2021 A Blockchain Definition to Clarify its Role for the Internet of Things

Last updated Dec 22, 2022

• Source zotero journal article
• Type subject
• Status needs revision
• On blockchain
• On networks
• On io t

Metadata

• CiteKey:: ghiroBlockchainDefinitionClarify2021
• Type:: journalArticle
• Author:: Lorenzo Ghiro Francesco Restuccia Salvatore D’Oro Stefano Basagni Tommaso Melodia Leonardo Maccari Renato Lo Cigno
• Editor::
• Publisher::
• Series::
• Series Number::
• Journal::
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• Pages::
• Year:: 2021
• DOI:: 10.1109/medcomnet52149.2021.9501280
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• ISBN::
• Format:: PDF

Abstract

The term blockchain is used for disparate projects, ranging from cryptocurrencies to applications for the Internet of Things (IoT). The concept of blockchain appears therefore blurred, as the same technology cannot empower applications with extremely different requirements, levels of security and performance. This position paper elaborates on the theory of distributed systems to advance a clear definition of blockchain allowing us to clarify its possible role in the IoT. The definition binds together three elements that, as a whole, delineate those unique features that distinguish the blockchain from other distributed ledger technologies: immutability, transparency and anonymity. We note that immutability-which is imperative for securing blockchains-imposes remarkable resource consumption. Moreover, while transparency demands no confidentiality, anonymity enhances privacy but prevents user identification. As such, we raise the concern that these blockchain features clash with the requirements of most IoT applications where devices are power-constrained, data needs to be kept confidential, and users to be clearly identifiable. We consequently downplay the role of the blockchain for the IoT: this can act as a ledger external to the IoT architecture, invoked as seldom as possible and only to record the aggregate results of myriads of local (IoT) transactions that are most of the time performed off-chain to meet performance and scalability requirements.

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Tags and Collections

• Keywords:: Blockchain, IoT, Definition, Consensus, ⭐⭐⭐
• Collections:: PoN

• ["] This position paper elaborates on the theory of distributed systems to advance a clear definition of blockchain that allows us to clarify its role in the IoT. This definition inextricably binds together three elements that, as a whole, provide the blockchain with those unique features that distinguish it from other distributed ledger technologies: immutability, transparency and anonimity. Page 1
• ["] immutability comes at the expense of remarkable resource consumption, transparency demands no confidentiality and anonymity prevents user identification and registration. Page 1
• ["] The blockchain features observed in Bitcoin, i.e., decentralization, resistance to powerful cyberattacks and preservation of users privacy, raised the enthusiasm of many research communities. Page 1
• ["] Considering that the Bitcoin blockchain currently supports the validation of less than 10 Transactions per Second (TPS) and exhibits a power consumption similar to that of an industrialized country such as Ireland [59], it is dubious that it can support the millions of IoT TPS [60] and meet the typical IoT power constraints. Page 1
• ["] This position paper analyzes the multiple technologies proffered under the term blockchain and proposes a clear definition of blockchain that allows us to argue about its role in the IoT. Building on the theory of distributed systems and on the critical analysis of current blockchain applications, the definition identifies three elements that, only when combined together, give to blockchains their specific features of openness, decentralization, security and ability to preserve user privacy. These three elements are: relying on a STRONG DISTRIBUTED CONSENSUS PROTOCOL, which makes the blockchain immutable, hence secure from tampering attacks, and further frees the system from centralized trusted authorities (e.g., banks); maintaining a FULL & PUBLIC HISTORY OF TRANSACTIONS, which permits their distributed and completely transparent validation, and being OPEN TO ANONYMOUS USERS, thus allowing blockchains to preserve users privacy. Page 1
• ["] blockchains are beneficial only for a limited range of applications, and that their integration into the IoT domain is not appropriate. Page 2
• ["] Everything starts when a transaction is issued, e.g., because a smart device is querying a remote service and pays to access the data. The transaction is announced in the P2P network and received by validator nodes. These nodes run a consensus protocol to decide about the validity of the transaction. If they reach a consensus on the fact that the device really owns the resources that is about to spend, then the transaction is considered valid. If so, it is grouped with others recently approved, forming a new block of transactions that will be registered in the ledger by appending it to the blockchain. At the end, the success of the transaction is notified to the users and the data is transferred to the device. Page 2
• ["] a blockchain can be considered a distributed system that in general includes:…A Peer-to-Peer (P2P) network made of all those nodes that either read or cooperatively write transactions in the blockchain, and Page 2
• ["] a consensus protocol, namely, a set of policies agreed upon and implemented by all nodes, which are the rules that regulate which and how new transactions can be added to the blockchain. Page 2 Page 2
• ["] In a public (permissionless) ledger, the record of transactions is public and the consensus protocol is open to anybody. This means that i) anyone in the world can verify the correctness of the ledger and ii) even anonymous strangers without explicit permission can join the network and participate in the validation process of transactions, provided only that they comply with the consensus protocol. Page 2
• ["] A permissionless blockchain can thus be considered as a trust builder in a trustless network and the enabler of an open, privacy-preserving, disintermediated marketplace. Page 3
• ["] Double Spending Problem Two transactions that spend the same resources may be processed in different order by distinct validators spread across a P2P network. This is because of different propagation delays in the network (Figure 2). At this point, it is crucial for validators to find an agreement on the order of transactions to determine which of the two came in first, and should be considered valid, and which came in second and should be rejected. Page 3
• ["] This fundamental problem is also known as Distributed Consensus Problem. Page 3 Page 3
• ["] To fend off falsification attacks a blockchain must be: • Tamper-proof : i.e., made so that is easy to verify that the registered transactions have not been manipulated after their recording, and it should be likewise easy to determine if these have been actually altered in a second instance of time. • Immutable: a blockchain-based ledger should adequately word off tampering attacks. Page 3
• ["] The tamper-proof property of blockchains is achieved by a clever embedding of Cryptographic Hash Functions (CHFs) into the blockchain data structure Page 3

