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Where Is Current Research on Blockchain Technology?—A Systematic Review

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Blockchain is a decentralized transaction and data management technology developed first for Bitcoin cryptocurrency. The interest in Blockchain technology has been increasing since the idea was coined in 2008. The reason for the interest in Blockchain is its central attributes that provide security, anonymity and data integrity without any third party organization in control of the transactions, and therefore it creates interesting research areas, especially from the perspective of technical challenges and limitations. In this research, we have conducted a systematic mapping study with the goal of collecting all relevant research on Blockchain technology. Our objective is to understand the current research topics, challenges and future directions regarding Blockchain technology from the technical perspective. We have extracted 41 primary papers from scientific databases. The results show that focus in over 80% of the papers is on Bitcoin system and less than 20% deals with other Blockchain applications including e.g. smart contracts and licensing. The majority of research is focusing on revealing and improving limitations of Blockchain from privacy and security perspectives, but many of the proposed solutions lack concrete evaluation on their effectiveness. Many other Blockchain scalability related challenges including throughput and latency have been left unstudied. On the basis of this study, recommendations on future research directions are provided for researchers.
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RESEARCH ARTICLE
Where Is Current Research on Blockchain
Technology?—A Systematic Review
Jesse Yli-Huumo
1
, Deokyoon Ko
2
, Sujin Choi
4
*, Sooyong Park
2
, Kari Smolander
3
1Dept. of Innovation and Software, Lappeenranta University of Technology, Lappeenranta, Finland, 2Dept.
of Computer Science & Engineering, Sogang University, Seoul, South Korea, 3Dept. of Computer Science,
Aalto University, Helsinki, Finland, 4Sogang Institute of Advanced Technology, Sogang University, Seoul,
South Korea
*sujinchoi@sogang.ac.kr
Abstract
Blockchain is a decentralized transaction and data management technology developed first
for Bitcoin cryptocurrency. The interest in Blockchain technology has been increasing since
the idea was coined in 2008. The reason for the interest in Blockchain is its central attri-
butes that provide security, anonymity and data integrity without any third party organiza-
tion in control of the transactions, and therefore it creates interesting research areas,
especially from the perspective of technical challenges and limitations. In this research, we
have conducted a systematic mapping study with the goal of collecting all relevant research
on Blockchain technology. Our objective is to understand the current research topics, chal-
lenges and future directions regarding Blockchain technology from the technical perspec-
tive. We have extracted 41 primary papers from scientific databases. The results show that
focus in over 80% of the papers is on Bitcoin system and less than 20% deals with other
Blockchain applications including e.g. smart contracts and licensing. The majority of
research is focusing on revealing and improving limitations of Blockchain from privacy and
security perspectives, but many of the proposed solutions lack concrete evaluation on their
effectiveness. Many other Blockchain scalability related challenges including throughput
and latency have been left unstudied. On the basis of this study, recommendations on
future research directions are provided for researchers.
Introduction
Currency transactions between persons or companies are often centralized and controlled by a
third party organization. Making a digital payment or currency transfer requires a bank or
credit card provider as a middlemanto complete the transaction. In addition, a transaction
causes a fee from a bankor a credit card company. The same process applies also in several
other domains, such as games, music, software etc. The transaction system is typically central-
ized, and all data and information are controlled and managed by a third party organization,
rather than the two principal entities involved in the transaction. Blockchain technology has
PLOS ONE | DOI:10.1371/journal.pone.0163477 October 3, 2016 1 / 27
a11111
OPEN ACCESS
Citation: Yli-Huumo J, Ko D, Choi S, Park S,
Smolander K (2016) Where Is Current Research on
Blockchain Technology?—A Systematic Review.
PLoS ONE 11(10): e0163477. doi:10.1371/journal.
pone.0163477
Editor: Houbing Song, West Virginia University,
UNITED STATES
Received: May 10, 2016
Accepted: September 9, 2016
Published: October 3, 2016
Copyright: ©2016 Yli-Huumo et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
within the paper.
Funding: The author(s) received no specific
funding for this work.
Competing Interests: The authors have declared
that no competing interests exist.
been developed to solve this issue. The goal of Blockchain technology is to create a decentral-
ized environment where no third party is in control of the transactions and data.
Blockchain is a distributed database solution that maintains a continuously growing list of
data records that are confirmed by the nodes participatingin it. The data is recordedin a public
ledger, including information of every transaction ever completed. Blockchain is a decentral-
ized solution which doesnot require any third party organization in the middle. The informa-
tion about every transaction ever completed in Blockchain is shared and available to all nodes.
This attribute makes the system more transparent than centralized transactions involving a
third party. In addition, the nodes in Blockchain are all anonymous, which makes it more
secure for other nodes to confirm the transactions. Bitcoin was the first application that intro-
duced Blockchain technology. Bitcoin created a decentralized environment for cryptocurrency,
where the participants can buy and exchange goods with digital money.
However, even though Blockchain seems to be a suitable solution for conducting transac-
tions by using cryptocurrencies, it has still some technical challenges and limitations that need
to be studied and addressed. High integrity of transactions and security, as well as privacy of
nodes are neededto prevent attacks and attempts to disturb transactions inBlockchain [1]. In
addition, confirming transactions in the Blockchain requires a computational power.
It is important to identify what topics have been already studied and addressed in Block-
chain and what are currently the biggest challenges and limitations that need further studies.
To address these questions, we decided to use a systematic mapping study process [2] to iden-
tify relevant papersrelated to Blockchain. In the systematic mapping study, we applied a well-
designed research protocol to search for material in scientific databases. The produced map of
current research on Blockchain will help other researchers and practitioners in identifying pos-
sible research areas and questions for future research.
Although cryptocurrencies are also a business and management topic, we decided to narrow
down the research topic to the technical perspective of Blockchain. Our objective was to find
and map all papers with technical viewpoints on Blockchain. We were interested in finding
Blockchain research topics related to various technical areas, such as security, performance,
data integrity, privacy, and scalability.
The rest of the paper is organized as follows. Section 2 introduces the backgroundof Block-
chain and Bitcoin. In addition, we present some already identified challenges and technical lim-
itations of Blockchain technology. In Section 3, we describethe applied research methodology
and the process of collectingrelevant research papers. Section4 presents the results of the gath-
ered papers and extracted data. Section 5 presents the identified classification schemes. Section
6 discusses the study and answers the research questions. Section7 concludes the paper.
Background
Blockchain, mostly known as the technology running the Bitcoin cr yptocurrency, is a public
ledger system maintaining the integrity of transaction data [1]. Blockchain technology was first
used when the Bitcoin cryptocurrencywas introduced. To this day, Bitcoin is still the most
commonly used application using Blockchain technology [3]. Bitcoin is a decentralized digital
currency payment system that consists of a public transaction ledger called Blockchain [4]. The
essential feature of Bitcoin is the maintainability of the value of the currency without any orga-
nization or governmental administration in control. The number of transfers and users inthe
Bitcoin network is constantly increasing [5]. In addition, the conversions with traditional cur-
rencies, e.g. KRW, EUR and USD, occur constantly in currency exchange markets [6][7]. Bit-
coin has therefore gained the attention of various communities and is currently the most
successful digital currency using Blockchain technology [6].
