Even though the blockchain technology was created for the cryptocurrency Bitcoin, its current uses seem practically unlimited, and some speculate that it can transform the entire information technology (IT) world. However, the blockchain technology remains at its heart a protocol for implementing distributed consensus, that is, an algorithm for allowing a set of independent, faulty, and maybe dishonest distributed agents to (eventually) reach an agreement on something such as the value of a variable. To put it another way: it might also be seen as an eventual consistency replicated storage system.

This concise book covers the algorithmic issues that blockchain technologists face, namely consensus, cryptography, quorum systems, authentication, and eventual consistency distributed stores. Each of the ten chapters covers the basic concepts and protocols related to one of the aforementioned topics. Following an introductory chapter 1, chapters 2 to 4 study state replication. Chapter 2 introduces the famous Paxos protocol, and chapter 3 explains why Paxos (or any deterministic protocol) may fail due to crash failures. Motivated by this fact, the same chapter describes a fast randomized protocol for achieving consensus. Chapter 4 then presents protocols able to work with Byzantine nodes, that is, nodes not simply crashing but showing arbitrary behavior. Chapter 5 presents basic concepts related to cryptography, such as key exchange, public-key cryptography, and authentication. Chapter 6, building on previous chapters, explains how state replication can be implemented with message authentication. The next chapter presents a different concept for implementing replicated systems, namely quorum systems that address the scalability problem. Chapters 8 and 9 describe the elements of Bitcoin and blockchain, including miners, proof-of-work, and smart contracts. Finally, chapter 10 presents concepts relevant to modern distributed storage systems, such as consistent hashing, and some overlays such as hypercube networks and specific peer-to-peer systems, that is, distributed hash tables and churn.

Written by a world-renowned expert on distributed systems theory and practice, the book is a must-read for advanced undergrad and graduate students, as well as engineers and software developers who use a blockchain as the underlying “storage” medium.

The book combines algorithmic thinking with mathematical rigor, interweaving algorithm pseudocode and proofs in a unique and elegant way, and achieving self-completeness despite its conciseness. Readers can further improve their understanding of Bitcoin by reading Narayanan et al.’s book [1], a 2017 PROSE Award in Computing and Information Sciences Honorable Mention, or Judmayer et al.’s excellent book on the actual blockchain structure [2].

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