Page 3

• ["] Those nodes that are constantly at work looking for valid nonces are metaphorically called “miners.” Page 4
• ["] The difficulty of the PoW is tuned so that, in the whole P2P network of miners, a valid block is produced on average every 10 minutes. To keep a constant Block Generation Interval (BGI ), the difficulty of the PoW must be tuned according to the miners computing power…The BGI must be kept high to avoid the simultaneous production of two blocks as far as possible. Two blocks are considered “simultaneous” if the second one is generated within the average Block Propagation time (BP ), which for any P2P overlay based on transactions is in the orders of seconds to maximum tens of seconds [67]. Simultaneous blocks are problematic because their almost contemporaneous proposal divides the nodes of the Bitcoin network in two parties that will append the two different blocks to the blockchain, forming two branches. This situation can be represented by a bifurcation of the blockchain and is called a “fork.” When a fork occurs, it means that there is no distributed consensus on block order anymore, thus there is no agreement on the order of transactions. Without this agreement, the system is exposed again to Double Spending attacks. Page 4
• ["] the PoW is key to make blockchains immutable. Page 4
• ["] Some believe that owning the majority of the computing power ensures the control of the network through the so called “50+1% attack.” The simple analysis above shows clearly that this is not enough Page 5
• ["] Bitcoin uses block depth as confirmation, and recommends users to consider a transaction to be final only if it has been confirmed by 6 subsequent blocks [70], a number that guarantees it is almost unassailable. Page 5
• ["] The Average Mining Overhead (AMO) denotes the percentage of the network computing power that, on average, does not contribute to the growth of the main chain, but rather leads to the generation of stale blocks. Page 5

