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An Overview of Blockchain Technology:
Architecture, Consensus, and Future Trends
Zibin Zheng1, Shaoan Xie1, Hongning Dai2, Xiangping Chen4, and Huaimin Wang3
1School of Data and Computer Science, Sun Yat-sen University Guangzhou, China
2Faculty of Information Technology, Macau University of Science and Technology, Macau, SAR
3National Laboratory for Parallel & Distributed Processing
National University of Defense Technology, Changsha 410073 China
4Institute of Advanced Technology,National Engineering Research Center of Digital Life
Sun Yat-sen University, Guangzhou, China
Email: zhzibin@mail.sysu.edu.cn
Abstract—Blockchain, the foundation of Bitcoin, has received
extensive attentions recently. Blockchain serves as an immutable
ledger which allows transactions take place in a decentralized
manner. Blockchain-based applications are springing up, cov-
ering numerous fields including financial services, reputation
system and Internet of Things (IoT), and so on. However,
there are still many challenges of blockchain technology such
as scalability and security problems waiting to be overcome.
This paper presents a comprehensive overview on blockchain
technology. We provide an overview of blockchain architechture
firstly and compare some typical consensus algorithms used
in different blockchains. Furthermore, technical challenges and
recent advances are briefly listed. We also lay out possible future
trends for blockchain.
Index Terms—Blockchain, decentralization, consensus, scala-
bility
I. INTRODUCTION
Nowadays cryptocurrency has become a buzzword in both
industry and academia. As one of the most successful cryp-
tocurrency, Bitcoin has enjoyed a huge success with its capital
market reaching 10 billion dollars in 2016 [1]. With a spe-
cially designed data storage structure, transactions in Bitcoin
network could happen without any third party and the core
technology to build Bitcoin is blockchain, which was first
proposed in 2008 and implemented in 2009 [2]. Blockchain
could be regarded as a public ledger and all committed
transactions are stored in a list of blocks. This chain grows
as new blocks are appended to it continuously. Asymmetric
cryptography and distributed consensus algorithms have been
implemented for user security and ledger consistency. The
blockchain technology generally has key characteristics of
decentralization, persistency, anonymity and auditability. With
these traits, blockchain can greatly save the cost and improve
the efficiency.
Since it allows payment to be finished without any bank or
any intermediary, blockchain can be used in various financial
services such as digital assets, remittance and online payment
[3], [4]. Additionally, it can also be applied into other fields
including smart contracts [5], public services [6], Internet of
Things (IoT) [7], reputation systems [8] and security services
[9]. Those fields favor blockchain in multiple ways. First of all,
blockchain is immutable. Transaction cannot be tampered once
it is packed into the blockchain. Businesses that require high
reliability and honesty can use blockchain to attract customers.
Besides, blockchain is distributed and can avoid the single
point of failure situation. As for smart contracts, the contract
could be executed by miners automatically once the contract
has been deployed on the blockchain.
Although the blockchain technology has great potential for
the construction of the future Internet systems, it is facing a
number of technical challenges. Firstly, scalability is a huge
concern. Bitcoin block size is limited to 1 MB now while
a block is mined about every ten minutes. Subsequently, the
Bitcoin network is restricted to a rate of 7 transactions per
second, which is incapable of dealing with high frequency
trading. However, larger blocks means larger storage space
and slower propagation in the network. This will lead to
centralization gradually as less users would like to maintain
such a large blockchain. Therefore the tradeoff between block
size and security has been a tough challenge. Secondly, it has
been proved that miners could achieve larger revenue than
their fair share through selfish mining strategy [10]. Miners
hide their mined blocks for more revenue in the future. In
that way, branches could take place frequently, which hinders
blockchain development. Hence some solutions need to be put
forward to fix this problem. Moreover, it has been shown that
privacy leakage could also happen in blockchain even users
only make transactions with their public key and private key
[11]. Furthermore, current consensus algorithms like proof of
work or proof of stake are facing some serious problems. For
example, proof of work wastes too much electricity energy
while the phenomenon that the rich get richer could appear in
the proof of stake consensus process.
There is a lot of literature on blockchain from various
sources, such as blogs, wikis, forum posts, codes, confer-
ence proceedings and journal articles. Tschorsch et al. [12]
made a technical survey about decentralized digital currencies
2017 IEEE 6th International Congress on Big Data
978-1-5386-1996-4/17 $31.00 © 2017 IEEE
DOI 10.1109/BigDataCongress.2017.85
557
including Bitcoin. Compared to [12], our paper focuses on
blockchain technology instead of digital currencies. Nomura
Research Institut made a technical report about blockchain
[13]. Contrast to [13], our paper focuses on state-of-art
blockchain researches including recent advances and future
trends.
