be a decentralized system. In [13], a decentralized
smart contract system that does not store or show financial transactions is presented. Normally, all
transactions, including flow of money, are exposed on the Blockchain. Such information might be useful by
hackers and network attackers to track money and assets. Using such decentralized smart contract system
retains transactional privacy from the public’s view. In [14], a highlight of the weaknesses and limitations
of Bitcoin technology is presented. This includes the theft or loss of Bitcoins due to malware attacks,
structural problems and scalability issues, like delayed transaction confirmation, data retention and
communication failures. A fair exchange protocol that improves the users' anonymity is used to improve the
quality of the existing Bitcoin technology. In [15], a new mechanism for securing Blockchains' contracts is
presented. By introducing a credibility score measure, a hybrid Blockchain that prevents an attacker from
monopolizing resources is introduced. Credibility is a vital factor in any contract or transaction. Contractors
must develop a good knowledge about each other to build up credibility and trust. The more contracts a
contractor has with different people, the more credibility he/she gains. The mechanism proposed creates a
hybrid Blockchain based on the proof-of-stake and credibility score methods. The proposed system uses the
proof-of-work algorithm to introduce a hybrid solution in which power and storage requirements are
minimized while improving the network's scalability and security.
Since Blockchain is a decentralized distributed ledger, it has to be managed by a peer-to-peer network
adhering to a common protocol for communication and appending new blocks. Therefore, each peer in the
network will have a copy of the Blockchain. Any alteration in the Blockchain will involve the alteration of
all subsequent blocks, which requires consensus of the majority of the network. In its current form, the hash
value of each block is generated by miners. Mining is a process at which specialized computers compete to
solve a complex computational problem and produce a valid hash. Hence, miners secure the network and
process every transaction. The SHA-256 cryptographic hash chooses any 256-bit number ranging from 0 to
2256. The target is a 256-bit number that all Blockcahin clients share. The SHA-256 hash of a block's header
must be lower than or equal to the current target for the block to be accepted by the network. The lower the
target, the more difficult it is to generate a block. The maximum target defined by SHA256 mining devices
is illustrated in equation 1.
TMax= 0x00000000FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF (1)
We define (D) to be the difficulty of finding a valid hash for a given block. D can be defined as in Equation
2 [16]-[17]:
(2)
where, TMax is the maximum target of the hash value and TCur is the current target hash that the miner found.
Difficulty can be simply defined as the ratio between the maximum target and the current target. TMax and
TCur can be expressed as in Equations 3 and 4 [16]-[17]. The maximum target is defined as (216 - 1) × 2208
or approximately 2224. Since there are 2256 different values that a hash can take, a random hash has a chance
of about 2-32 to be lower than the maximum target.
TMax = (3)
TCur =
(4)
The expected number of hashes (N) that a miner needs to calculate to find a block with a given difficulty
(D) is illustrated in Equation 5 [2]:
(5)
Thus, every hash produced by a given miner has a probability (P) to validate a given block, as seen in
Equation 6:
(6)
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Hybrid Blockchain, H. W. Marar and R. W. Marar.
Miners from all around the world compete using powerful dedicated mining computers to generate a valid
hash. From the equations above, it is evident that such process imposes high levels of power consumption.
Currently, adding a single transaction to the Bitcoin Blockchain platform consumes about 600 kWh [18]-
[19] and the annual power consumption for the Bitcoin network is about 77.78 TWh, which is comparable
to the power consumption of Chile. Furthermore, every node in the public Blockchain network has a local
copy of the entire Blockchain. This drains huge amount of storage. Currently, the Bitcoin database exceeds
250GB and is growing up in a sharp exponential rate, as seen in Figure 2. The overall Bitcoin network is
consuming more than 1000 TiB of storage per year [20] and is increasing in a sharp rate. This is due to the
increase in the number of users of the Bitcoin network. Such properties will restraint the use of this
technology in banking and commercial businesses.
Figure 2. Size of blockchain database.
3. PROPOSED HYBRID BLOCKCHAIN
To minimize the computational power and storage requirements, a hybrid Blockchain model that combines
the centralized feature of servers with the distributed nature of Blockchains is proposed. Figure 3 illustrates
the network infrastructure of the proposed system.
Figure 3. Proposed hybrid blockchain network infrastructure.
0
50000
100000
150000
200000
250000
300000
Blockchain Size in Megabytes
Year
2010 2011 2012 2013 2014
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Jordanian Journal of Computers and Information Technology (JJCIT), Vol. 06, No. 04, December 2020.
