With the growth in acceptance of computers over the past few decades, records have mostly migrated from being physical paper documents to digitized versions, created and managed on a computer.
This is one of the many cyber applications, the ones enabled by computers. While such records are created and stored on computers, it still involves a human being entering the information. Financial transactions, health records, insurance records are some of the many examples in this category. So, one can say that humans were still the primary source of data collection in these applications. Over the past few years, fueled by the emergence of IoT and driven by the proliferation of sensing technology, sensors are now replacing humans as the primary source of data collection in many systems. Such systems, called CPS, combine physical processes, software, and communication to provide an integrated system with abstractions, design, and analysis capabilities. The technology spans research across multiple disciplines, having core components, such as embedded systems, real time communication, computer, networking, and physical systems dynamics. The use of blockchain for making a financial transaction has been well researched and documented. Advances in this technology have helped in sending money directly to the authorized people without including centralized authorities. Application of blockchain as smart contracts minimizes the possibility of delays, suppression, or any other outside influence. It applies comprehensive financial security, monitors the terms of the contract and is unbreakable. It also makes it easier to track and monitor digital identities using blockchain. The usage of blockchain as a cheap notary system has been described in [15
], thereby avoiding different types of scams by creating unique certificates which would be easy to verify. In similar lines, a recent review of blockchain in education is given in [16
]. This paper mainly focuses on an emerging application of blockchain for cyber-only systems, namely health records and four representative applications of CPS, namely implantable medical devices, industrial control systems, smart grid systems, and connected cars (Figure 6
). Table 1
outlines the application domains of various systems discussed in the paper, along with the societal impact in each system.
3.2. Blockchain Applications in Industrial Control Systems
Industrial control systems (ICS) refers to such systems that control and monitor the physical entities that can be used in a diverse set of industries, ranging from mission-critical nuclear plants and commonplace irrigation systems. ICS senses and collects data through sensors and passes the information to the controller, which in turns sends the feedback through the actuator, as shown in Figure 7
The key components in the ICS environment are:
A plant capable of data acquisition, communication, and local processing, using operational technology. This is called as sensors. It is a device which measures physical quantity. Examples of sensors are cameras, accelerators, gyroscope, Lidar, Radar, etc.
A computing device, typically referred to as Programming Logic Control (PLC), that can be programmed to perform operations based on programmable logic. It has been traditionally used both in Distributed Control System and Supervisory Control and Data Acquisition systems to control the overall system. A data historian is maintained on the controller to log information related to all the process. The same can be used for algorithms, parameter configuration, monitoring, and set-point configuration.
A control loop that enables the controller to execute on different tasks by interpreting sensor signals. Actuators are a part of this system, which modifies the physical quantity observed by the plant. Typical examples of actuators are motor controllers, LEDs, lasers, loudspeakers, switches, valves, etc.
The Industrial Internet of Things (IIoT) is a significant component of the future transformation of industrial systems. Similar to ICS, the interconnection and intelligence is provided through sensing devices and actuators with ubiquitous networking and computing capabilities. It is speculated that by the year 2020, billions of devices capable of generating data will be connected to the internet. This will benefit various applications, such as infrastructure, transportation, and agriculture. In such systems, transactions include readings of data acquired from various sensors with a spatio-temporal stamp indicating where and when the reading was taken. Such data is then shared among various players in the network. Similar to the concept of financial transactions, it is critical to maintaining a historical record of these transactions as they are used to impact mission critical decisions. This mandates that the records are not tampered with illegally and a trace is maintained in case such attempts are made.
Blockchain introduces a robust and efficient next generation of techniques for the transactions generated by the physical resources. The mix of blockchain and IoT gives us a versatile, truly distributed, peer-to-peer system and the capacity to interact with distributed sensors in a trust-less, auditable way. A framework using ethereum to communicate electricity usage, including air conditioners and bulbs, was proposed in [46
]. The ethereum notifies the network to update devices from normal to energy saving mode to make efficient usage of the electricity. Another such effort for making smart homes more efficient by significantly lowering overhead due to traffic, processing time and energy consumption was presented in [47
]. Furthermore, to support fast and secure energy trading in IoT applications, a consortium of blockchain providers is proposed in [48
]. The authors propose using available information and energy interaction perspective to create a network that can make decisions that are context-aware. A technique based on distributed consensus to obtain proof of work, using the frequency of data and the amount of energy contribution, is presented in [49
]. The work in [50
] presents the usage of blockchain technology as a platform for hierarchical and distributed control systems based on the IEC-61499 standard. In this work, Hyperledger Fabric, where functional blocks are implemented as smart contracts on a supervisor level, was selected as the blockchain solution.