The rest of this paper is organized as follows. Section II
introduces blockchain architecture. Section III shows typical
consensus algorithms used in blockchain. Section IV summa-
rizes the technical challenges and the recent advances in this
area. Section V discusses some possible future directions and
section VI concludes the paper.
II. BLOCKCHAIN ARCHITECTURE
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Fig. 2: Block structure
Blockchain is a sequence of blocks, which holds a complete
list of transaction records like conventional public ledger
[14]. Figure 1 illustrates an example of a blockchain. With
a previous block hash contained in the block header, a block
has only one parent block. It is worth noting that uncle blocks
(children of the block’s ancestors) hashes would also be stored
in ethereum blockchain [15]. The first block of a blockchain
is called genesis block which has no parent block. We then
explain the internals of blockchain in details.
A. Block
A block consists of the block header and the block body as
shown in Figure 2. In particular, the block header includes:
(i) Block version: indicates which set of block validation
rules to follow.
(ii) Merkle tree root hash: the hash value of all the transac-
tions in the block.
(iii) Timestamp: current time as seconds in universal time
since January 1, 1970.
(iv) nBits: target threshold of a valid block hash.
(v) Nonce: an 4-byte field, which usually starts with 0
and increases for every hash calculation (will be explained
in details in Section III).
(vi) Parent block hash: a 256-bit hash value that points to
the previous block.
The block body is composed of a transaction counter and
transactions. The maximum number of transactions that a
block can contain depends on the block size and the size of
each transaction. Blockchain uses an asymmetric cryptography
mechanism to validate the authentication of transactions [13].
Digital signature based on asymmetric cryptography is used in
an untrustworthy environment. We next briefly illustrate digital
signature.
B. Digital Signature
Each user owns a pair of private key and public key.
The private key that shall be kept in confidentiality is used
to sign the transactions. The digital signed transactions are
broadcasted throughout the whole network. The typical digital
signature is involved with two phases: signing phase and
verification phase. For instance, an user Alice wants to send
another user Bob a message. (1) In the signing phase, Alice
encrypts her data with her private key and sends Bob the
encrypted result and original data. (2) In the verification phase,
Bob validates the value with Alice’s public key. In that way,
Bob could easily check if the data has been tampered or not.
The typical digital signature algorithm used in blockchains is
the elliptic curve digital signature algorithm (ECDSA) [16].
C. Key Characteristics of Blockchain
In summary, blockchain has following key characteristics.
•Decentralization. In conventional centralized transaction
systems, each transaction needs to be validated through
the central trusted agency (e.g., the central bank), in-
evitably resulting to the cost and the performance bottle-
necks at the central servers. Contrast to the centralized
mode, third party is no longer needed in blockchain.
Consensus algorithms in blockchain are used to maintain
data consistency in distributed network.
•Persistency. Transactions can be validated quickly and
invalid transactions would not be admitted by honest
miners. It is nearly impossible to delete or rollback
transactions once they are included in the blockchain.
Blocks that contain invalid transactions could be discov-
ered immediately.
•Anonymity. Each user can interact with the blockchain
with a generated address, which does not reveal the
real identity of the user. Note that blockchain cannot
guarantee the perfect privacy preservation due to the
intrinsic constraint (details will be discussed in section
IV).
558
TABLE I: Comparisons among public blockchain,consortium blockchain and private blockchain
Property Public blockchain Consortium blockchain Private blockchain
Consensus determination All miners Selected set of nodes One organization
Read permission Public Could be public or restricted Could be public or restricted
Immutability Nearly impossible to tamper Could be tampered Could be tampered
Efficiency Low High High
Centralized No Partial Yes
Consensus process Permissionless Permissioned Permissioned
•Auditability. Bitcoin blockchain stores data about user
balances based on the Unspent Transaction Output (UTX-
O) model [2]: Any transaction has to refer to some previ-
ous unspent transactions. Once the current transaction is
recorded into the blockchain, the state of those referred
unspent transactions switch from unspent to spent. So
transactions could be easily verified and tracked.