Page 5

• ["] The stale rate is considered an indicator of the security level of a PoW-based blockchain, because a higher stale rate means that a blockchain is more exposed to chain replacement and eclipse attacks Page 5
• ["] The effort of setting up an exceptionally secure blockchain resulted, in Bitcoin, in a PoW that has become extraordinarily power-hungry [59, 73]. The power consumption of the Bitcoin network in 2018 were estimated to be 2.55 GW, and forecast to reach 7.67 GW in the future. This requirement is comparable to the energy demand of a whole country such as Ireland [59], so it is hard to think that a blockchain secured by the PoW could be integrated in the constrained domain of the IoT. Page 5
• ["] The PoW advantages are many: it is extraordinarily secure, fully distributed, and user-agnostic. In fact, users can participate to a PoW-based consensus without registering their identity with some trusted registrar or bank, but just providing some computing power. Ultimately the PoW i) protects the user privacy and ii) free users from trusted authorities. Page 5
• ["] However, the PoW imposes also serious limitations in terms of transaction latency, throughput and power consumption. Page 5
• ["] It can be observed that consensus protocols are a crucial component for a Shared Ledger: performance, consistency, policies of governance, security, and tolerance to failures are all properties of a Shared Ledger that depend on the selected consensus protocol, rather than on the data structure used to record transactions. Page 5
• ["] it is impossible to achieve consensus in distributed systems in the presence of faulty nodes and unreliable communication channels Page 6
• ["] This impossibility proof is in tight relation with another fundamental pillar of distributed systems, i.e., the Consistency, Availability and Partition tolerance (CAP) theorem [75]. The CAP theorem states that whenever a system gets Partitioned, then only two options are available: i) grant Consistency by safely blocking the system to fix the failures; or ii) keep processing transactions favoring Availability, with the risk that the two conflicting transactions (e.g., Double Spending ones) could be recorded, one per partition. Page 6
• ["] Both the impossibility proof and the CAP theorem may be considered only mildly relevant, as they are valid only for illbehaving systems, while in practice a system is built to work properly for most of its lifetime. Page 6
• ["] The idea to define partitions of the systems that can process subsets of commutative (non-conflicting) transactions is also known as Sharding Page 6

Page 6

• ["] An IoT developer should therefore choose a consensus protocol and a blockchain-based system only after having clearly identified the application requirements, choosing the most appropriate trade-off. Page 6
• ["] Consensus protocols are commonly partitioned into two broad families: Voting and lottery-based protocols. Page 6
• ["] Consensus protocols address the metaphor of the Byzantine Generals problem [81], i.e., the challenge for an ensemble of commanders to coordinate to perform a successful attack despite the potential betrayal of messengers, where betrayals model nodes/links failures and malicious attacks. Page 6
• ["] The most popular protocol implementing this scheme is the Practical Byzantine Fault Tolerance (PBFT) protocol [82]. It tolerates up to f malicious entities in a system with 3f + 1 nodes, this is obtained requiring a quorum threshold of 2f + 1 that ensures a majority of working/honest nodes. Page 7
• ["] Leader-based and PBFT variants are protocols that typically block (i.e., stop the decision process) when communication is asynchronous…By blocking, safety is ensured at the cost of an increased latency, while progress (liveness) is granted only when the network recovers from the transient asynchronous phase. Page 7
• ["] The potential blocks due to the failure of the leader are apparently inescapable. A unique leader must be part of the design; otherwise, a unique order of transactions cannot be defined. This observation leads to the conjecture that the distributed consensus problem is equivalent to the problem of unique leader election Page 7
• ["] Unfortunately, the need of a leader comes with severe consequences [93, 94]. First of all, a single point of failure is introduced, so that it is sufficient to attack the leader via Distributed Denial of Service (DDoS) to block consensus. Secondly, the decisions taken by a single leader are not validated by any peer: having a leader becomes therefore a concern for the protocol fairness. Page 7
• ["] In a pure voting system, each node sends its vote to all others, letting everybody perform the counting operations locally. Page 7
• ["] Pure voting systems avoid single point of failure, but their quadratic complexity prevents their deployment at the scale of an IoT network. Page 7
• ["] ure voting systems are impractical while leader based protocols, albeit efficient, introduce a single point of failure. An hybrid approach can mitigate these issues. Page 7
• ["] With Federated Voting or Sharding the network is partitioned and the consensus protocols are decomposed in smaller subproblems, solved within the federated enclaves (shards). Page 7
• ["] Another way to address the consensus problem is to organize a lottery, and the lottery winner becomes the leader. The winner must provide a sort of winning ticket, a “proof” to be shown to claim the consensus. Page 7
• ["] For the PoW, the nonce is the winning ticket that miners need to show to propose a block, imposing their order of transactions. Page 7
• ["] The PoS is an energy-aware alternative to PoW that relies on economical rationality to achieve consensus. In PoS, a randomized process selects a leader, and the key property of the random process is to bias those entities that own more cryptocoins, or whatever resource is at stake. The reasoning is that the owners of many cryptocoins (the richest stakeholders) have a vested interest in keeping the network working well and trusted, so that the system could be perceived as valuable, and the value of the owned cryptocoins is safeguarded and enhanced. Page 7
• ["] A variant of PoS that recently became very popular is DPoS, which is implemented for example by EOS, Tron, Steem, and Bitshares, and outperforms all other consensus protocols in terms of scalability [101]. With DPoS stakeholders vote to elect delegates, and their votes are weighted according to the fraction of owned coins. Sometimes delegates need to show commitment with a deposit (escrow) that can be confiscated if they do not run the internal consensus protocol honestly. The result is that delegates are chosen according to an economic rational criterion, and given that delegates are few and trustable (they are committed and accountable), they can achieve consensus much faster. Page 7