A blockchain-based system for secure mutual authentication, to enforce access control policies is discussed in [51
]. This system uses a triangulation with integrated attribute signature, multi-receivers encryption, and message authentication code and is designed to provide privacy and security guarantees. Another work along similar lines is presented in [52
], wherein the authors use blockchain to create virtual zones to present a robust method for identification and authentication of devices. These virtual zones form a distributed system called Bubble of Trust, which ensures a robust identification and authentication of devices and protects the integrity and availability of the data. Authors in [53
] analyzed unique functions and open challenges of blockchain, as well as discuss a potential application that stands at the intersection of blockchain and IoT. A fully distributed access control system for IoT based on blockchain technology, used for arbitrating roles and permissions in IoT, is discussed in [54
]. A blockchain-based privacy protection management scheme for IoT devices, which combines attribute-based encryption with time-limited key management technology to achieve privacy protection and device management, is described in [55
]. Authors in [56
] propose a distributed fair access control framework based on cryptocurrency. It provides granular level access to data through smart contracts by using the consistency of blockchain technology to manage access control on behalf of constrained devices. A theoretical lightweight architecture based on private blockchain in the context of smart home, which reduces the communication overhead of workload proof mechanism by introducing the central miners, is discussed in [57
]. Table 3
presents the use cases, design challenges, and future directions in ICS.
When all the devices in an IoT network are connected to each other and have decision-making capabilities, one can automate time- and human resource-consuming work processes. However, this mandates the need to maintain a historical ledger of these actions and the data that led to the actions. Active scientific research, such as the one cataloged above, indicates that the incorporation of blockchains in the IoT area will fulfill the need for cryptographic verifiability, thereby affecting critical changes over several industries.
3.3. Applications in Transportation
Autonomous vehicles are the future of transportation and will play a crucial part in how society evolves. These vehicles play an important role in improving the connectivity and providing road safety, better traffic management and driver comfort. Blockchain can be used to build up a verified, trusted and decentralized self-governing intelligent transport system, making better use of the heritage intelligent transport systems (ITS) framework and assets, particularly successful for crowd-sourcing of innovation. Figure 8
presents the ITS national architecture proposed by department of transportation. ITS-oriented, a seven-layer conceptual model for blockchain is proposed in [58
]. The seven layers are physical, data, network, consensus, incentive, and application layer, respectively. Additionally, a distributed key management in heterogeneous intelligent transport systems is proposed in [59
]. It includes the key transfer between heterogeneous networks and the dynamic key management scheme to decrease the key transfer time.
Refueling scenario for autonomous electric vehicles using blockchain to guarantee the execution of energy recharges is discussed in [60
]. A reward-based intelligent vehicle communication based on blockchain technology is presented in [61
]. It improves the privacy and provides fast, secure communication between vehicles.