D. Taxonomy of blockchain systems
Current blockchain systems are categorized roughly into
three types: public blockchain, private blockchain and con-
sortium blockchain [17]. In public blockchain, all records are
visible to the public and everyone could take part in the con-
sensus process. Differently, only a group of pre-selected nodes
would participate in the consensus process of a consortium
blockchain. As for private blockchain, only those nodes that
come from one specific organization would be allowed to join
the consensus process.
A private blockchain is regarded as a centralized network
since it is fully controlled by one organization. The consortium
blockchain constructed by several organizations is partially
decentralized since only a small portion of nodes would be
selected to determine the consensus. The comparison among
the three types of blockchains is listed in Table I.
•Consensus determination. In public blockchain, each n-
ode could take part in the consensus process. And only
a selected set of nodes are responsible for validating the
block in consortium blockchain. As for private chain, it is
fully controlled by one organization and the organization
could determine the final consensus.
•Read permission. Transactions in a public blockchain are
visible to the public while it depends when it comes to a
private blockchain or a consortium blockchain.
•Immutability. Since records are stored on a large number
of participants, it is nearly impossible to tamper trans-
actions in a public blockchain. Differently, transactions
in a private blockchain or a consortium blockchain could
be tampered easily as there are only limited number of
participants.
•Efficiency. It takes plenty of time to propagate transac-
tions and blocks as there are a large number of nodes
on public blockchain network. As a result, transaction
throughput is limited and the latency is high. With fewer
validators, consortium blockchain and private blockchain
could be more efficient.
•Centralized. The main difference among the three types
of blockchains is that public blockchain is decentralized,
consortium blockchain is partially centralized and private
blockchain is fully centralized as it is controlled by a
single group.
•Consensus process. Everyone in the world could join
the consensus process of the public blockchain. Different
from public blockchain, both consortium blockchain and
private blockchain are permissioned.
Since public blockchain is open to the world, it can at-
tract many users and communities are active. Many public
blockchains emerge day by day. As for consortium blockchain,
it could be applied into many business applications. Cur-
rently Hyperledger [18] is developing business consortium
blockchain frameworks. Ethereum also has provided tools for
building consortium blockchains [19].
III. CONSENSUS ALGORITHMS
In blockchain, how to reach consensus among the untrust-
worthy nodes is a transformation of the Byzantine Generals
(BG) Problem, which was raised in [20]. In BG problem,
a group of generals who command a portion of Byzantine
army circle the city. Some generals prefer to attack while
other generals prefer to retreat. However, the attack would
fail if only part of the generals attack the city. Thus, they
have to reach an agreement to attack or retreat. How to reach
a consensus in distributed environment is a challenge. It is
also a challenge for blockchain as the blockchain network
is distributed. In blockchain, there is no central node that
ensures ledgers on distributed nodes are all the same. Some
protocols are needed to ensure ledgers in different nodes are
consistent. We next present several common approaches to
reach a consensus in blockchain.
A. Approaches to consensus
PoW (Proof of work) is a consensus strategy used in the
Bitcoin network [2]. In a decentralized network, someone has
to be selected to record the transactions. The easiest way is
random selection. However, random selection is vulnerable to
attacks. So if a node wants to publish a block of transactions, a
lot of work has to be done to prove that the node is not likely
to attack the network. Generally the work means computer
559
TABLE II: Typical Consensus Algorithms Comparison
Property PoW PoS PBFT DPOS Ripple Tendermint
Node identity management open open permissioned open open permissioned
Energy saving no partial yes partial yes yes
Tolerated power <25% <51% <33.3% <51% <20% <33.3%
of adversary computing
power
stake faulty replicas validators faulty nodes in
UNL
byzantine voting
power
Example Bitcoin [2] Peercoin [21] Hyperledger
Fabric [18]
Bitshares [22] Ripple [23] Tendermint [24]
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Fig. 3: An scenario of blockchain branches (the longer branch
would be admitted as the main chain while the shorter one
would be deserted)
calculations. In PoW, each node of the network is calculating
a hash value of the block header. The block header contains
a nonce and miners would change the nonce frequently to
get different hash values. The consensus requires that the
calculated value must be equal to or smaller than a certain
given value. When one node reaches the target value, it would
broadcast the block to other nodes and all other nodes must
mutually confirm the correctness of the hash value. If the block
is validated, other miners would append this new block to
their own blockchains. Nodes that calculate the hash values
are called miners and the PoW procedure is called mining in
Bitcoin.