Page 8

• ["] Many other lottery based protocols have been proposed in the last years. They all evolve around the concept of proving commitment either by showing to be willing to sacrifice resources, or by the fact that the user owns a considerable stake. Page 8
• ["] Proof of Elapsed Time (PoET) [102]: where the sacrificed resource is the (random) time spent in a waiting queue; Page 8
• ["] Proof of Importance (PoI)5 and Proof of Networking (PoN) [103]: where the node commitment in the network is computed on the base of a different mix of metrics, including network topological information Page 8
• ["] Proof of Burn (PoB): a node must burn coins, sending them to a dead address, to gain the privilege of leading the consensus Page 8
• ["] Proof of Capacity (PoC): if memory is the main resource necessary to solve a cryptographical problem, then PoW becomes PoC Page 8
• ["] Proof of Deposit (PoD): with PoS, nodes with “nothing at stake” can behave maliciously without any punishment. In Casper [105], entities have to deposit some coins that are confiscated in case of malicious activities. Page 8
• ["] Round-Robin A straight-forward mechanism to address the distributed consensus protocol is to let all nodes succeed each other, in consecutive turns, as leader. Full trust among nodes is required to fairly run the successions procedure, which can be blocked indefinitely by any malicious participant. Page 8
• ["] The node-to-node keyword is used to indicate two ore more mutually trusting neighbors that agree to privately perform transactions. Usually these neighbors open a fast-payment channel to handle their frequent private transactions, so that they avoid paying the multiple fees that would incur if a public blockchain were used instead. Node-to-node transactions are not visible on a main blockchain, so they are said to be off-chain Page 8
  • [n] Althea uses this consensus mechanism.

