1
Introduction

Sachin S. Shetty,1 Laurent Njilla,2 and Charles A. Kamhoua3

1Old Dominion University, Virginia Modeling, Analysis and Simulation Center, Norfolk, VA, USA

2US Air Force Research Lab, Cyber Assurance Branch, Rome, NY, USA

3US Army Research Laboratory, Network Security Branch, Adelphi, MD, USA

1.1 Blockchain Overview

Blockchain technology has attracted tremendous interest from a wide range of stakeholders, which include finance, healthcare, utilities, real estate, and government agencies [1-5]. Examples of potential applications of this technology are claims processing, transparency and auditing of operations, identity management, supply chain provenance to address the threat of counterfeit products, and integrity of the information acquired from Internet of Things (IoT) devices. Blockchains are a shared, distributed, and fault-tolerant database that every participant in the network can share, but no entity can control. The technology is designed to operate in a highly contested environment against adversaries who are determined to compromise. Blockchains assume the presence of adversaries in the network and nullify the adversarial strategies by harnessing the computational capabilities of the honest nodes, and the information exchanged is resilient to manipulation and destruction. Blockchains facilitate the development of trustworthy networks in a trustless environment.

The premise of blockchain is that applications do not need a trusted central authority to operate and can function in a decentralized fashion. Blockchain enables exchange of information among distrusting entities. Blockchain enables trustless networks and allows entities to engage in transactions in the absence of mutual trust. There is an assumption that a communication medium could be compromised by insiders or outsiders. The reconciliation process between entities is sped up due to the absence of a trusted central authority or intermediary. Tampering of blockchains is extremely challenging due to the use of a cryptographic data structure and no reliability of secrets. Blockchain networks are fault tolerant, which allows nodes to eliminate compromised nodes.

Blockchains have the following advantages over centralized databases: (i) ability to directly share a database across diverse boundaries of trust in situations where it is difficult to identify a trusted, centralized arbitrator to enforce constraints of proof of authorization and validity. In a blockchain, transactions leverage their own proof of validity and authorization based on a verification process managed by multiple validating nodes and a consensus mechanism that ensures synchronization; and (ii) ability to provide robustness in an economical fashion without the need for expensive infrastructure for replication and disaster recovery. Blockchain requires no configuration to connect and synchronize nodes in a peer-to-peer (p2p) fashion, with built-in redundancy and no need for close monitoring. It can tolerate multiple communication link failures, allows external users to transmit transactions to any node, and ensures disconnected nodes will be caught up on missed transactions.

Blockchain's distributed database maintains a continuously growing list of records, called blocks, secured from tampering and revision by distributed storage and continuous verification. The blocks contain a temporal listing of transactions that are stored in a public ledger using a persistent, immutable, and append-only data structure that is globally viewable by every participant in the underlying p2p network. When such an elegant data structure is considered to track data transactions in a distributed environment, the block structure contains attributes such as the set of user transactions, a timestamp, a reference to a previous block in the blockchain, Merkle root of the transactions, and so on. In this manner, the blocks are linked together to form a chain, where the hash of the previous blocks helps to maintain the integrity of the whole blockchain (Figure 1.1).

Figure 1.1 Block structure.

1.1.1 Blockchain Building Blocks

Blockchain technology's effectiveness hinges on the following three main components: a decentralized network, distributed consensus, and cryptographically secure algorithms. Figure 1.2 illustrates the basic blockchain architecture.

Figure 1.2 Blockchain architecture.

The key features of each of the components are as follows:

• Decentralized Network-The function of the decentralized network is to ensure the propagation of transaction messages among the nodes responsible for maintaining the distributed ledger. The network protocol allows the transaction message to be broadcast from any node to all nodes in the decentralized network. However, the network is not a pure broadcast medium and allows nodes to propagate messages that represent valid transactions. The network can be part of a private or public blockchain that has ramifications on network performance and security. Irrespective of whether the blockchain is public or private, the decentralized network is based on a p2p architecture. The nodes can join and leave freely. There is no centralized arbitrator. The network has built-in redundancy and robustness to mitigate node and link failures.
• Distributed Consensus-Blockchain uses consensus protocols over a decentralized p2p network for verification of transactions prior to adding blocks to the public ledger. The consensus protocol receives messages from the p2p network and inserts transactions in the distributed ledger. The consensus protocol is responsible for mining blocks and reaching consensus on their integration in the blockchain. The consensus protocol chooses the set of transactions that is accepted after passing a verification process. The verification process is determined by users and does not require a centralized administrator. The consensus protocols ensure that the newly added transactions are not at odds with the confirmed transactions in the blockchain and maintain the correct chronological order. The newly added transactions that are waiting to be confirmed are packed in a block and submitted to the blockchain network for validation.
• Cryptographically Secure Algorithm-The foundational component of blockchain technology is the cryptosystem. State-of-the art blockchains' cryptography systems use public key algorithms such as Elliptic Curve Cryptography, and message digests such as SHA3-256. In a typical blockchain application, an Elliptic Curve key pair that contains a public key and private key is generated based on Secp256K1 curves. The private key has the traditional usage of being kept secret and utilized to sign transactions. For instance, in the case of bitcoin use, when a user exchanges bitcoins with another user, the user will sign the transaction with their private key prior to announcing to the network. Once the transaction is signed, the miners in the network will use consensus algorithms to verify the validity of the transaction signature, and validation is achieved.

1.1.2 Blockchain Commercial Use Cases

Home IoT-"Smart home" is an emerging Internet of Things (IoT) application that aims to provide higher accessibility to all home accessories and personalized user experience for the appliances. To operate efficiently, IoT hubs in smart homes collect and analyze a lot of sensible data from the home area network of all smart devices. With the gathered data, it is easy to derive usage patterns and user behaviors in the home environment, thus creating a digital trail of families in smart homes. However, this information could easily fall into the wrong hands, or the vendors could use the information to promote additional products. Therefore, when more smart technologies are added to smart homes, there is an increased possibility of a severe privacy breach. Conventional approaches may fail to achieve credible security and privacy in IoT because the IoT framework has particular characteristics. These characteristics include decentralized topology, resource-constrained devices, limited network performance, and minimal security standards for IoT devices. The introduction of blockchain technology could potentially address smart home challenges in an efficient manner because of the following facts: blockchain does not rely on a centralized control; instead, it works in a distributed network setting that is similar to IoT. As a result, blockchain avoids the problem of single-point failure and improves scalability. Moreover, blockchain inherently offers the anonymity that is required in the IoT environment, where identities must be kept private. Irrespective of the several advantages produced by blockchain technology, integration in the smart home environment may create the following obstacles: first, blockchain mining is computationally intensive in nature, and this requires decent computing capabilities in the participating devices; however, IoT devices have a heterogeneous computational power that may not be sufficient to mine blocks in a desirable amount of time. The second problem is that of data storage since blockchain is nothing but a distributed ledger stored locally to verify transactions; however, smart devices, such as various sensors, have a limited storage capacity. Finally, blockchain protocols also consume significant network capacity for internal communication, which may be undesirable for...