Where Is Current Research on Blockchain Technology?—A Systematic Review
PLOS ONE | DOI:10.1371/journal.pone.0163477 October 3, 2016 2 / 27
Bitcoin uses the public key infrastructure (PKI) mechanism [8]. In PKI, the user has one
pair of public and private keys. The public key is used in the address of the user Bitcoin wallet,
and the private key is for the authentication of the user. The transaction of Bitcoin consists of
the public key of the sender, multiple public keys of the receiver, and the value transferred. In
about ten minutes, the transaction will be written in a block. This new block is then linked to a
previously written block. All blocks, including information about every transaction made, are
stored in the disk storage of the users, called nodes. All the nodes store information about all
recorded transactions of the Bitcoin network and check the correctness of each new transaction
made by using previous blocks. The nodes are rewarded by checking the correctness of transac-
tions. This method is called mining, and it is confirmed with Proof-of-Work, which is one of
the main concepts of Blockchain technology. When all transactions are successfully confirmed,
a consensus exists between all the nodes. The new blocks are linked to previous blocks and all
the blocks are aligned in one continuous chain. This chain of blocks is the public ledger tech-
nique of Bitcoin, called Blockchain.
Blockchain is the decentralized managing technique of Bitcoin, designed for issuing and
transferring money for the users of the Bitcoin currency. This technique can support the public
ledger of all Bitcoin transactions that have ever been executed, without any control of a third
party organization [1]. The advantage of Blockchain is that the public ledger cannot be modi-
fied or deleted afterthe data has been approved by all nodes. This is why Blockchain is well-
known of its data integrity and security characteristics. Blockchain technology can also be
applied to other types of uses. It can for example create an environment for digital contracts
and peer-to-peer data sharing in a cloud service [1]. The strong point of Blockchain technique,
data integrity, is the reason why its use extends also to other services and applications.
Blockchain technology has also some technical challenges and limitations that have been
identified. Swan [1] presents seven technical challenges and limitations for the adaptation of
Blockchain technology in the future:
Throughput: The potential throughput of issues in the Bitcoin network is currently maxi-
mized to 7tps (transactions per second). Other transaction processing networks are VISA
(2,000tps) and Twitter (5,000tps). When the frequency of transactions in Blockchain
increases to similar levels, the throughput of the Blockchain network needs to be improved.
Latency: To create sufficient security for a Bitcoin transaction block, it takes currently
roughly 10 minutes to complete one transaction. To achieve efficiency in security, more time
has to be spent on a block, becauseit has to outweigh the cost of double spending attacks.
Double-spending is the result of successful spending of money more than once [9]. Bitcoin
protects against double spending by verifying each transaction added to the block chain, to
ensure that the inputs for the transaction have not been spent previously [9]. This makes
latency a big issue in Blockchain currently. Making a block and confirming the transaction
should happen in seconds, while maintaining security. To complete a transaction e.g. in
VISA takes only a few seconds,which is a huge advantage compared to Blockchain.
Size and bandwidth: At the moment, the size of a Blockchain in the Bitcoin network is over
50,000MB (February 2016). When the throughput increases to the levels of VISA, Blockchain
could grow 214PB in each year. The Bitcoin community assumes that the size of one block is
1MB, and a block is created every ten minutes [10]. Therefore, there is a limitation in the num-
ber of transactions that can be handled (on average 500 transaction in one block) [11]. If the
Blockchain needs to control more transactions, the size and bandwidth issues have to be solved.
Security: The current Blockchain has a possibility of a 51% attack. In a 51% attack a single
entity would have full control of the majority of the networks mining hash-rate and would
Where Is Current Research on Blockchain Technology?—A Systematic Review
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be able to manipulate Blockchain. To overcome this issue, more research on security is
necessary.
Wasted resources: Mining Bitcoin wastes huge amounts of energy ($15million/day). The
waste in Bitcoin is causedby the Proof-of-Work effort. There are some alternatives in indus-
try fields, such as proof-of-stake. With Proof-of-Work, the probability of mining a block
depends on the work done by the miner [12]. However, in Proof-of-Stake, the resource that
is compared is the amount of Bitcoin a miner holds [12]. For example, someone holding 1%
of the Bitcoin can mine 1% of the “Proof-of-Stake blocks” [12]. The issue with wasted
resources needs to be solved to have more efficient mining in Blockchain.
Usability: The Bitcoin API for developingservicesis difficult to use. There is a need to
develop a more developer-friendly API for Blockchain. This could resemble REST APIs.
Versioning, hard forks, multiple chains: A small chain that consists of a small number of
nodes has a higher possibility of a 51% attack. Another issue emerges when chains are split
for administrative or versioning purposes.
Overall, Blockchain as a technology has the potential to change the way how transactions
are conducted in everyday life. In addition, the applications of Blockchain are not limited to
cryptocurrencies, but the technology could be possibly applied in various environments where
some forms of transactions are done. The research on the possibilities of Blockchain in applica-
tions is certainly an interesting area for future research, but at the moment Blockchain suffers
from technical limitations and challenges. Anonymity, data integrity and security attributes set
a lot of interesting challenges and questions that needto be solved and assessed with high qual-
ity research. Scalability is also an issue that needs to be solved for future needs. Therefore, to
identify and understand the current status of research conducted on Blockchain, it is important
to gather all relevant research. It is then possible to evaluate what challenges and questions
have been tackled and answered,and what are the most problematic issues in Blockchain at the
moment.
Research methodology
Systematic mapping study was selected as the research methodology for this study. The goal of
a systematic mapping study is to providean overview of a research area,to establish if research
evidence exists, and quantify the amount of evidence [2]. In this study we follow the systematic
mapping process describedby Petersen et al. [13]. We also use guidelines for a systematic liter-
ature review described by Kitchenham and Charters [2] to search for relevant papers. We
chose the systematic mapping process as our research methodologybecause our goal was to
explore the existing studies related to Blockchain technology. The results of the mapping study
would help us to identify and map research areas related to Blockchain technologyand possible
research gaps. The process for the systematic mapping study is presented in Fig 1, and consists
of five process steps and outcomes. The Prisma Checklist is provided in S1 Checklist.
Definition of research questions
The first stage of the systematic mapping process is the definitionof the research questions.
The goal of this study was to provide an overview of the current research on Blockchain tech-
nology. Therefore, we defined four research questions:
RQ1: What research topics have been addressed in current research on Blockchain?
The main research question of this mapping study is to understand the current research top-
ics on Blockchain. By collecting all the relevant papers from scientific databases, we would be
Where Is Current Research on Blockchain Technology?—A Systematic Review
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able to create an overall understanding of Blockchain research and map the current research
areas. Mapping the current research done on Blockchain technology will help other research-
ers and practitioners to gain better understanding on the currentresearch topics, which will
help to take the research on Blockchain even further.
RQ2: What applications have been developed with and for Blockchain technology?
Blockchain is mostly known for its relation to Bitcoin cryptocurrency. Bitcoin uses Block-
chain technology in currency transactions. However, Bitcoin cryptocurrency is not the only
solution that uses Blockchain technology. Therefore, it is important to find the current appli-
cations developed by using Blockchain technology. Identifying other applications can help to
understand other directions and ways to use Blockchain.
RQ3: What are the current research gaps in Blockchain research?
A systematic mapping of research enables understanding the current research gaps. The
identification of research gaps will help other researchers and practitioners to focus their
research on areas that require more research. Finding research gaps will help to understand
and find unanswered research questions in current Blockchain technology.
RQ4: What are the future research directions for Blockchain?
Understanding the potential future research directions for Blockchain technology is a conse-
quence of RQ1-RQ3. Answering this research question is beneficial when deciding where the
research on Blockchain technology should be directed and what issues need to be solved.