Page 9

• ["] A centrally managed DB is maintained by a central administrator, such as a trusted employee or a single company in charge of keeping the DB well maintained. The recorded data can be shared among various clients upon request. The central manager can, at his own discretion, authorize or deny the access to the DB. According to the described paradigm a centrally managed DB represents a possible implementation of a Shared Ledger. Page 9
• ["] The greatest advantage of one such implementation is the high level of efficiency in terms of transaction rate, communication effort and power consumption. Page 9
• ["] If advantages are many, disadvantages are numerous too. For example, the trust in the administrator must be absolute because this administrator can in principle tamper, censor or even resell users data. A centrally managed DB is not considered transparent as well, because nobody controls nor validates the admin operations. Similarly it cannot even be considered immutable, as the admin is free to delete data. Page 9
• ["] Distributedly managed DBs, i.e., DBs cooperatively maintained by a group of administrators, represented the only option to implement a decentralized Shared Ledger before the rise of blockchains. Redundant DB copies are introduced: nodes chose and run a consensus protocol to agree on writing operations Page 9
• ["] This distributed architecture provides a varying degree of tolerance to failures which depends on the strength of the consensus protocol and on the number of redundant DB copies. Page 9
• ["] The price paid by distributed DBs for decentralization is the increased coordination effort necessary to run the consensus protocol, that also slows down the transaction rate. Page 9
• ["] A distributed DB is harder to tamper compared to a centralized one, since an attacker must corrupt more nodes. All write operations are validated by a quorum of peers: this mechanism enhances transparency as no absolute trust in the admin is required anymore. Page 9
• ["] Nonetheless, the system is secure only if a majority of peers is honest. The maintainers of the distributed DB are free to record data in any data structure Page 9
• ["] Classic blockchains turns out to be a particular case of decentralized DB where transparency and immutability are constitutional and brought to their extremes. The only data structure used in a classic blockchain is, unquestionably, a block-chain, i.e., a special linked list characterized by cryptographic links, and blocks of transactions as items of the list. Page 9
• ["] In a blockchain, data can only be appended and it is never deleted or modified. All append operations are public and transparent, so that the validity of all transactions can be verified at anytime by any peer. A classic blockchain is open to any anonymous user, therefore a very strong consensus protocol is necessary to safeguard the ledger. Page 9
• ["] The well known drawbacks: slow transactions rate, high latency, and huge power consumption. Page 9
• ["] Def. 4.1: Characteristics of a Classic Blockchain
  1. OPENNESS TO ANONYMOUS USERS
  3. FULL & PUBLIC HISTORY OF TRANSACTIONS
  5. STRONG DISTRIBUTED CONSENSUS PROTOCOL Page 9
• ["] The OPENNESS TO ANONYMOUS USERS is the first, essential feature of a blockchain….The openness to anonymous users is also fundamental for making blockchains decentralized. Page 9
• ["] The openness to anonymous users is thus constitutional for a blockchain, but introduces also a new problem about the disputation of transactions, because it is not possible to prosecute an anonymous, untraceable user in case of fraud: users must accept that transactions are, de facto, indisputable. Page 10
• ["] Involving a trusted authority, such as a bank, this user could trust the private and opaque internal ledger of this bank to manage transactions, but removing trusted intermediaries implies the necessity of keeping a complete record of all transactions on a ledger open to the public, otherwise the distributed validation of transactions becomes impossible….the solution offered by blockchains is to record the PUBLIC & FULL HISTORY OF TRANSACTIONS, so that anyone can verify that no previous transactions in the whole history already spent the resources being transacted. Page 10
• ["] the ledger (i.e., the blockchain) must be safeguarded by a STRONG DISTRIBUTED CONSENSUS PROTOCOL, otherwise nobody would trust the system. Page 10
• ["] This kind of immutability is so fundamental for the concept of blockchain that [sic] for cryptocurrency activists and blockchain proponents even simply questioning the immutable nature of blockchain is tantamount to heresy Page 10
• ["] the blockchain alone is meaningless without a mechanism that safeguard the historical records of transactions from tampering attacks, this is why a STRONG DISTRIBUTED CONSENSUS PROTOCOL becomes essential. Page 10
• ["] Iansiti and Lakhani propose a wider definition which is the following: “[The] blockchain is an open, distributed ledger that can record transactions between two parties efficiently and in a verifiable and permanent way” Page 10
• ["] the blockchain is not a universal technology, but it rather has precise characteristics advantageous only for a limited number of applications. Page 10
• ["] permissioned ledgers are not a truly decentralized nor an open technology. Page 10
• ["] Permissioned ledgers are therefore less immutable and less secure than iconic blockchains such as Bitcoin or Ethereum Page 10
• ["] Permissioned platforms seem to be not much different from traditional ledgers that existed also before Bitcoin Page 11
• ["] we will consider permissioned platforms as belonging to the broader class of traditional DB-based ledgers, rather than blockchain representatives. Page 11
• ["] The PoS design deliberately priorities the richest stakeholders, these last tend to accumulate voting power damaging the network decentralization. This tendency is usually condensed in the motto “the richer get richer” Page 11
• ["] the economics of stake-based systems suggest that their equilibrium is not always granted Page 11
• ["] A PoS-based blockchain is reversible by means of longrange or stake-bleeding attacks [130, 131], thus its immutaiblity is considered questionable. Page 11
• ["] In light of the discussions on the limits of PoW and PoS we claim that, essentially, they both empower a census suffrage system. In the case of PoW only rich users that can afford the sophisticated mining equipment can participate in the protocol. In PoS a similar restriction on voting by census is directly embedded in the protocol, with the remarkable advantage of saving a huge amount of energy, but with the risk of long term instabilities. Page 11
• ["] This position paper concludes that a PoW-based system is more stable and predictable than a PoS-based one, and therefore suggests that a platform that candidates itself to be the most secure one must be PoW-based. Page 11
• ["] we recommend to use the blockchain technology only when needed, opting for a different technology whenever possible, especially for the IoT. Page 11