Connected cars have built-in sensing features, through which they monitor their surroundings and build a comprehensive 360-degree view of what is around them. For instance, they have built-in navigation systems, cameras, proximity detection sensors, light and radio-frequency detection sensors, to name a few. They have the ability to synchronize information from multiple sensors, a technique known as sensor fusion, with real-time data to keep the vehicles and infrastructure elements informed, in case of accidents. Such features have led to an increase in the number of advanced driver assistance functions, such as adaptive cruise control, lane change warnings, and collision avoidance mechanisms. To enable all of these functions, such cars are also equipped with communication devices and protocols that are used to share information between all the entities in the vehicular network. Dedicated Short Range Communications (DSRC) is the currently approved protocol for 5.9 GHz Intelligent Transportation Systems (ITS) band to handle Vehicle to Vehicle (V2V) safety applications [62
]. It uses IEEE 1609.2-4 message protocol and security services to enable use-cases for V2V Communication, which includes emergency electronic brake lights, forward collision warning, blind-spot detection. Information is shared between vehicles using SAE J2735 Basic Safety Messages (BSMs). BSMs provide position, size, velocity information to other vehicles, thereby creating awareness about the environment. Such information is encrypted using PKI-based certificates, thereby ensuring that the safety message is from a trusted source. For V2V communications, basic safety messages are trusted but not encrypted because they are broadcast to all the neighboring vehicles, while certificate messages are both trusted and encrypted. While BSMs have built-in encryption, it has been demonstrated that they can be tampered with, thereby leading to serious safety and security concerns. As an example, Charlie Miller and Chris Valasek have demonstrated attacks on a Toyota Prius and a Ford Escape using simple off-the-shelf components, such as Uconnect head unit, which is used for remote access [63
]. Likewise, researchers at the University of South Carolina, Chinas Zhejiang University, and the Chinese security firm Qihoo 360 demonstrated that they could jam such sensors from a popular electric vehicle, thereby making objects invisible to its navigation system. They were able to precisely jam the radio signals by simply by using two commercially available off-the-shelf radio equipment and basic signal generation and frequency control instruments [64
]. It is conceivable that the units inside the vehicle, such as engine control unit, brake control unit, and wheel control unit, are also vulnerable to such malicious attacks since they are also heavily dependent on communication. While the occurrence of such attacks has been largely limited to date, with the drastic increase in the number of connected vehicles, the surface area of such attacks is bound to increase much. It is indisputable that security in such types of vehicles is imperative and critical.
While Public Key Infrastructure (PKI) today handles the security aspects of these messages, it suffers from the same limitations as any centralized authentication system. Additionally, a centralized PKI lacks true information about ground reality as it does not have the sensing capabilities available on vehicles. Blockchain holds a cutting edge potential for these cars. In the case of connected cars, the transactions shared between vehicles are the basic safety messages, which contain information about the size, position, velocity and heading of the car. These messages are digitally signed and the signature is validated by the PKI. They have to be maintained in a time sequenced historical manner in the folders for use-cases related to law enforcement and insurance claims. It is critical that these transactions are validated in real-time for immediate use-cases related to higher levels of automated driving. Additionally, these transactions should not be tampered with any time in the future, as they may be needed for judicial and insurance claim reasons. All of these requirements, make blockchains a viable option to consider for transaction management in connected vehicles.
Authors in [65
] proposed a decentralized technique to protect against attacks on sensors and communication channels. They do so for securely sharing messages between connected vehicles based on a blockchain architecture. Trust bit [66
] uses the blockchain approach to implement intelligent vehicle communication using a reward based scheme. It exchanges trust bits as rewards during successful communication. For recording and maintaining historical evidence of transactions involving such trust bits, they used blockchain technology in the vehicular cloud. This allows all the trust bit details to be securely accessed by vehicles independent of space and time constraints. The concept of a local dynamic blockchain and main blockchain has been explored in a branch based blockchain technology presented in [67
]. The underlying new idea is the definition of a secure and unique crypto ID, called an intelligent vehicle trust point, to ensure trustworthiness among vehicles. Vehicles use the local dynamic blockchain to verify the IDs while they are communicating with other vehicles. A new secure blockchain-based communication scheme for connected vehicles is presented in [68
]. In this scheme, the identity of the vehicles joining the network is first verified by a ring-signature based scheme. Next, the consensus among the vehicles is achieved using a blockchain-based mechanism prior to sharing the information, created by multi-party smart contracts, using secure communication channels. A blockchain technology which uses multi-signature mechanism proposed in [69
]. It provides emerging vehicular services, such as remote software updates, without revealing any of the vehicles’ private information. Yuan et al. [58
] proposed a seven-layer conceptual model for intelligent transportation using blockchain technology, thereby creating a secure and trust-worthy decentralized ecosystem. Leiding et al. [70
] have combined ethereum-based smart contracts with vehicular network technology. As vehicles are increasingly becoming software-dependent, a key question which must be answered is about updates to the software as new features are added. A design of how blockchain technology can be used to do this has been shown in [71
]. In this approach, an overlay network is used to transfer messages between software providers, cloud storage mechanisms, and vehicular interfaces. Such messages are used to initialize the blockchain system and handle the software distribution processes. Vehicle system with vehicle report generation and methods for use are presented in [72
] a processor is configured to perform data-driven operations, such as report generation. Such operations are generated using a vehicle-specific digital currency record using cryptocurrency protocol. The value of this digital currency is adjusted based on the price of goods or services purchased. It is stored in memory and communicated while the purchase of goods or services is in action. Using visible light and acoustic side-channels, Rowan et al. [73
] proposed a new blockchain technology for securing communication in cars. Table 4
presents the use cases, design challenges, and future directions in transportation sector.