In the decentralized network, valid blocks might be gen-
erated simultaneously when multiple nodes find the suitable
nonce nearly at the same time. As a result, branches may be
generated as shown in Figure 3. However, it is unlikely that
two competing forks will generate next block simultaneously.
In PoW protocol, a chain that becomes longer thereafter is
judged as the authentic one. Consider two forks created by
simultaneously validated blocks U4 and B4. Miners keep
mining their blocks until a longer branch is found. B4,B5
forms a longer chain, so the miners on U4 would switch to
the longer branch.
Miners have to do a lot of computer calculations in PoW,
yet these works waste too much resources. To mitigate the
loss, some PoW protocols in which works could have some
side-applications have been designed. For example, Primecoin
[25] searches for special prime number chains which can be
used for mathematical research.
PoS (Proof of stake) is an energy-saving alternative to PoW.
Miners in PoS have to prove the ownership of the amount
of currency. It is believed that people with more currencies
would be less likely to attack the network. The selection
based on account balance is quite unfair because the single
richest person is bound to be dominant in the network. As a
result, many solutions are proposed with the combination of
the stake size to decide which one to forge the next block.
In particular, Blackcoin [26] uses randomization to predict the
next generator. It uses a formula that looks for the lowest
hash value in combination with the size of the stake. Peercoin
[21] favors coin age based selection. In Peercoin, older and
larger sets of coins have a greater probability of mining the
next block. Compared to PoW, PoS saves more energy and
is more effective. Unfortunately, as the mining cost is nearly
zero, attacks might come as a consequence. Many blockchains
adopt PoW at the beginning and transform to PoS gradually.
For instance, ethereum is planing to move from Ethash (a kind
of PoW) [27] to Casper (a kind of PoS) [28].
PBFT (Practical byzantine fault tolerance) is a replication
algorithm to tolerate byzantine faults [29]. Hyperledger Fabric
[18] utilizes the PBFT as its consensus algorithm since PBFT
could handle up to 1/3 malicious byzantine replicas. A new
block is determined in a round. In each round, a primary would
be selected according to some rules. And it is responsible for
ordering the transaction. The whole process could be divided
into three phase: pre-prepared,prepared and commit. In each
phase, a node would enter next phase if it has received votes
from over 2/3 of all nodes. So PBFT requires that every
node is known to the network. Like PBFT, Stellar Consensus
Protocol (SCP) [30] is also a Byzantine agreement protocol.
In PBFT, each node has to query other nodes while SCP gives
participants the right to choose which set of other participants
to believe. Based on PBFT, Antshares [31] has implemented
their dBFT (delegated byzantine fault tolerance). In dBFT,
some professional nodes are voted to record the transactions.
DPOS (Delegated proof of stake). The major difference
between PoS and DPOS is that PoS is direct democratic while
DPOS is representative democratic. Stakeholders elect their
delegates to generate and validate blocks. With significantly
fewer nodes to validate the block, the block could be confirmed
quickly, leading to the quick confirmation of transactions.
Meanwhile, the parameters of the network such as block size
and block intervals could be tuned by delegates. Additionally,
560
users need not to worry about the dishonest delegates as they
could be voted out easily. DPOS is the backbone of Bitshares
[22].
Ripple [23] is a consensus algorithm that utilizes
collectively-trusted subnetworks within the larger network. In
the network, nodes are divided into two types: server for
participating consensus process and client for only transferring
funds. Each server has an Unique Node List (UNL). UNL is
important to the server. When determining whether to put a
transaction into the ledger, the server would query the nodes
in UNL and if the received agreements have reached 80%, the
transaction would be packed into the ledger. For a node, the
ledger will remain correct as long as the percentage of faulty
nodes in UNL is less than 20%.
Tendermint [24] is a byzantine consensus algorithm. A new
block is determined in a round. A proposer would be selected
to broadcast an unconfirmed block in this round. It could be
divided into three steps: 1) Prevote step. Validators choose
whether to broadcast a prevote for the proposed block. 2)
Precommit step. If the node has received more than 2/3 of
prevotes on the proposed block, it broadcasts a precommit for
that block. If the node has received over 2/3 of precommits,
it enters the commit step. 3) Commit step. The node validates
the block and broadcasts a commit for that block. if the
node has received 2/3 of the commits, it accepts the block.
Contrast to PBFT, nodes have to lock their coins to become
validators. Once a validator is found to be dishonest, it would
be punished.