Conducting the search
The second stage of a mapping study is to search for all the relevant scientific paperson the
research topic. A search protocol defines the methods that will be used to undertake a specific
systematic literature search. A pre-defined protocol is needed to reduce the possibility of
researcher bias [2].
We created a search protocol that we used for scientific databases to gather all the papers rele-
vant for our research topic. The terms used in the search string were chosen after pilot searches,
where we tested possiblekeywords. After the pilot search we decided to useonly the term Block-
chainas the search string, even though Bitcoin could also have been a possible one. However, in
the pilot search we used also Bitcoin as a search term, but we identifieda huge number of papers
Fig 1. The systematic mapping process.
doi:10.1371/journal.pone.0163477.g001
Where Is Current Research on Blockchain Technology?—A Systematic Review
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that were related to economic topics in cryptocurrencies,rather than technological aspects of
Blockchain technology. Therefore, since our goal in this mapping study process was to find and
map the papers related to technical aspects of Blockchain technology, we decided to drop the
term Bitcoin. We believe that by using only the term Blockchain as the search string, the majority
of Bitcoin-related papers with a technical perspectiveon Blockchain were still included. In addi-
tion, it seemed that if a Bitcoin-related paper did not have the term Blockchain anywhere in its
meta-data, the paper was related to the economics of a cryptocurrency.
After designing and testing the search protocol, we chose the scientific databases for the
searches. We decided to concentrate on peer-reviewed,high quality papers published in confer-
ences, workshops, symposiums, books and journals related to the research topic. We used six sci-
entific databases for paper retrieval. The chosen databases were (1) IEEE Xplore, (2) ACM Digital
Library, (3) SpringerLink, (4) ScienceDirect, (5) Ebsco, and (6) PLOS One. We decided not to
use grey literature e.g. from Google searches, and kept scientific peer review as the criterion.
Screening of relevant papers
Because all papers in the searchers were not necessarily related to the research questions, they
needed to be assessed for their actual relevance [2]. After using the search protocol in the scien-
tific databases, the next stage was the screeningof papers. For screening the relevant papers, we
used a process inspired by Dybåand Dingsøyr [14]. At the first screening phase, we screened
the papers based on their titles and excluded studies that were not relevant to the research
topic. For example, the search protocol returned papers related to Blockchain in other scientific
fields, which had different meaning than the Blockchain technology used in computer science.
These papers were clearly out of the scope of this mapping study, which was a valid reason to
exclude them. However, in some cases it was difficult to determine the relevancy of the paper
on the basis of the title of the paper. In these situations, we passed the paper through to the
next stage for further reading. In the second phase, the authors read the abstracts of every
paper that passed the previous phase. In addition, we used specific inclusion and exclusion cri-
teria to screen each paper. We decided to exclude the following types of papers: (1) papers
without full text availability, (2) papers where the main language was not English, (3) papers
that had some other meaning than Blockchain used in computer science, (4) papers that were
duplicates, and (5) papers that were posters. When a paper passed all the five exclusion criteria,
and after reading the abstract it was considered as focusing on Blockchain, we decided to
include it in the next screening stage.
Keywording on the basis of the abstract
The next stage in a mapping study process after finding the relevant papers through abstracts is
keywording. For this stage, we used the process defined by Petersen et al. [13] (Fig 2). Key-
wording was done in two steps. In the first step we read the abstract and identified keywords
and concepts that reflected the contribution of the paper [13]. The second step was to develop
a higher level of understanding based on these keywords [13]. We used the keywords to cluster
and form categories for the mapping of the studies. After the categories had beenclustered, we
read all the selected papers. Afterthe reading we also updated the categories or created new
ones, if the paper revealed something new. This resulted in a systematic map of clustered cate-
gories formed from all the relevant papers on the research topic.
Data extraction and mapping process
A data extraction form (Table 1) was designed to collect the informationneeded to address the
research questions of this mapping study [2]. Data items DI0 to DI6 gathered basic
Where Is Current Research on Blockchain Technology?—A Systematic Review
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information of the papers. These items included e.g. the title of the paper, the name(s) of the
author(s), the country of the author(s), and publication type/place.The rest of the data items
(DI7-DI10) were gathered after reading the papers. These data items included e.g. study goals
and major findings of each paper. We collected the extracted data items to Excel, which helped
us to organize and analyze the data.
Fig 2. Building the classification scheme.
doi:10.1371/journal.pone.0163477.g002
Table 1. Data extraction items.
# Data item Description
DI0 Study identifier Study id (e.g. ID01)
DI1 Title Title of the paper
DI2 Authors Name of the author(s)
DI3 Country Country of authors
DI4 Publication info Name of the publication place
DI5 Publication type Type of publication (e.g. conference/workshop/journal)
DI6 Publication source Academia / Industry
DI7 Abstract Abstract of the paper
DI8 Study aim Aim of the paper
DI9 Research question/goal Research questions/goals defined for the paper
DI10 Study findings Major findings of study
doi:10.1371/journal.pone.0163477.t001
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Basic information of the papers
In this section, the search and selection results of the systematic mapping study are presented.
Out of the extracted data items (Table 1), this section reports on data items DI0-DI6.
Search and selection results
The search and selection results are presented in Fig 3. The PRISMA flow diagram is also pro-
vided in S1 Diagram. 121 papers were initially retrieved when the designed search protocol was
applied to the selected scientific databases. The first inclusion and exclusion round was based
on the titles of the retrievedpapers. All the paper titles were examined by two authors, which
led to the selection of 55 papers. The reason for the high number of excluded papers (66) was
that they were not related to theresearch topic. For example, many excluded papers discussed
the business perspective of Bitcoin, and therefore they did not belong to our study. We also
Fig 3. Search and selection process of the papers.
doi:10.1371/journal.pone.0163477.g003
Where Is Current Research on Blockchain Technology?—A Systematic Review
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retrieved multiple papers related to other scientific areas, such as chemistry and mathematics,
where the keyword Blockchain had another meaningthan the technology used in computer
science.
After the selection of 55 papers, we removed duplicates and used the next round exclusion
and inclusion criteria defined in section 3.3. This round resulted in the selection of 48 papers.
After this, three authors read the abstracts of all the selected papers. This did not result in the
exclusion of any papers, however. Based on the abstracts, all the selected papers had a topic
related to Blockchain with a technical viewpoint.
However, we decided to pass some unclear papers to the next selection round for more in-
depth analysis. In the last stage of paper selection, three authors read all the papers. This
resulted in the selection of 41 papers, which we included in this study as primary papers. Three
papers were dropped due to their focus on the economic perspective of Blockchain and Bitcoin.
Additional four papers were excluded for being only reports describing Blockchain and how it
works without providing any actual new research findings or evidence. The full list of the
selected papers withthe extracted data items is presented in S1 Table.
Publication year, source and geographic distribution
Fig 4 shows the publication year distribution of the selectedprimary papers. Interestingly, all
the selected papers were published after the year 2012. This shows that Blockchain as a research
area is a very recent and new one. When looking at the publication year distribution more
closely, out of all the selected papers, 2 papers (5%) were published in 2013, 16 papers (39%) in
2014 and 23 papers (56%) in 2015. This shows an increasing number of publications each year,
which suggests also a growing interest in Blockchain technology. This is not a surprise, because
the idea of Blockchain and Bitcoin was first coined only in 2008 [4].
Fig 5 shows the source of each selected primary paper. The possible sources for a paper are
the academia, industry, or both. Our results showed that 30 papers (73.1%) were published by
an academic sourceand only 3 papers (7.3%) were published by an industry source. In 8 papers
(19.5%), the authors were from both academia and industry. It is, however, highly possible that
most of the papers published by the industry are not included in scientific databases. Most
industry papers can be found as white papers and are not often published in peer-reviewed
conferences or journals.