The proposed model can be highly beneficial to modern online businesses. Banks and e-commerce platforms
have several servers distributed over a large geographical area. Using the proposed model, two sets of
networks are defined: a personal Blockchain through which users can interface with the network servers
and a global Blockchain consisting of interconnecting servers. The global Blockchain can be viewed as a
chain of Blockchains that is distributed among servers and being updated periodically. In such scheme, users
can access their respective personal Blockchain, hence accessing their private information only. No user can
get hold of other users' personal data, which improves data privacy. Moreover, single or multiple private
miners owned or governed by the local business policies are distributed among the network. Miners control
the appending of new blocks and mapping hashes throughout the network without competing to solve a
complex mathematical formula. This scenario can improve the network's security, since securing a
Blockchain might require knowing participating miners' computational capabilities. This, in turn, aids in
detecting potential selfish mining attacks [21-[22]. Normally, miners use the Proof-of-Work (PoW) as the
security algorithm and compete trying to solve a complex computational challenge imposed by the PoW
protocol. As a result, miners consume a huge amount of power [23]-[24], [15]. The proposed design includes
single or multiple private trusted miners distributed throughout the network. Furthermore, the mining
process can be fairly distributed among miners, hence immensely reducing the power consumption
requirements of maintaining a Blockchain. Figure 4 illustrates the detailed architecture of the proposed
Hybrid Blockchain.
Figure 4. Proposed hybrid blockchain architecture.
In this scheme, every user has a copy of his/her personal Blockchain consisting of his/her personal
transactions and information. Whenever a user executes a transaction, a broadcast request message is sent
from the user's device to the miners throughout the network. Upon receiving the message, a specific miner,
based on an election algorithm, will compute and produce a global hash and a private hash which is sent
back to the user. Transactions and data blocks of the user's personal Blockchain are linked together through
the received private hash. The user's data is saved locally on his device and also broadcasted to the data
servers throughout the network. Hence, the locally saved user's Blockchain represents 50% of his/her overall
personal network. The other 50% is represented by the business's servers. Each server holds a copy of a
global Blockchain consisting of blocks from all connected users. The blocks are linked through a global
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hash generated by the network's miner. Users can only access their respective information. Therefore, no
user can access or view other users' personal data, hence improving data privacy.
Since miners are responsible for the generation of private and global hashes, a mapping algorithm should
be used to track any network change. Mapping algorithms, in their simplest form, can be look-up-tables that
are controlled by miners to translate and govern any change in the network. Furthermore, to reduce power
consumption for maintaining the Blockchain, the mining process can be fairly distributed among miners.
The distribution process can be based on an election algorithm to assign a miner. The election algorithm
can be chosen upon the miner's geographical area, the miner's workload, user's ping signal latency or even
randomly. Figure 5 illustrates an example of an election algorithm based on a specific user's ping signal
latency.
Figure 5. A sample election algorithm based on ping signal latency.
As seen in Figure 4, the network of each user is composed of the user's personal device, representing 50%
of the network, along with the servers' network representing the remaining 50%. This scheme offers high
levels of data security. This is due to the fact that hackers need to control the majority of the network's
servers in addition to the client's device to achieve network consensus to alter or access only a single block.
Assuming that there are several servers in a network, Equation 7 describes the minimal amount of devices
"K" needed to alter a given user block, whereas Equation 8 illustrates the minimum amount of devices "K"
needed to control several blocks within the network.
(7)
(8)
where,
S is a server holding the chain of Blockchains.
n: Number of different users within a selected portion of the Blockchain.
To reach consensus in conventional Blockchains, attackers only need to control a half of the nodes holding
the data. Figure 6 illustrates the improved security of the proposed design against the current Blockchain
system by comparing the minimal number of devices "K" needed to control the network as the number of
users "n" increases.
Figure 7 presents the consensus algorithm used in the proposed design. Miners initiate the consensus
protocol in the network through a 'proposed' function illustrated at lines 7–14 of the algorithm, allowing
them to propose new blocks. Afterwards, processes within the network decide whether a new block at a
given index is valid at lines 16-23. Appending a new block is shown at line 24 depending on the function
get-main-branch( ). Line 11 maps a global hash used in the servers' network with a local hash used within
the personal network.
7
10
11
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Jordanian Journal of Computers and Information Technology (JJCIT), Vol. 06, No. 04, December 2020.
Figure 6. Number of devices to achieve network majority.
Figure 7. Consensus algorithm.
4. CONCLUSION
As centralized systems need a central owner to connect and govern all other users and devices, the system
structure is highly dependent on the network connectivity. Hence, abrupt failure of the entire system due to
connectivity or security issues is likely to occur. Therefore, decentralized systems emerged as an alternative
solution to resolve the security issues. One of the most recent promising decentralized architectures is
Blockchain technology. Ever since, Blockchain has been adopted by businesses, e-commerce platforms and
digital currencies like Bitcoin. However, as Bitcoin and related crypto-currencies have become increasingly
popular, they have hit scalability and reliability issues. The process of improving scalability has been
obstructed by an inherent trade-off between performance metrics and security requirements of the system
10
16
17
18
19
20
21
22
23
24
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Hybrid Blockchain, H. W. Marar and R. W. Marar.
architecture. This paper proposes a hybrid Blockchain system that is suitable for banks and e-commerce
businesses. Users can connect to servers using personal Blockchains, while servers share a chain of
Blockchains throughout the business's private network. Miners governed by the business's policies control
the appending of new blocks and mapping hash values. Having private miners will dramatically reduce
power consumption demands and improve the overall quality of the Blockchain technology. This solution
diminishes the space requirements of end users and enhances the security of the system by introducing
personal networks. The proposed hybrid solution inherits the simple deployment and affordable
maintenance in centralized systems while promoting resource sharing and improved scalability and fault-
tolerance in decentralized systems.
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