B. Consensus algorithms comparison
Different consensus algorithms have different advantages
and disadvantages. Table II gives a comparison between d-
ifferent consensus algorithms and we use the properties given
by [32].
•Node identity management. PBFT needs to know the
identity of each miner in order to select a primary in every
round while Tendermint needs to know the validators in
order to select a proposer in each round. For PoW, PoS,
DPOS and Ripple, nodes could join the network freely.
•Energy saving. In PoW, miners hash the block header
continuously to reach the target value. As a result, the
amount of electricity required to process has reach an
immense scale. As for PoS and DPOS, miners still have
to hash the block header to search the target value but
the work has been largely reduced as the search space
is designed to be limited. As for PBFT, Ripple and
Tendermint, there is no mining in consensus process. So
it saves energy greatly.
•Tolerated power of adversary. Generally 51% of hash
power is regarded as the threshold for one to gain control
of the network. But selfish mining strategy [10] in PoW
systems could help miners to gain more revenue by only
25% of the hashing power. PBFT and Tendermint is
designed to handle up to 1/3 faulty nodes. Ripple is
proved to maintain correctness if the faulty nodes in an
UNL is less than 20%.
•Example. Bitcoin is based on PoW while Peercoin is
a new peer-to-peer PoS cryptocurrency. Further, Hyper-
ledger Fabric utilizes PBFT to reach consensus. Bitshares,
a smart contract platform, adopts DPOS as their con-
sensus algorithm. Ripple implements the Ripple protocol
while Tendermint devises the Tendermint protocol.
PBFT and Tendermint are permissioned protocols. Node
identities are expected to be known to the whole network,
so they might be used in commercial mode rather than public.
PoW and PoS are suitable for public blockchain. Consortium
or private blockchain might has preference for PBFT, Tender-
mint, DPOS and Ripple.
C. Advances on consensus algorithms
A good consensus algorithm means efficiency, safty and
convenience. Recently, a number of endeavors have been made
to improve consensus algorithms in blockchain. New con-
sensus algorithms are devised aiming to solve some specific
problems of blockchain. The main idea of PeerCensus [33] is
to decouple block creation and transaction confirmation so that
the consensus speed can be significantly increased. Besides,
Kraft [34] proposed a new consensus method to ensure that
a block is generated in a relatively stable speed. It is known
that high blocks generation rate compromise Bitcoin’s security.
So the Greedy Heaviest-Observed Sub-Tree (GHOST) chain
selection rule [35] is proposed to solve this problem. Instead
of the longest branch scheme, GHOST weights the branches
and miners could choose the better one to follow. Chepurnoy
et al. [36] presented a new consensus algorithm for peer-to-
peer blockchain systems where anyone who provides non-
interactive proofs of retrievability for the past state snapshots
is agreed to generate the block. In such a protocol, miners
only have to store old block headers instead of full blocks.
IV. CHALLENGES &RECENT ADVANCES
Despite the great potential of blockchain, it faces numerous
challenges, which limit the wide usage of blockchain. We
enumerate some major challenges and recent advances as
follows.
A. Scalability
With the amount of transactions increasing day by day,
the blockchain becomes bulky. Each node has to store all
transactions to validate them on the blockchain because they
have to check if the source of the current transaction is unspent
or not. Besides, due to the original restriction of block size and
the time interval used to generate a new block, the Bitcoin
blockchain can only process nearly 7 transactions per second,
which cannot fulfill the requirement of processing millions of
transactions in real-time fashion. Meanwhile, as the capacity of
blocks is very small, many small transactions might be delayed
since miners prefer those transactions with high transaction
fee.
There are a number of efforts proposed to address the
scalability problem of blockchain, which could be categorized
into two types:
561
•Storage optimization of blockchain. Since it is harder for
node to operate full copy of ledger, Bruce proposed a
novel cryptocurrency scheme, in which the old transaction
records are removed (or forgotten) by the network [37].
A database named account tree is used to hold the
balance of all non-empty addresses. Besides lightweight
client could also help fix this problem. A novel schem
named VerSum [38] was proposed to provide another
way allowing lightweight clients to exist. VerSum allows
lightweight clients to outsource expensive computations
over large inputs. It ensures the computation result is
correct through comparing results from multiple servers.
•Redesigning blockchain. In [39], Bitcoin-NG (Next Gen-
eration) was proposed. The main idea of Bitcoin-NG is
to decouple conventional block into two parts: key block
for leader election and microblock to store transactions.