Fig 4. Publication year of the selected primary papers.
doi:10.1371/journal.pone.0163477.g004
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The geographical distribution of the selected papers is shown in Fig 6. The largest number
of papers (13, 31%) were published by universities or companies in the USA. Afterthis, the two
most common publication countries were Germany with 6 papers (14.6%) and Switzerland
with 5 papers (12.2%). The rest of the countrieshad four or less papers published. The geo-
graphical distribution of the selected primary papers shows that Blockchain technology has
gathered research interest around the world.
Publication type and channel
Fig 7 shows the publication type of the selected papers. Publication type means the channel
where the paper has been published. The publication types included in this mapping study
were conference, journal, workshop, symposium, and book chapter. Most of the papers were
published in conferences (23) (56%) and workshops (12) (29.2%). The rest of the papers were
published in symposiums (4) (9.7%), as a book chapter (1) (2.4%), or in a journal (1) (2.4%). In
addition, Table 2 shows the publication channel of each selected paper. Most papers were pub-
lished in conferences and workshops in the International Conference on Financial
Fig 5. Source of the selected primary papers.
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Fig 6. Geographic distribution of the selected primary papers.
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Cryptography and Data Security (FC) (13) (31.7%). 3 or less of the selected papers had used
other publication channels.
Classification of the relevant papers
In this section, the classification of the selected primary papers is presented, including extracted
data items DI7-DI10 (Table 1). After reading all the selected papers and creating classifications
based on the findings, we identified that a majority of the papers were related to the technical
challenges and limitations presented by Swan [1]. Therefore, we decided to use these challenges
and limitations for the classification to map the existingresearch on Blockchain. The chal-
lenges and limitations presented by Swan are throughput, latency, size and bandwidth, security,
wasted resources, usability, versioning, hard forks, and multiple chains. In addition, we identi-
fied a new classification type, privacy. Privacy in an essential attribute in the Blockchain envi-
ronment, becauseof its anonymity characteristic. In addition, we also usedthe class others to
map papers that were not related to any of the classes mentioned above.
We also identified that there were three different paper types for each class, Blockchain
report, Blockchain improvement and Blockchain application. A Blockchain reportincludes
papers that report previously identified solutions and ideas in Blockchain and Bitcoin. A Block-
chain improvement includes papers that suggest new solutions and improvements to the cur-
rent Blockchain or Bitcoin technology. A Blockchain application includes papers that present an
application based on Blockchain technology. The final map of this study is presented in Fig 8.
We also decided to examine the papers based on their relation to Bitcoin (Fig 9), because it
is considered so far the most important and commonly used solution based on Blockchain
technology. As expected, a great number of papers were related to Bitcoin, rather than other
applications. In 33 (80.5%) of the selectedpapers, the research was conducted in the Bitcoin
environment. We found only 8 papers (19.5%) that did focus on Bitcoin, but on other applica-
tions using Blockchain technology.
We also made a comparison between the paper type (Blockchain report, Blockchain
improvement, and Blockchain application) and the publication year. The comparison is shown
in Fig 10. The figure shows an increasing number of papers in both report and application cate-
gories over the threeyears. Improvement papers had a significant increase in 2014, but a
decrease in 2015.
Fig 7. Publication type.
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Security
Security was the one of the major research topics in the selectedprimary papers. 14 out of the
41 papers (34%) were related to challenges and limitations in Blockchain and Bitcoin security.
We identified various topics in security, including trends and impacts of security incidents,
51% attack, data malleability problems, and authentication and cryptography issues.
Trends and impacts of security incidents: With the increasing use of Bitcoin as a way to
conduct payments and transfers, security incidents and their impact on the economic losses of
Bitcoin users have increased. Some of the identified papers presented security incidents that
had occurred in the Bitcoin network, such as economic losses by several Bitcoin scams and dis-
tributed denial-of-service (DDoS) attacks on exchanges and mining pools. Vasek et al. [33]
investigated four types of Bitcoin scams (Ponzi scams, mining scams, scam wallet and fraudu-
lent exchanges) by tracking online forums and voluntary vigilantes. The authors noted that $11
million had been contributed to scams by 13000 victimsin Bitcoin from September 2013 to
September 2014. Lim et al. [48] analyzed the trend of security breaches in Bitcoin and their
countermeasures. According to the authors, all possible types of security breaches had
Table 2. Publication channels.
Canadian Conference on Electrical and Computer Engineering (CCECE) [15]
International Conference on Information Systems Security (ICISS) [16]
International Workshop on Security and Trust Management (STM) [17][18]
International Conference on Applied Cryptography and Network Security
(ACNS)
[19]
International Conference on Software Engineering, Artificial Intelligence,
Networking and Parallel/Distributed Computing (SNPD)
[20]
International Conference on Passive and Active Measurement (PAM) [21]
International Conference on Intelligence in Next Generation Networks
(ICIN)
[22]
International Conference on Financial Cryptography and Data Security
(FC)
[23][24][25][26][27][28][29][30]
[31][32][33][34][35]
European Symposium on Research in Computer Security (ESORICS) [36][37][38]
International Conference on the Theory and Applications of
Cryptographic Techniques (EUROCRYPT)
[39]
International Cryptology Conference (CRYPTO) [40]
International Conference on Trust & Trustworthy Computing (TRUST) [41][42]
International Conference on Network and System Security (NSS) [43][44]
Book: Programming Languages with Applications to Biology and Security [45]
ACM Conference on Computer and Communications Security (ACM
CCS)
[46]
ACM Conference on Data and Application Security and Privacy
(CODASPY)
[47]
International Conference on Computational Science and Applications
(ICCSA)
[48]
eCrime Researchers Summit (eCRS) [49]
International Conference on Big Data and Cloud Computing (BDCloud) [50]
IEEE Symposium on Visualization for Cyber Security (VizSec) [51]
International Conference on Peer-to-Peer Computing (P2P) [52]
International Workshop on Secure Peer-to-Peer Intelligent Networks &
Systems (SPINS)
[53]
IEEE Micro Magazine [54]
International Workshop on Data Privacy Management (DPM) [6]
doi:10.1371/journal.pone.0163477.t002
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PLOS ONE | DOI:10.1371/journal.pone.0163477 October 3, 2016 12 / 27
occurred, including DDoS attacks, private account hacking using Trojan horses, or viruses
from ads. The authors introduce some security countermeasures for individual users and safe
Bitcoin transactions(e.g. a hardware wallet and a hardware authentication device). Vasek et al.
[27] present evidence on DDoS attacks in the Bitcoin network using DDoS-related posts in the
popular Bitcointalk.org forum. The authors figured out that the most targeted service category
was the use of anti-DDoS protection,influencing factors such as the mining pool size. The
major findings of the study were that the most often targeted service was currency exchange
(41%), followed by mining pools (38%). According to the paper, 54% of the services that had
experienced DDoS attacks had anti-DDoS protection, although it was not certain whether they
had the protection on at the time of attack. In addition,of the services that had not yet experi-
enced a DDoS attack, only 15% had anti-DDoS protection. The paper concludes that over 60%
of large mining poolshave suffered DDoS attacks, compared to 17% of small pools.