3.4. Applications in Smart Grid Systems
Access to electricity is a fundamental need for modern society and the economy. An estimated $
48 trillion investment is required in the energy infrastructure over the period of the next fifteen years [74
]. This poses an imminent need and opportunity to shift towards an efficient and clean energy system with a low carbon footprint. Smart grid systems play the role of a necessary enabler for this transition. A smart grid is an intelligent, digitized energy network delivering electricity in an optimized way between source and consumption, as shown in Figure 9
. This is accomplished by integrating information, telecommunication, and power technologies with the existing electricity system. Smart grid systems incorporate sensors and software on the existing grids, thereby equipping utilities and personal users with information that enables them to react to changes quickly. In addition to improvements in the efficiency and reliability of electricity supply, smart grids play a catalyst role in the integration of renewable energy into existing networks, thereby reducing carbon emissions.
Different ways in which blockchain can be used to modernize the grid have been discussed in [75
]. A framework for information exchange and buy-sell transaction mechanisms between energy providers and citizens using blockchains is proposed in [76
]. Existing power grids do not provide resilience against cyberattacks on distributed energy resources and grid edge devices. Authors in [77
] have discussed a business model to reduce costs by cutting out third parties. They also discuss techniques to increase arbitrage opportunities to produce and sell energy at an individual level. In [78
], energy usage information is collected in a distributed fashion from smart metering devices. The expected energy flexibility at the consumer level can be controlled in a programmatic manner using self-enforcing smart contracts. By tracking the flexibility between energy consumption and demand response signal, authors have shown how energy demand and production can be matched at a smart grid level. They have also combined the reward and penalty mechanism to balance energy demand. Blockchain based smart contracts presented in [79
], provide security and resilience, an immutable transaction history, and the ability to enable transactions, and automation at a micro-level in an effective and profitable manner. Authors in [80
] present a local energy market design and simulation, implemented on a private blockchain with artificial agents, to offer real time pricing information. It simulates optimum decisions based on production capacity prediction, thereby automating informed tariff decisions. Similar work, wherein production and consumption load profiles are transformed in a distance-preserving embedding in order to find a matching tariff, is presented in [81
]. It uses blockchain to make the calculations for tariff matching publicly available, while still maintaining the privacy through embedding. This work has been further extended and validated on electric vehicles [82
]. A similar work, where a blockchain-based privacy preserving payment mechanism is proposed for a vehicle to grid networks, enables data sharing while securing sensitive user information [83
A modular platform-based approach for applying cryptocurrency features to the renewable market has been shown in [84
]. This work also includes a robot which advises users on the best selling strategy. A smart replicable district model which uses new technology to build an efficient energy management system integrated into a platform based on an IoT and blockchain approach is presented in [85
]. A blockchain-based method for power grid communications in smart communities which preserves privacy and manages efficient aggregation is proposed in [86
]. Here, a blockchain based solution is proposed to avoid application usage patterns by analyzing the user’s electricity consumption profile. Along similar lines, a sovereign blockchain which provides transparency and provenance is utilized to mitigate the security and privacy concerns on smart grid [87
]. A proof-of-concept for decentralized energy trading system using blockchain technology, multi-signatures, and anonymous encrypted messaging streams, enabling peers to anonymously negotiate energy prices and securely perform trading transactions is presented in [88
]. A control strategy using proportional fairness to create incentives for distributed energy resources to operate at sub-optimal capacity has been described in [89
]. In this scheme, a subset of the network actors decreases their power output and revenue to aid the overall system performance. A historical track record of these transactions is managed using blockchains. Table 5
presents the use cases, design challenges, and future directions in smart grid domain.