Blockchain Technology

Blockchain is a distributed decentralized peer-to-peer network aiming to facilitate the immutability and security of data. Towards the service orientation, blockchain is a collection of distributed blocks having unique hash codes without any point of failure. Each block is stored on distributed ledgers, and transactions with them are secure, transparent, immutable, and traceable. To create a new block and allow a transaction to complete, an agreement between all parties is required. To reach an agreement in a blockchain network, consensus algorithms are used. In this chapter, fundamental principles and algorithms of blockchain networks have been discussed, and a detailed review of the blockchain consensus algorithms PoW, PoS, DPoS, PoET, PoWeight, PoB, PoA, and PoC have been provided including the merits and demerits of consensus algorithms with analysis to provide a deep understanding of the current research trends and future challenges.

Crystal: A Privacy-Preserving Distributed Reputation Management

This paper investigates the situation in which exists the unshared Internet in specific areas while users in there need instant advice from others nearby. Hence, a peer-to-peer network is necessary and established by connecting all neighbouring mobile devices so that they can exchange questions and recommendations. However, not all received recommendations are reliable as users may be unknown to each other. Therefore, the trustworthiness of advice is evaluated based on the advisor's reputation score. The reputation score is locally stored in the user’s mobile device. It is not completely guaranteed that the reputation score is trustful if its owner uses it for a wrong intention. In addition, another privacy problem is about honestly auditing the reputation score on the advising user by the questioning user. Therefore, this work proposes a security model, namely Crystal, for securely managing distributed reputation scores and for preserving user privacy. Crystal ensures that the reputation score can be verified, computed and audited in a secret way. Another significant point is that the device in the peer-to-peer network has limits in physical resources such as bandwidth, power and memory. For this issue, Crystal applies lightweight Elliptic Curve Cryptographic algorithms so that Crystal consumes less the physical resources of devices. The experimental results prove that our proposed model performance is promising.

Solving the Max-Diversity Orthogonal Regrouping Problem by an Integer Linear Programming Model and a GRASP/VND with Path-Relinking Approach

Students from Master of Business Administration (MBA) programs are usually split into teams. In light of the generalistic nature of MBA programs, diversity within every team is desirable in terms of gender, major, age and other criteria. Many schools rotate the teams at the beginning of every term so that each student works with a different set of peers during every term, thus training his or her adaptation skills and expanding the peer network. Achieving diverse teams while avoiding–or minimizing—the repetition of student pairs is a complex and time-consuming task for MBA Directors. We introduce the Max-Diversity Orthogonal Regrouping (MDOR) problem to manage the challenge of splitting a group of people into teams several times, pursuing the goals of high diversity and few repetitions. We propose a hybrid Greedy Randomized Adaptive Search Procedure/Variable Neighborhood Descent (GRASP/VND) heuristic combined with tabu search and path relinking for its resolution, as well as an Integer Linear Programming (ILP) formulation. We compare both approaches through a set of real MBA cohorts, and the results show that, in all cases, the heuristic approach significantly outperforms the ILP and manually formed teams in terms of both diversity and repetition levels.

May I sleep in your bed? Getting permission to book

Demand for tourist accommodation offered on peer-to-peer networks is skyrocketing. In such networks tourists can only book if the accommodation provider (host) gives their permission. Needing permission to book accommodation is radically new in tourism. No hotel, motel or B&B assesses a booking inquiry in detail before accepting their booking. But do peer-to-peer network hosts actually refuse permission to book and, if so, why? A choice experiment with Airbnb hosts shows that refusing permission to book is common and that specific attributes of the booking inquiry—such as the purpose of their trip—affect the likelihood of getting permission to book.

Socioeconomic Status, Time Scarcity and Well-Being in Retirement

We tend to think of retirement as a great equalizer when it comes to relief from the pernicious time scarcity characterizing the lives of many individuals in the labor force. Puzzlingly, this is not entirely the case. Using data from the MTUS (N=15,390) in combination with long-term participant observation (980 hours) and in-depth interviews (N=53), I show that socioeconomic characteristics are important determinants of retiree time scarcity. Neighborhood disadvantage gets under the skin via time exchanges that are forged by both neighborhood and peer network characteristics. The SES-based ‘time projects of surviving and thriving’ undergirding the experience of time scarcity lead to divergent strategies of action and differing consequences for well-being. For the advantaged, the experience of time scarcity is protective for well-being in later life, as it emerges from the ‘work of thriving’ and managing a relative abundance of choices. For the disadvantaged, the later life experience of time scarcity is shaped by cumulative inequality, further exacerbating inequalities in well-being. The final section of the article offers an analysis and interpretation of these results, putting retiree time scarcity in conversation with the broader literature on socioeconomic status and well-being.

Peer Network Processes in Adolescents’ Health Lifestyles

Combining theories of health lifestyles—interrelated health behaviors arising from group-based identities—with those of network and behavior change, we investigated network characteristics of health lifestyles and the role of influence and selection processes underlying these characteristics. We examined these questions in two high schools using longitudinal, complete friendship network data from the National Longitudinal Study of Adolescent to Adult Health. Latent class analyses characterized each school’s predominant health lifestyles using several health behavior domains. School-specific stochastic actor-based models evaluated the bidirectional relationship between friendship networks and health lifestyles. Predominant lifestyles remained stable within schools over time, even as individuals transitioned between lifestyles. Friends displayed greater similarity in health lifestyles than nonfriend dyads. Similarities resulted primarily from teens’ selection of friends with similar lifestyles but also from teens influencing their peers’ lifestyles. This study demonstrates the salience of health lifestyles for adolescent development and friendship networks.

A theory of transaction parallelism in blockchains

Decentralized blockchain platforms have enabled the secure exchange of crypto-assets without the intermediation of trusted authorities. To this purpose, these platforms rely on a peer-to-peer network of byzantine nodes, which collaboratively maintain an append-only ledger of transactions, called blockchain. Transactions represent the actions required by users, e.g. the transfer of some units of crypto-currency to another user, or the execution of a smart contract which distributes crypto-assets according to its internal logic. Part of the nodes of the peer-to-peer network compete to append transactions to the blockchain. To do so, they group the transactions sent by users into blocks, and update their view of the blockchain state by executing these transactions in the chosen order. Once a block of transactions is appended to the blockchain, the other nodes validate it, re-executing the transactions in the same order. The serial execution of transactions does not take advantage of the multi-core architecture of modern processors, so contributing to limit the throughput. In this paper we develop a theory of transaction parallelism for blockchains, which is based on static analysis of transactions and smart contracts. We illustrate how blockchain nodes can use our theory to parallelize the execution of transactions. Initial experiments on Ethereum show that our technique can improve the performance of nodes.

Influence of Bottleneck Nodes on Malicious Packet Dropping Nodes Mitigation in Wireless Infrastructure-less Networks

Mobile ad hoc networks as an infrastructure free, and constrained resource environment network. The network aim is to establish internet connectivity everywhere regardless of location. The applications of network are healthcare, disaster relief and military, where reliable communication is major concern. Communication in the network is initiated by establishing the communication route between source and destination and sending the information through it. One of the characteristics of MANETs is a peer-to-peer network, where intermediate nodes have to cooperate for reliable communication by acting as routers. In literature number of routing protocols have been designed based on the MANET’s peer to peer characteristic. However, it may not be every time true that the intermediate nodes act as faithful routers, and they may untrustworthy either due to malicious behavior or bottleneck. Number of secure protocols have been designed to mitigate malicious behavior by neglecting the bottleneck. The paper aims to define the bottleneck, and its importance in communication. Finally, how bottleneck influence on the MANETs performance during malicious nodes mitigation