Fig 8. Classification of the relevant papers.
doi:10.1371/journal.pone.0163477.g008
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51% Attack: The Blockchain mechanism is designed with the assumption that honest nodes
control the network [4]. If attacker nodes collectively control more computational power than
the good ones, the network is vulnerable to the so called 51% Attack. Beikverdi et al. [20] argue
that although the Bitcoin itself is designed as a fully decentralized network, market-based cen-
tralization of mining power by a few large mining pools increase the risk of a 51% Attack.
Their study shows that the centralization factor of Bitcoin has been continuously increasing
from 2011 (0.26) to 2014 (0.33).In this context, 0 means purely decentralizedand 1 means
fully centralized. Moreover, there are studies claiming that the 1/2 assumption of computa-
tional power is not enough for security. Garay et al. [39] propose applications built on the core
of the Bitcoin protocol focusing on the Byzantine agreement(BA), which is the fundamental
scientific problem for decentralized transaction agreement in the Bitcoin network. The sug-
gested application presents a simple BA protocol with the assumption that the adversary’s
hashing power is bounded by 1/3. Eyal and Sier [30] introduce a Selfish Mine attack where col-
luding miners obtain a revenue larger than a fair share by keeping their discovered blocks
Fig 9. Bitcoin related research.
doi:10.1371/journal.pone.0163477.g009
Fig 10. Paper types by year.
doi:10.1371/journal.pone.0163477.g010
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private. The authors propose a protocol modificationwhich commands less than 1/4 of the
total computation power.
The more recent Blockchain-based systems, such as Ethereum, allow users to specify scripts
in transactions and contracts to support applications beyond simple cash transactions. In this
case, the required computational resources for verification could be larger, depending on the
user-specified script size. Luu et al. [46] present a security attack called the verifier’s dilemma,
which drives rational miners to skip verification where the verifying transactions require signif-
icant computational resources in Bitcoin and especiallyin Ethereum. The authors formalize a
consensus model to give incentives to miners by limiting the amount of work required to verify
a block.
Armknecht et al.[42] explain how to support security and privacy in the Ripple system,
which is one of the consensus-based distributed payment protocols. The paper discusses the
basic difference between the protocol of Ripple and Bitcoin-focused Blockchain fork. A fork
can occur if two conflicting ledgers get a clear majority of votes, and could lead to double
spending attacks. According to Decker and Wattenhofer [52], the propagation delay in the Bit-
coin network is the primary cause for Blockchain forks and inconsistencies among replicas,
which was done by analyzing Blockchain synchronization mechanism.
Data malleabilityproblems: Data integrity is an essential issue in the Blockchain environ-
ment. It is necessary that when data gets sent and verified, it has not been altered or tampered
with. We found two studies related to data integrity that studied malleability attacks in Block-
chain. Malleability describesthe fact that the signatures that prove the ownership of Bitcoin
being transferred in a transaction do not provide any integrity guarantee for the signatures
themselves [36]. Therefore, in a malleability attack an attacker intercepts, modifies, and
rebroadcasts a transaction, causing the transaction issuer to believe that the original transaction
was not confirmed [36].
Decker & Wattenhoffer [36] studied transaction malleability in Bitcoin environment and
used a real-life case as an example. According to the paper, the transaction malleability prob-
lem is real and should be considered when implementing Bitcoin clients. Andrychowicz et al.
[31] made a similar study by conducting practical experimentswhich presented a high possibil-
ity of a malleability attack and its impact. In their study, the malleability attack caused incorrect
balance computing, application crashes, and a deadlock which stopped new transactions in sev-
eral well-known Bitcoin wallets. The paper suggests a depositprotocol with a timed commit-
ment scheme to enable a malleability-resilient refund transaction as a solution to the
malleability problem.
Authentication and cryptographyissues: In Bitcoin, the private key is the major authenti-
cation element. Authentication in cryptocurrency controls self-certification.There have been
some incidents with authentication. For example, there is the well-known case in Mt.Gox,
where a Bitcoin wallet company was attacked. In the attack, Mt.Goxs storage that included pri-
vate keys of their customer was stolen. This incident has motivated some studies in strengthen-
ing authentication in Bitcoin. In addition to the Mt.Gox case, Bos et al. [26] state that the use of
elliptic curve cryptography (ECC), which is usedto derive Bitcoin addresses to users,is insuffi-
cient and does not have the required randomness.
We identified a numberof papers that had the goal to address the issues in the Bitcoin
authentication process. Bamert et al. [18] suggest a Bitcoin hardware token, the BlueWallet.
The device communicates by using Bluetooth Low Energy, and is able to secure and sign Bit-
coin transactions. Ateniese et al. [19] propose a certificationsystem for Bitcoin that offers an
opt-in guaranteeto send and receive Bitcoins only to/ fromcertified users, and control of the
creation of Bitcoins addresses by trusted authorities. According to the paper, this approach
improves the trustworthiness of real-world entities into the system, which mitigates the
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existing reservationsto the adoption of Bitcoin as a legitimatecurrency. Mann et al. [17] sug-
gest two-factor authentication for a Bitcoin wallet. The authors used a smart phone as the sec-
ond authentication factor. The solution can be used with hardware already available to most
users, and the user experience/interfacehas similarities to the existing online bankingauthenti-
cation methods.
Wasted resources
The energy efficiency problem is not handled in the computer engineeringfield at the moment.
However, in special domains like mobile cloud computing, it might be one of the major issues
in the future [55]. Mining Bitcoins requires a high amount of energy to compute and verify
transactions securely and with trustworthiness [1]. However, for the efficiency of mining and
Proof-of-Work, it is important to decrease the amount of wasted resources.
We identified some papers related to the wasted resource problems in Bitcoin. Wang and
Liu [21] present the evolution of Bitcoin miners in termsof volume of solo and poolminers
and their productivity. In the early stages, the computation power was evenly distributed
among the solo miners. As the Bitcoin network evolved, the computation power of some pool
miners increased. The study notes that all miners play a zero-sum-computation race game:
each miner increases their computation power, and then the total computation power in the net-
work increases; consequently the system increases the difficultyvalue to maintain a steady Bit-
coin creation speed, which in turn reduces the Bitcoin mining rate of individual miners [21].
We also identified some papers that proposed solutions for the wasted resources problem in
Blockchain and Bitcoin. Wang and Liu [21] suggest an economic model for getting high eco-
nomic returns inconsideration of the useof mining hardware with high computation-over-
power efficiency and electricity price. Paul et al. [16] have calculated and show how a new
scheme can lead to an energy-efficient Bitcoin. The authors modified the present block header
by introducing some extra bytes to utilize the timestamp more effectively. The suggested
scheme uses less computing power, and thus the mining is more environment-friendly. Anish
[15] proposes methods of achieving contextually higher speeds of Bitcoin mining, involving
simultaneous usage of CPUs and GPUs in individual machines in mining pools. The results
presented in the paper show how standard hardware miners in large mining pools could quite
significantly add to the overall hash rate. Barkatullah et al. [54] describe the architecture and
implementation details of a CoinTerras first-generation Bitcoin mining processor, Goldstrike
1, and how this processor was used to design a Bitcoin mining machine called Terraminer IV,
especially about how high power density issues were solved and energy efficiency increased.
Usability
The original definitionof the challenges and limitations in theusability of Blockchain by Swan
[1] describes Bitcoin API as hard and difficult to use. This definition can be viewed mainly
from the developer’s perspective, where Bitcoin API is hard to implement and use in and with
other services and applications. We did not find any papers related to the usability issue from
the software developer’s perspective. However, we found several papers that considered the
usability of Bitcoin from the cryptocurrency user’s perspective. Therefore, we decided to
expand the original definitionof Blockchain usability to take usability into account also from
the point of view of the cryptocurrency user.