The protocol divides time into epoches. In each epoch,
miners have to hash to generate a key block. Once the key
block is generated, the node becomes the leader who is
responsible for generating microblocks. Bitcoin-NG also
extended the heaviest (longest) chain strategy in which
microblocks carry no weight. In this way, blockchain
is redesigned and the tradeoff between block size and
network security has been addressed.
B. Privacy Leakage
Blockchain can preserve a certain amount of privacy through
the public key and private key. Users transact with their
private key and public key without any real identity exposure.
However, it is shown in [40], [5] that blockchain cannot
guarantee the transactional privacy since the values of all
transactions and balances for each public key are publicly
visible. Besides, the recent study [41] has shown that a user’s
Bitcoin transactions can be linked to reveal user’s information.
Moreover, Biryukov et al. [11] presented an method to link
user pseudonyms to IP addresses even when users are behind
Network Address Translation (NAT) or firewalls. In [11], each
client can be uniquely identified by a set of nodes it connects
to. However, this set can be learned and used to find the
origin of a transaction. Multiple methods have been proposed
to improve anonymity of blockchain, which could be roughly
categorized into two types:
•Mixing [42]. In blockchain, users addresses are pseudony-
mous. But it is still possible to link addresses to user real
identity as many users make transactions with the same
address frequently. Mixing service is a kind of service
which provides anonymity by transferring funds from
multiple input addresses to multiple output addresses. For
example, user Alice with address A wants to send some
funds to Bob with address B. If Alice directly makes a
transaction with input address A and output address B,
relationship between Alice and Bob might be revealed. So
Alice could send funds to a trusted intermediary Carol.
Then Carol transfer funds to Bob with multiple inputs c1,
c2, c3, etc., and multiple output d1, d2, B, d3, etc. Bob’s
address B is also contained in the output addresses. So it
becomes harder to reveal relationship between Alice and
Bob. However, the intermediary could be dishonest and
reveal Alice and Bob’s private information on purpose.
It is also possible that Carol transfers Alice’s funds to
her own address instead of Bob’s address. Mixcoin [43]
provides a simple method to avoid dishonest behaviours.
The intermediary encrypts users’ requirements including
funds amount and transfer date with its private key. Then
if the intermediary did not transfer the money, anybody
could verify that the intermediary cheated. However, theft
is detected but still not prevented. Coinjoin [44] depends
on a central mixing server to shuffle output addresses to
prevent theft. And inspired by Coinjoin, CoinShuffle [45]
uses decryption mixnets for address shuffling.
•Anonymous. In Zerocoin [46], zero-knowledge proof
is used. Miners do not have to validate a transaction
with digital signature but to validate coins belong to
a list of valid coins. Payment’s origin are unlinked
from transactions to prevent transaction graph analyses.
But it still reveals payments’ destination and amounts.
Zerocash [47] was proposed to address this problem.
In Zerocash, zero-knowledge Succinct Non-interactive
Arguments of Knowledge (zk-SNARKs) is leveraged.
Transaction amounts and the values of coins held by users
are hidden.
C. Selfish Mining
Blockchain is susceptible to attacks of colluding selfish
miners. In particular, Eyal and Sirer [10] showed that the
network is vulnerable even if only a small portion of the
hashing power is used to cheat. In selfish mining strategy,
selfish miners keep their mined blocks without broadcasting
and the private branch would be revealed to the public only
if some requirements are satisfied. As the private branch is
longer than the current public chain, it would be admitted
by all miners. Before the private blockchain publishment,
honest miners are wasting their resources on an useless branch
while selfish miners are mining their private chain without
competitors. So selfish miners tend to get more revenue.
Based on selfish mining, many other attacks have been
proposed to show that blockchain is not so secure. In stubborn
mining [48], miners could amplify its gain by non-trivially
composing mining attacks with network-level eclipse attacks.
The trail-stubbornness is one of the stubborn strategy that
miners still mine the blocks even if the private chain is left
behind. Yet in some cases, it can result in 13% gains in
comparison with a non-trail-stubborn counterpart. [49] shows
that there are selfish mining strategies that earn more money
and are profitable for smaller miners compared to simple
selfish mining. But the gains are relatively small. Furthermore,
it shows that attackers with less than 25% of the computational
resources can still gain from selfish mining. To help fix the
selfish mining problem, Heilman [50] presented an novel
approach for honest miners to choose which branch to follow.
