2.2.1. Nakamoto Protocols

The Nakamoto protocol is implemented as the consensus foundation of Bitcoin [7], and it is widely adopted by many cryptocurrency-based blockchain networks such as Ethereum [17]. The Nakamoto protocol adopts a computation-intensive PoW, which requires all participants to compete for rewards through a cryptographic block-hash value

discovery racing game. The PoW consensus demonstrates security and scalability in an asynchronous open-access network as long as an adversary does not control the majority (51%) of the miners. However, the brute-force PoW mining process also incurs a high demand in terms of computation and energy consumption such that it is not affordable on resource-constrained IoT devices.

In order to improve performance and resource usage efficiency in PoW, a number of alternative Proof of X-concept (PoX) schemes have been proposed. Permacoin [8] repurposes mining resources in PoW to achieve distributed storage of archival data. The Permacoin adopts PORs [18], which require miners to present random access to a copy of a file from local storage as valid proof for successfully minting money. Permacoin requires participants to invest in its storage capacity rather than solo computational power. It could reduce unnecessary wastage of computational resources in PoW and mitigate centralized mining pools issue.

Similar to Permacoin, a Resource-Efficient Mining (REM) [19] scheme is proposed to achieve security and resource efficiency based on the partially decentralized trust models inherent in Intel Software Guard Extensions (SGX). The REM utilizes a Proof-of-Useful-Work (PoUW) consensus protocol, which requires miners to provide trustworthy measurements on CPU cycles used by its useful workloads in SGX-protected enclave. Compared with Proof-of-Elapsed-Time (PoET) in Sawtooth [20] that uses random idle CPU time as proofs, PoUW in REM not only prevents the stale chip problem but also yields the smallest amount of mining waste.

In order to reduce energy consumption caused by intensive hash value calculating in PoW, Peercoin [21] adopts Proof-of-Stake (PoS), which leverages the distribution of token ownership to simulate a verifiable random function to propose new blocks. Such a process of efficient "virtual mining" manner allows PoS miners to only consume limited computational resources in order to generate new blocks. Similarly to PoW, PoS guarantees security as long as an adversary owns no more than half of the total stakes in the network.

Unlike PoW and its variants, the PoENF consensus scheme neither requires high demand of computation and storage for mining nor depends on security guarantees supported by trusted hardware or monetary deposit stake. It is suitable for heterogeneous IoVT devices connected to the power grid.