An important factor in Blockchain usability from the user’s perspective is the ability to ana-
lyze Blockchain. In Blockchain, new blocks are created constantly and confirmed by miners,
which creates an interesting environment of transaction flows. It is therefore essential to have
supporting tools to help users analyze the wholeBlockchain network to improve the usability.
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We found applications that had been developed for this purpose. BitConeView [51] is a system
for the visual analysis of Bitcoin flows in Blockchain. BitIodine [23] parses Blockchain, clusters
addresses that are likely to belong to the same user or group of users, classifiessuch users and
labels them, and finally visualizes the complex information extracted from the Bitcoin network.
Both these systems were tested successfully with experiments and cases, and showed effective-
ness in analyzing and detecting patterns in the Bitcoin network. These systems can help also in
improving security and privacy -related issues.
Bankruptcy and the closure of Bitcoin exchanges can cause economical damage to the cus-
tomers [38]. Decker et al. [38] propose an audit software to improve usability in Bitcoin
exchanges. The goal of the software is to prove the exchange participants’ solvency without
publishing important information. In addition, Vandervort [25] discusses the link between a
buyer and a seller with a layer of limitedanonymity, thus preventing buyers from findingor
validating information in Bitcoin. The paper presents three different models by which a reputa-
tion/rating system could be implemented in conjunction with Bitcoin transactions, and consid-
ers the pros and cons of each. Improving these aspects of exchanges done in the Bitcoin
network can improve the usability by providing additional information for the users making
the transactions.
Throughput, latency, size and bandwidth, and versioning, hard forks,
multiple chains
Interestingly, we did not identify any papers that were related to other technical challenges and
limitations, such as throughput, latency, size and bandwidth, versioning, hard forks, and multi-
ple chains.
Privacy
In a Blockchain network, a distributed consensus network without a trusted party, all the trans-
actions are transparent and announced to the public. Therefore, privacy in Blockchain is main-
tained by breaking the flow of information. The public can see all transactions, but without
information linking the transaction to identities [4]. For this security model, 10 studies out of
41 (24%) proposed privacy issues and countermeasures to increase anonymity in Blockchain.
Meiklejohn and Orlandi [32] present a definitional framework of anonymity focusing on
the ownership of the coin. There are also studies that show experimental evidenceon the lack
of anonymity in the Bitcoin network. Koshy et al. [35] analyzed a traffic pattern in Bitcoin and
conclude that some subset of Bitcoin addresses can be mapped to an IP address simply by
observing the transaction relay traffic. Feld et al. [53] introduce a framework to traverse the Bit-
coin network and generate statistics based on that. By using the tool, the authors figured out
that an average peer-list contains addresses that mostly reside in the own autonomous systems
of the peers. Taking this information into account, the authors claim that transaction linking
could be possible.
Similar to our mapping study, Herrera-Joancomartí [6] provide an exhaustive review of
papers on Bitcoin anonymity research. according to the author, very few papers have been pub-
lished regarding the traffic of Bitcoin that may reveal private information. In order to solve the
anonymity reduction, a mix of services has been proposed in some papers. A number of studies
have applied a transaction mixing technique to increase privacy. A mixing transaction allows
the users to move Bitcoins from one user address to another without a clear trace linking
between the addresses. Such transactions can act as a primitive to help improve anonymity
when transaction linking becomes more challenging.
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Valenta and Rowan [24] have modified the Mixcoin protocol to prevent the mix from learn-
ing the input/output address mappings of participating users. The authors propose a system,
Blindcoin, which modifies the Mixcoin mixing protocol by using blind signatures and a public
append-only log. The log makes it possible for a third party to verify the validity of accusations
when blind signatures are used. Ziegeldorf et al. [47] present CoinParty, a decentralized mixing
service for Bitcoin based on a combination of decryption mix-nets with threshold signatures.
According to the authors, CoinParty is secure against malicious adversaries, and the evaluation
of their prototype shows that it scales easily to a great number of participantsin real-world net-
work settings. Ruffing et al. [37] propose CoinShuffle, a completely decentralized Bitcoin mix-
ing protocol that allows theusers to utilize Bitcoin in a truly anonymous manner. It does not
require any (trusted, accountable or untrusted) third party and it is compatible with the current
Bitcoin system. CoinShuffle introduces only a small communication overhead for its users,
while avoiding additional anonymization fees and minimizing the computation and communi-
cation overhead for the rest of the Bitcoin system. Androulaki et al. [41] propose a solution, an
extension of ZeroCoin (EZC), to hide transaction value and address balances in Bitcoin for
increased privacy. ZeroCoin acts as a temporary currency to impede the traceability of coins,
but it does nothide the number of transactionsand balances of Bitcoin addresses. The pro-
posed improvements include mixing Bitcoins from various sources before sending them to a
destination and enabling payments in the form of EZC without the need to transform them
back to Bitcoin. For the effectiveness of mixing techniques in improving anonymity, Möser
et al. [49] present analysis results on some available Bitcoin mixing services. The test results
showed that linking the input and output transactions was possible in 1 out of 3 tested services.
Other than mixing techniques, Saxena et al. [28] suggest use of composite signatures to prevent
linking between sending and receiving addresses.
Smart contracts, new cryptocurrencies, botnet, broadcast protocol,
trustworthiness
We also identified other classifications that were not included in the seven technical challenges
and limitations defined by Swan [1]. Three of the papers were related to the use of Smart con-
tracts in the Blockchain environment. A smart contract is a solution that utilizes Blockchain
technology to create contracts between twoor more participants. Similarly to the useof Bitcoin
Blockchain, smart contracts are done in a decentralized environment, where contract terms are
executed by the Blockchain systemwhen the terms are fulfilled. Bigi et al. [45] introduce a
decentralized smart contract protocol inspired by BITHALO and validated the feasibility of the
protocol based on the protocols of Bitcoin. The approach is a combination of the game theory
and formal models. The authors argue that a decentralized smart contract system can be a
promising approach and worthy of beingstudied and developedfurther. Wan et al. [43] pro-
pose an electronic signing protocol between two parties using the Bitcoin network as a way of
providing a time-stamping service. In addition, smart contracts can be possibly used in various
environments and industries for different purposes. For example, Kishigami et al. [50] provide
a Blockchain-baseddigital content distributionsystem and show a prototype of theconcept.
The idea was presented to one hundred people including creators, content owners and digital
content stake holders. The feedback showed that the most impressive point was the decentral-
ized mechanism for Digital Right Management. However, the proposed system has no incen-
tive mechanism for mining calculation, which can make it a challenge to adopt at the moment.
Even though Bitcoin is the most famous and commonly used cryptocurrency adopting
Blockchain technology, there has also been research on developing other cryptocurrencies.
Zhang and Wen [22] have designed a new generation cryptocoin called IoTcoin, based on the
Where Is Current Research on Blockchain Technology?—A Systematic Review
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protocol of Bitcoin and Blockchain. In IoT-coin, people can use keys and scripts which are
obtained in them to exchange paid sensor data or smart property. IoTcoins can be used to pres-
ent the ownership of many IoT commodities, such as smart property, paid data and digital con-
trolled energy. Another cryptocurrency has been proposed by Vandervort et al. [29] as a model
of a community cryptocurrencywith a community fundfeature.
