# A Lightweight Hash-Based Blockchain Architecture for Industrial IoT
## 2 Related Work
### Hash Functions on Cryptocurrency  
Cryptocurrency is one of the typical applications of blockchain. There are many cryptocurrencies in the real world, and each cryptocurrency has different features; recently, a cryptocurrency changed the hash function or modified the consensus protocol, improving its performance. Bitcoin [1] is the most popular and first-established cryptocurrency. Bitcoin uses PoW that is the first consensus protocol for cryptocurrency. PoW protocol is a costly computer computation involving hashing (SHA-256, scrypt, etc.), Merkle Tree, and peer-2-peer networking for creating, broadcasting, and validating a block on the blockchain network. The bitcoin uses the SHA-256 function; other cryptocurrencies using SHA-256 function are Bitcoin Cash, Counterparty, MazaCoin, etc. An alternative hash function used in many other cryptocurrencies is scrypt [24]. However, it shows the lower hash rate than SHA-256 function. Cryptocurrencies that uses Scrypt are Bitconnect, Bitcoin Gold, Litecoin [25], etc. There is another aggressively grown platform called Ethereum [26] which also uses blockchain technology to facilitate smart contracts and uses PoS protocol. The PoS is more energy efficient and secure protocol compared to PoW counterparts. Ethereum-based coins use the Ethash function, which uses a Keccak hash function [27] that is eventually standardized to SHA-3. SHA-3 is the latest hash function based on sponge function. It reduced the risk of all standard cryptographic algorithms being broken simultaneously. Moreover, blockchain-based coins use various hash functions, and are summarized in Table 2.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ref.** & **Description** & \multicolumn{3}{c}{**Strategy for Lightweight**} \\ \cline{3-4}  & **Architecture** & **Consensus** & **LW Cryptography** \\ \hline [PERSON] et al. [16] & \begin{tabular}{c} Using private blockchain and \\ eliminate consensus \\ \end{tabular} & & ✓ \\ \hline [PERSON] et al. [17] & \begin{tabular}{c} Using SRN to improve \\ PBFT consensus \\ \end{tabular} & & ✓ \\ \hline [PERSON] et al. [18] & \begin{tabular}{c} IoT software update \\ architecture \\ \end{tabular} & ✓ & \\ \hline [PERSON] et al. [19] & \begin{tabular}{c} Decentralized data storage \\ for IoT \\ \end{tabular} & ✓ & & ✓ \\ \hline [PERSON] et al. [20, 21] & \begin{tabular}{c} software-defined IoT \\ architecture with blockchain \\ \end{tabular} & ✓ & \\ \hline [PERSON], [PERSON] et al. [22] & \
\begin{tabular}{c} SDN based network \\ using blockchain \\ \end{tabular} & ✓ & \\ \hline [PERSON] et al. [23] &
\begin{tabular}{c} Synergistic Multiple Proof and \\ lightweight data structure \\ \end{tabular} & ✓ & ✓ \\ \hline \hline \end{tabular}
\end{table}
Table 1: _Cont._  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Hash Function** & \multicolumn{2}{c}{**Example Coins**} & **Number of Coins** \\ \hline Scrypt & \begin{tabular}{c} Auroracoin, Bitconnect, Bitcoin Gold, Coinye, Dogecoin, \\ Gridcoin, Litecoin, PotCoin \\ \end{tabular} & 261 \\ \hline X11 & Dash, Petro & 106 \\ \hline SHA-256 & \
\begin{tabular}{c} Bitcoin, Bitcoin Cash, Counterparty, MazaCoin, Namecoin, \\ NeuCoin, Nxt, Peercoin, Titcoin \\ \end{tabular} & 99 \\ \hline Quark & PIVX, LockChain, DimeCoin & 35 \\ \hline CryptoNight &
\begin{tabular}{c} ByteCoin, Electroneum, Fantom \\ \end{tabular} & 30 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Top 10 hash functions for blockchain-based coins [28].