With random beacons and timestamps, honest miners would
select more fresh blocks. However, [50] is vulnerable to
562
forgeable timestamps. ZeroBlock [51] builds on the simple
scheme: Each block must be generated and accepted by the
network within a maximum time interval. Within ZeroBlock,
selfish miners cannot achieve more than its expected reward.
V. P OSSIBLE FUTURE DIRECTIONS
Blockchain has shown its potential in industry and academi-
a. We discuss possible future directions with respect to four
areas: blockchain testing,stop the tendency to centralization,
big data analytics and blockchain application.
A. Blockchain testing
Recently different kinds of blockchains appear and over
700 cryptocurrencies are listed in [52] up to now. However,
some developers might falsify their blockchain performance
to attract investors driven by the huge profit. Besides that,
when users want to combine blockchain into business, they
have to know which blockchain fits their requirements. So
blockchain testing mechanism needs to be in place to test
different blockchains.
Blockchain testing could be separated into two phases:
standardization phase and testing phase. In standardization
phase, all criteria have to be made and agreed. When a
blockchain is born, it could be tested with the agreed criteria
to valid if the blockchain works fine as developers claim. As
for testing phase, blockchain testing needs to be performed
with different criteria. For example, an user who is in charge
of online retail business cares about the throughput of the
blockchain, so the examination needs to test the average time
from a user send a transaction to the transaction is packed into
the blockchain, capacity for a blockchain block and etc.
B. Stop the tendency to centralization
Blockchain is designed as a decentralized system. However,
there is a trend that miners are centralized in the mining pool.
Up to now, the top 5 mining pools together owns larger than
51% of the total hash power in the Bitcoin network [53]. Apart
from that, selfish mining strategy [10] showed that pools with
over 25% of total computing power could get more revenue
than fair share. Rational miners would be attracted into the
selfish pool and finally the pool could easily exceed 51% of
the total power. As the blockchain is not intended to serve a
few organizations, some methods should be proposed to solve
this problem.
C. Big data analytics
Blockchain could be well combined with big data. Here
we roughly categorized the combination into two types: data
management and data analytics. As for data management,
blockchain could be used to store important data as it is
distributed and secure. Blockchain could also ensure the data
is original. For example, if blockchain is used to store patients
health information, the information could not be tampered and
it is hard to stole those private information. When it comes to
data analytics, transactions on blockchain could be used for
big data analytics. For example, user trading patterns might
be extracted. Users can predict their potential partners’ trading
behaviours with the analysis.
D. Blockchain applications
Currently most blockchains are used in the financial domain,
more and more applications for different fields are appearing.
Traditional industries could take blockchain into considera-
tion and apply blockchain into their fields to enhance their
systems. For example, user reputations could be stored on
blockchain. At the same time, the up-and-coming industry
could make use of blockchain to improve performance. For
example, Arcade City [51], a ridesharing startup offers an
open marketplace where riders connect directly with drivers
by leveraging blockchain technology.
A smart contract is a computerized transaction protocol that
executes the terms of a contract [54]. It has been proposed
for long time and now this concept can be implemented with
blockchain. In blockchain, smart contract is a code fragment
that could be executed by miners automatically. Smart contract
has transformative potential in various fields like financial
services and IoT.
VI. CONCLUSION
Blockchain has shown its potential for transforming tradi-
tional industry with its key characteristics: decentralization,
persistency, anonymity and auditability. In this paper, we
present a comprehensive overview on blockchain. We first give
an overview of blockchain technologies including blockchain
architecture and key characteristics of blockchain. We then dis-
cuss the typical consensus algorithms used in blockchain. We
analyzed and compared these protocols in different respects.
Furthermore, we listed some challenges and problems that
would hinder blockchain development and summarized some
existing approaches for solving these problems. Some possible
future directions are also proposed. Nowadays blockchain-
based applications are springing up and we plan to conduct
in-depth investigations on blockchain-based applications in the
future.
ACKNOWLEDGEMENT
The work described in this paper was supported by the
National Key Research and Development Program (2016YF-
B1000101), the National Natural Science Foundation of China
under (61472338), the Fundamental Research Funds for the
Central Universities and Macao Science and Technology De-
velopment Fund under Grant No. 096/2013/A3. The authors
would like to thank Gordon K.-T. Hon for his constructive
comments.
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