## 4 Blockchain-Assisted Cloud Frameworks: Security Perspective
### Comparison of Schemes Discussed in Sections 4.1-4.3
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Year, Ref.**} & \multirow{2}{*}{\begin{tabular}{c} **Type of Cloud** \\ **Storage** \\ \end{tabular} } & \multirow{2}{*}{**Mechanism Used**} & \multirow{2}{*}{**Computation Cost**} & \multirow{2}{*}{**Communication Cost**} & \multirow{2}{*}{\begin{tabular}{c} **Storage** \\ **Overhead** \\ \end{tabular} } \\ \hline \multirow{9}{*}{\(2019,\ [80]\)} & \multirow{9}{*}{Single} & \multirow{9}{*}{Tag} & 1. Server side: exponent in & & \\  & & & group (G) + group operations in & & \\  & & & G + multiplication in \(Z_{p}\) + & & \\  & & & addition in \(Z_{p}\) & & \\  & & & 2. TPA side: computing e(g,q), & & \\  & & & where g,q \(\in\) G + & & \\  & & & exponent in G + & & \\  & & & group operations in G + & & \\  & & & mapping a value into G + & & \\  & & & hash a value into \(Z_{p}\) + & & \\  & & & multiplication in \(Z_{p}\) & & \\ \hline \multirow{9}{*}{\(2021,\ [81]\)} & \multirow{9}{*}{Single} & \multirow{9}{*}{\begin{tabular}{c} Merkle tree and \\ cuckoo filter data \\ structure \\ \end{tabular} } & 1. Client side: 3 nMMU + & CSP’s response on & \\  & & & 2 nMAdd + nMMod + nMHash & & challenge: (no of blocks \\  & & & 2. CSP side: 2 cMul + cAdd + & X signature length of & \\  & & & cMod + cHash & & \\  & & & & & \\ \hline \multirow{9}{*}{\(2021,\ [82]\)} & \multirow{9}{*}{Multi} & \multirow{9}{*}{\begin{tabular}{c} Homomorphic \\ verifiable tag \\ \end{tabular} } & 1. Client side: 3 nMMU + & & \\  & & 2 nMAdd + nMMod + nMHash & & \\  & & & 2. CSP side: 2 cMul + cAdd + & (no. of blocks X \\  & & & & size of each file block) & \\ \hline \multirow{9}{*}{\(2023,\ [83]\)} & \multirow{9}{*}{Single} & \multirow{9}{*}{\begin{tabular}{c} Tag-based \\ challenge, \\ response method \\ \end{tabular} } & 1. User to organizer: no. & \\  & & & of challenged blocks X & \\  & & & & size of each \\  & & & & 2. Organizer to CSPs: no. & not addressed \\  & & & & of challenged blocks X & \\  & & & & size of each \\ \hline \multirow{9}{*}{\(2024,\ [84]\)} & \multirow{9}{*}{Single} & \multirow{9}{*}{\begin{tabular}{c} Tag-based \\ challenge, \\ response method \\ \end{tabular} } & 1. Encryption and & \\  & & & signing key generation: & \\  & & & (2n + 2) X no. of & \\  & & & key servers & \\  & & & & 2. For auditing: (n tags X \\  & & & & size) + (n public keys \\  & & & X size) & \\ \hline \multirow{9}{*}{\(2024,\ [84]\)} & \multirow{9}{*}{Single} & \multirow{9}{*}{\
\begin{tabular}{c} Rank-based \\ Merkle hash tree \\ \end{tabular} } & Tag generation: no. of blocks X &
\begin{tabular}{c} Audit challenge: (no. of \\ sampled blocks X size) + \\ (no. of sample leaf \\ nodes X size) + (no. of \\ sampled blocks in the \\ sampled files X size) \\ \end{tabular} & \\  & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Cont. decentralized networks because they have no central point to control and some of them have followed partially decentralized networks, which have still a point of control. There are two main types of blockchain networks: public and private. A user who wants to be a participant in a public blockchain network can carry out the login process; after that, they can be a participant. There is no restriction for this kind of blockchain network, known as a permissionless network such as Bitcoin, Ethereum, Litecoin, etc. Privacy is the main concern for this public network because there is no restriction over participants; hence, the status of the network is visible to all participants. A private blockchain network is just the opposite of the public one. Only a specific group of nodes are allowed to be a participant in this kind of network. For this reason, it is not a fully decentralized network. Some of the private blockchain networks are Hyperledger, Corda, Ripple, etc. Different kinds of blockchain networks used in the selected research papers are defined under this category. Some of them have used public blockchains and some have used Hyperledger Fabric, which is a distributed ledger technology framework used to build business blockchain applications.