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• ["] We note that a blockchain as defined by Definition 4.1 contrasts the needs of voting because, although it is true that the voter’s identity should be kept secret, still users cannot be anonymous. Their identity must be in fact uniquely determined to ensure uniform eligibility, namely, nobody should be able to cast multiple votes (Sybil attack), “hence an identity provider is required one way or another” Page 12
• ["] The idea behind PoET is the following. A random waiting time is distributed to all nodes competing to become the next block miner. When the waiting time expires, the node proves that it waited by providing the PoET generated by its Intel chip. The first node that exhibits a valid PoET is elected as block-miner. This protocol is much more energy efficient since the Intel chip consumes much less than a Bitcoin miner to generate a PoET….However, in this protocol users must trust the server distributing random waiting times and must also trust the proprietary Intel SGX technology Page 13
• ["] In general, all fast or low-energy proofs facilitate attacks, so that mining-difficulty must be artificially kept high (as described for Bitcoin in Section 2.4) to ensure high level of security. Page 13
• ["] We claim that sensors cannot be trusted not just because they can be compromised, but also because of the inescapable uncertainty of measurements, independently on the source—whether benign or malicious—of the uncertainty. Page 13
• ["] Confidentiality, namely, providing data secrecy, is critical for many applications. There is no reason to publish and record confidential data on a public blockchain as confidentiality is in clear contrast with a key characteristic of blockchains: Transparency Page 13
• ["] Registering user credentials and account information on a blockchain is an irreversible operation, as data cannot be deleted. Services implemented on top of a blockchain will not be able to delete user data; not even upon legitimate request. Page 13
• ["] We stress that the digital signature is the technology for verifying the authenticity of transactions or digital documents but the blockchain alone, instead, cannot prove the authenticity of products or certificates. Page 14
• ["] Any blockchain project should describe a sound incentive mechanism for miners and consider transaction costs or it will be destined to failure, as no actors of the system will bear the cost of mining. Page 14
• ["] Even accounting and billing does not require the entire history, but only a previous reading and recent invoicing. Page 14
• ["] the blockchain can still be used as an external service supporting the decentralized validation of IoT transactions, offering a complementary or alternative paradigm to centralized cloud services. Page 14

Page 14

• ["] Classic blockchains are in contrast with IoT requirements Page 14
• ["] We identify node-to-node consensus as a means to build trust among operators/systems without established relations….To settle a transaction it is sufficient for the transacting parties to agree on the transaction protocol, and this agreement can be reached privately by the two (or few more) parties in any fashion. What makes node-to-node consensus appealing for IoT is its efficient support of local consensus, which is natural for many IoT applications such as those with groups of sensors or a platoon of vehicles. Page 15
• ["] Transaction Channels are all those techniques used to group off-chain transactions between the same small group of users to speed them up and avoid paying multiple transaction fees. A Transaction Channel is therefore a node-to-node consensus protocol where the two transacting parties establish a fast “payment” method and agree to postpone the clearing of the transactions’ balance. Recording the status of the channel on the blockchain can be periodic or event-based. Page 15
• ["] The most notable implementation of Transaction Channels is the Lightning Network Page 15
• ["] The Lightning Network is “Bitcoin oriented,” but the concept of a network of payment channels may become the transaction platform enabling a global market at the IoT scale. Page 15

Page 15

• ["] IoT transactions are local and normally lightweight in nature, therefore calling for local and lightweight solutions for the platform to support them. From time to time, separate IoT domains, platforms and applications may need to carry out and record transactions with a global, final, and immutable nature. At this level blockchains will play an important role, freeing IoT systems from the need to subscribe to a global, centralized, expensive, trust-based service whose security and reliability have well-known limitations. Using blockchains externally would therefore bring added value to the IoT domain, responding to its requirements of extending beyond local, context-limited applications when needed. Page 15
• ["] In this paper we argue that the blockchain is not the appropriate technology for securing the IoT, albeit it can bring added value as an external service. Page 16
• ["] We raise the concern that stake-based protocols fully rely on the rationality assumption of their users and, compared to traditional voting based protocols, lack of mathematical stability properties. This means that stake-based systems are prone to market failures Page 16
• ["] We stress how the voting power has a tendency to consolidate in the hands of few great stakeholders with both PoS and PoW. Page 16
• ["] For this reason, we suggest to consider the two as census suffrage mechanisms, with PoS preferable over PoW to reduce the energy consumption. However, the immutability of a PoS-blockchain is questionable, so a PoW-based one is considered more secure. Page 16
• ["] In conclusion, we advocate using the blockchain only in those IoT scenarios where the transactions are supported by local, lightweight platforms whose consensus is tailored to the domain of application and the local context. We name these platforms “Transaction Channels.” These channels may (or may not, depending on the application) interact through aggregate, rare transactions to form a global network of Transaction Channels, which can be successfully based on the blockchain technology, freeing the IoT from the need to rely on global, centralized platforms to interact across diverse application, technology, and administrative domains. Page 16