We also found three papers that used Blockchain for Botnet networks, a P2P broadcast pro-
tocol, and a trustworthiness improvement. Ali et al. [34] ppresent Zombiecoin, which runs in
Bitcoin networks and offers a Botnet C&C (command-and-control) mechanism. Botnet net-
works include a number of computers communicating in an effort to compute representative
tasks. However, the weak point for botnet is the C&C infrastructure.The Bitcoin transaction
can be used as a communication vehicle. Andrychowicz and Dziembowsk [40] ppresent a for-
mal model for peer-to-peer communication and a Proof-of-Work concept used in Bitcoin, and
based on the model, propose a broadcast protocol which is more secure against an adversary
with arbitrary computational power. Wilson and Ateniese [44] have adopted the Bitcoin tech-
nology to enhance the Pretty Good Privacy (PGP) mechanism. In this mechanism, a Bitcoin
address, Bitcoin identity verification transactions, and a Blockchain key server are used to
improve the user’s trustworthiness.
Summary of the identified challenges/limitations and suggested
solutions in Blockchain
In Fig 11 we summarize the identified challenges and suggested solutions in Blockchain and
Bitcoin.
Discussion
In this chapter we discuss the results and answer thefour main research questions. In addition,
at the end of this chapter, we discuss the limitations and validity of the study.
RQ1: What research topics have been addressed in current research on
Blockchain?
The results of this mapping study showed that a majority of the current research on Blockchain
is focused on finding and identifying improvements to the current challenges and limitations
in Blockchain [1]. A large portion of the research concentrates on security and privacy issues in
Blockchain.
The security vulnerability of the Blockchain network and the growing interest in Bitcoin
have increased the economic losses of both miners and end users. The identified vulnerabilities
include computation power -based attacks, such as the 51% attack, selfish mine attack, transac-
tion data malleability problems, and deanonymization by transaction linking. Although several
solutions to address these issues have been presented, many of them are just brief idea sugges-
tions, lacking concrete evaluation of their effectiveness.
The research on other topics in challenges and limitations described by Swan [1], such as
wasted resources and usability, was rather limited. We found some research done on computa-
tional power and wasted resources in Bitcoin mining, and improvements on the usability of
Bitcoin. However, the number of papers was considerably small compared to those on security
and privacy issues. Computational power is one of the key attributes in Blockchain, and it
requires attention in the research. When Blockchain grows more complex, it also requires
more computational power to confirm more blocks. The Proof-of-Work concept is a rather
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new idea, which isthe reason why it has to be studied more, to make sure that it can work in
large-scale Blockchain environments.
Interestingly, we did not find many studies on challenges and limitations in latency, size
and bandwidth, throughput, versioning, hard forks, and multiple chains. It is surprising that
the attention paid to and research done on other challenges and limitations than security and
privacy was rather low. We assumed that especially topics like latency, size and bandwidth, and
wasted resources would have received more attention in the overall research map. When the
size of Blockchain increases, it has a direct impact on all these challenges and limitations in
scalability. It is possible that these issues have not been studied a lot because the Blockchain
concept is still rather new.
In addition to the identified research topics, the findings in this mapping study showed that
a majority of research was conducted in the Bitcoin environment. This was alsothe original
assumption of the authors, considering that Bitcoin is currently the most commonly used and
important technology using Blockchain, with the largest user base. However, we were quite sur-
prised that the number of other solutions than Bitco using Blockchain was so low. The results
showed that the research outside the Bitcoin environment was mostly focused on smart
Fig 11. Summary of the identified challenges and solutions of Blockchain.
doi:10.1371/journal.pone.0163477.g011
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contracts and other cryptocurrencies,but the research on Bitcoin and its security issues formed
the majority.
RQ2: What applications have been developed with and/to Blockchain
technology?
We originally defined a Blockchain application as a solution that has been developed with
Blockchain technology. By this definition, we identified some prototype applications developed
and suggested for using Blockchain in other environments, such as IoT, smart contracts, smart
property, digital content distribution, Botnet, and P2P broadcast protocols. This shows that
Blockchain technology is not limited to applications in cryptocurrencies.Instead, the idea of a
public ledger and a decentralized environment can be applied to various other applications in
different industries, which makes the whole Blockchain research more interesting.
However, we also found a set of different applications developed for the Bitcoin environ-
ment, rather than using Blockchain technology in some other environment. Some of the appli-
cations were developed for Bitcoin analysis. Applications like BitConeView [51] and BitIodine
[23] help users to analyze the Bitcoin network and study how Bitcoin transactions are com-
pleted, with a visual presentation. These types of applications can help to understand the
essence of Blockchain, and how a decentralized transaction environment actually works. Anal-
ysis applications can also help to identify frauds and possiblesecurity issues by following the
flows of transactions.
Another major direction for applications is security. We found applications where the focus
was on Bitcoin mixers.Bitcoin mixing applications, such as CoinParty [47] and CoinShuffle[37]
can help the Bitcoin network to become more secure, by adding an extra layer of privacy for the
users. These types of applications and solutions will likely increase in the future, considering that
security and privacy are the main attributes in a decentralized transaction environment.
RQ3: What are the current research gaps in Blockchain research?
We were able to identify a few major research gaps. The first gap is that the research on topics
such as latency, throughput, size and bandwidth, versioning, hard forks, and multiple forks
does not exist in the current literature. This is a major research gap, which requires more
research in the future. These topics are not possibly the mostinteresting topics for researchers
at the moment, because the sizes of the current Blockchain applications are relatively small. Bit-
coin is currently the largest solution with Blockchain. The number of transactions in Bitcoin is
considerably smallerthan e.g. in VISA. However, in the future, if Blockchainsolutions are used
by tens of millions of peopleand the number of transactions is multiplied drastically, more
research on e.g.latency, size and bandwidth, and wasted resources needsto be conducted to
ensure scalability.
The second research gap is the lack of research on usability. We identified only papers that
discussed usability from the user perspective, not from the developer perspective, as suggested
by Swan [1]. For instance, the difficulty of using Bitcoin API has not been tackled yet. This
needs to be studied and improved in the future. This could spark more applications and solu-
tions to the Bitcoin environment.
The third research gap is that the majority of current research is conducted in the Bitcoin
environment, rather than in other Blockchain environments. Research on e.g.smart contracts
needs to be carried out to increase knowledge outside cryptocurrencies. Even though Block-
chain was first introducedin the cryptocurrency environment, the same idea can be usedin
various other environments. Therefore, it is necessary to conduct research on the possibilities
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of using Blockchain in other environments, because it can reveal and produce better models
and possibilities for doing transactions in different industries.
The fourth research gap can be found in the low number of high quality publications in
journal level publication channels. Currently most of the research is published in conferences,
symposiums and workshops. There is a need for high quality journals where the focus is on
Blockchain.
RQ4: What are the future research directions for Blockchain?
The future research directionsfor Blockchain are not clear, and it is interesting to see where it
is heading. On theother hand, Bitcoin has receiveda lot of attention as a cryptocurrency, and
more people are trading and buying Bitcoins every day. Therefore, it is highly possible that Bit-
coin is important as one of the future research topics, and it willattract industry and academia
to conduct more research from both business and technical perspectives.
Bitcoin is only one solution using Blockchain technology. There are also a lot of other cryp-
tocurrencies at the moment, competing with Bitcoin to be the world’s primary cryptocurrency.
We believe that future research will also include research conducted on other cryptocurrencies.
However, at the moment it seems that Bitcoin has by far the largest market share, and it will be
a challenge for other cryptocurrencies to compete with it.
However, we believe that future research will not only focus on Bitcoin and other cryptocur-
rencies, but on other possible applications using Blockchain as a solution. We already found
some papers that studied the possibility of using smart contracts, licensing, IoT, and smart
properties in the Blockchain environment. We believe that this type of research will have a lot
of impact in the future, and can possibly be even more interesting than cryptocurrencies.To
use a decentralizedenvironment in e.g. sharing a virtual property could be a solution that revo-
lutionizes the way companies can sell their products. Taking this in consideration, we strongly
believe that when Blockchain technology gets adopted more by both industry and academia, it
will generate a significant amount of new research.
When more Blockchain solutions are taken in use with larger numbers of users, it will also
have an impact on the research done on technicallimitations and challenges.In the future,
increased sizes and user bases in various Blockchains will trigger the need to conduct more
research on the challenges and limitations in topics related to scalability. In addition, the secu-
rity and privacy of Blockchain will be always a topic for research, when new ways are invented
to disturb and attack Blockchain. Although Blockchain is a rather new technology, there already
exist profound studies in each problem domain including security and distributed system litera-
ture (for example, multi-level authentication technique [56], energy-efficient resource manage-
ment for distributed systems [55,57], and etc.). A closerlook and adoption of proven solutions
would accelerate overcoming current challenges and limitations of Blockchain technology.
Limitations of the systematic mapping study
The principal limitations of a systematic mapping study are related to publication bias, selec-
tion bias, inaccuracy in data extraction, and misclassification [58].
Publication bias refers to the problem that positive results are more likely to be published
than negative ones,since negative results take longer to be published or are cited in other publi-
cations to a lesser extent [2][58]. To address this issue, we used several well-known scientific
databases in the search protocol to find as many papers as possible. This increased the number
of papers we were able to find for this mapping study, which to some extent also increased the
possibility to find papers with negative results. However, considering that Blockchain technol-
ogy is rather a new topic in computer science industry and academia,it is possible that research
Where Is Current Research on Blockchain Technology?—A Systematic Review
PLOS ONE | DOI:10.1371/journal.pone.0163477 October 3, 2016 22 / 27
has been conducted in the industry and published as white papers or internally within compa-
nies. Therefore, all research conducted on the technicalaspects on Blockchain mightnot be
included in this mapping study. However, by using only scientific databases as a source for
finding relevant research, we were able to collect papers that were probably of a higher quality.
Selection bias refers to the distortionof statistical analysis owing to thecriteria used to select
the publications [58]. We addressed this issue by designingour search protocol carefully. We
also conducted a pilot search with different keywords, to ensure that we included as many
papers as possible in this mapping study. We defined rigorous inclusion and exclusion criteria,
to ensure that all the selected papers were part of our research topic, and answered the research
questions. However, there is one major limitation that needs to be addressed.Our search proto-
col included only the term Blockchain. There is a possibility that not all the research related to
Blockchain was found due to our search protocol for paper retrieval. Much of the research
related to Blockchain concerns economic, legal, or regulation aspects of Bitcoin and its possibil-
ities as a cryptocurrency. Our goal was to study the technical aspects of Blockchain, rather than
trying to understand how Bitcoin as a cryptocurrency can work in the real-world environment.
Based on our pilot search, we believethat we were able to retrieve a majority of the relevant
papers by using only Blockchain as the search term.
Inaccuracy in data extraction and misclassification refer to the possibility that information
is extracted differently by different reviewers [58]. We addressed this issue by using three
authors in the paper retrievalprocess. All three authors went through the abstracts of the
selected papers, and gave their opinion on including or excluding the paper. In a situation
where the opinions did not match, we had a discussion to address whether that specific paper
should be included or excluded. In addition, the classifications of the papers were done in sev-
eral face-to-face meetings, where the three authors discussed and created classifications and
mappings to all the 41 selected primary papers.
Conclusion
Blockchain technology runs the Bitcoin cryptocurrency. It is a decentralized environment for
transactions, whereall the transactions are recordedto a public ledger, visible to everyone.
The goal of Blockchain is to provide anonymity, security, privacy, and transparency to all its
users. However, these attributes set up a lot of technical challenges and limitations that need
to be addressed.
To understand where the current research on Blockchain technology positions itself, we
decided to map all relevant research by using the systematic mapping study process [2]. The
goal of this systematic mapping study was to examine the current status and research topics of
Blockchain technology. We excluded the economic, law, business, and regulation perspectives,
and included only the technical perspective. We extracted and analyzed 41 primary papers
from scientific databases. We provide recommendations on future research directions of Block-
chain technology based on the current research status as following:
Continue to identify more issues and propose solutions to overcome challenges and limitations
of Blockchain technology.
The interest on Blockchain technology has been drastically increased since 2013. The cumu-
lative number of papers is increased from 2 in 2013 to 41 in 2015. Majority of the studies has
been focused on addressing the challenges and limitations, but there still exist many issues
without proper solutions.
Conduct more studies on scalability issues of Blockchain.
Most of the currentresearch on the Blockchain technology is focusedon security and privacy
Where Is Current Research on Blockchain Technology?—A Systematic Review
PLOS ONE | DOI:10.1371/journal.pone.0163477 October 3, 2016 23 / 27
issues. To be ready for pervasive use of Blockchain technology, scalability issues such as per-
formance and latency have to be addressed.
Develop more Blockchain based applications beyond Bitcoin and other cryptocurrency systems.
The current research is focusedon Bitcoin system. However, the research also shows that
Blockchain technology is applicable for other solutions such as smart contracts, property
licensing, voting etc.
Evaluate the effectiveness of the proposed solutions with an objective evaluation criteria.
Although several solutions to challenges and limitations have been presented, many of them
are just brief idea suggestions and lack concrete evaluation on their effectiveness.
Supporting Information
S1 Table. The full list of selected primary papers.
(PDF)
S1 Checklist. PRISMA Checklist.
(DOC)
S1 Diagram. PRISMA Flow diagram.
(DOC)
Author Contributions
Conceptualization:JY DK SC SP KS.
Data curation: JY DK SC.
Formal analysis: JY DK SC.
Investigation: JY DK SC.
Methodology: JY DK SC.
Project administration: SC.
Resources: SP KS.
Supervision: SP KS.
Visualization: JY.
Writing – original draft: JY DK.
Writing – review & editing: JY DK SC.
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The objective of this report is to propose comprehensive guidelines for systematic literature reviews appropriate for software engineering researchers, including PhD students. A systematic literature review is a means of evaluating and interpreting all available research relevant to a particular research question, topic area, or phenomenon of interest. Systematic reviews aim to present a fair evaluation of a research topic by using a trustworthy, rigorous, and auditable methodology. The guidelines presented in this report were derived from three existing guidelines used by medical researchers, two books produced by researchers with social science backgrounds and discussions with researchers from other disciplines who are involved in evidence-based practice. The guidelines have been adapted to reflect the specific problems of software engineering research. The guidelines cover three phases of a systematic literature review: planning the review, conducting the review and reporting the review. They provide a relatively high level description. They do not consider the impact of the research questions on the review procedures, nor do they specify in detail the mechanisms needed to perform meta-analysis.
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The invention of the Bitcoin protocol has opened the door to new forms of financial interaction. One such form may be to adapt Bitcoin technology for use as a community currency. A community currency is a form of money issued by a non-government entity to serve the economic or social interests of a group of people, often in a small geographic area. We propose a model of a community cryptocurrency that includes a community fund from which community members may take out loans if the community votes to approve them. We consider possible vulnerabilities and mitigations to issues that would affect this community fund, including issues of identity, voting protocols and funds management. We conclude that these vulnerabilities are, in most cases, amenable to technological mitigations that must be adaptable to both community values and changing conditions, emphasizing the need for careful currency design.
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