# Blockchain and Smart Contracts for Digital Copyright Protection
## 6 Security Analysis  
As indicated in Section 4, the following assumptions are the basis of the analysis conducted on the proposed protocol. Firstly, all communications among the web entities involved in the protocol are protected by the TLS protocol to avoid man-in-the-middle attacks in point-to-point communications. Furthermore, digital contents are protected by adopting a watermark insertion technique that can resist the most common manipulations. This is a reasonable assumption since examples of such techniques are well-documented in the literature [34; 42; 43]. Finally, the adopted encryption scheme is privacy homomorphic with respect to watermark insertion. It is \"probabilistic\" and \"semantically secure\", meaning that a malicious user with polynomial computational resources cannot obtain useful information from a ciphertext [18]. In this regard, the analysis can be restricted to the asymmetric homomorphic encryption schemes that support addition operations [1; 39; 40] \begin{table}
\begin{tabular}{c l} \hline \hline
**Entities and** & **Actions and Data** \\
**Interactions** & \\ \hline \(\mathcal{CP}\) & : finds \(X^{\prime}\) in the market and extracts \(W^{\prime}=W^{\prime}_{\mathcal{CP}}\|N^{\prime}\) \\ \(\mathcal{CP}\) & : searches its databases for a possible match on \(W^{\prime}_{\mathcal{CP}}\) \\ \(\mathcal{CP}\xrightarrow{m_{1}}\mathcal{J}\) & : \(m_{1}=\{N^{\prime},X_{d},T_{X},pk^{X},\mathbb{E}_{pk^{X}}(N),SBlock\}\) \\ \(\mathcal{J}\xrightarrow{m_{2}}\mathcal{BC}\) & : \(m_{2}=\{X_{d},T_{X},pk^{X},\mathbb{E}_{pk^{X}}(N),SBlock\}\) \\ \(\mathcal{BC}\) & : \(\mathbb{A}_{sm_{v}}(m_{2})\) \\ \(\mathcal{BC}\) & : retrieves \(PBlock=[X_{d},T_{X},SBlock]\) \\ \(\mathcal{BC}\) & : decrypts \(SBlock\) and obtains \(B_{id},X_{d},T_{X},pk^{X},sk^{X},N,\mathbb{E}_{pk^{X}}(N)\) \\ \(\mathcal{BC}\xrightarrow{m_{3}}\mathcal{J}\) & : \(m_{3}=\{B_{id},N\}\) \\ \(\mathcal{BC}\) & : \(\mathbb{E}\mathbb{N}\mathbb{D}_{sm_{v}}\) \\ \(\mathcal{J}\) & : compares \(N^{\prime}\) with \(N\) and adjudicates \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Identification and arbitration protocol. since such schemes can be adopted in conjunction with both \"spread spectrum\" (SS) watermarking techniques [34] and data hiding schemes defined as \"informed embedding\", such as watermark insertion schemes based on \"quantization index modulation\" (QIM) [37; 42]. For the present analysis, the watermarking protocol can be synthetically described as follows: \(\mathcal{CP}\) cells the copyrighted digital content \(\bar{X}\) to \(\mathcal{B}\); \(\mathcal{BC}\) behaves as a ledger able to publish the information identifying the purchase transaction of \(\bar{X}\); \(sm_{s}\) and \(sm_{p}\) are the smart contracts that are automatically executed within \(\mathcal{BC}\) to generate and manage the tokens needed to conduct the purchase transactions; \(\mathcal{J}\) determines whether \(\mathcal{B}\) has infringed the \(\mathcal{CP}\)'s copyright by distributing pirated copies of \(\bar{X}\). Furthermore, the entities participating in the protocol behave as follows:  
* When uncorrupted, buyers and content providers do not infringe copyright by distributing pirated copies. * \(\mathcal{J}\) cannot be corrupted since it is implemented by a TTP. * \(\mathcal{BC}\) can be considered an \"honest-but-curious\" entity [44]. This means that \(\mathcal{BC}\) could obtain information about purchase transactions, but it cannot exploit it to collude with \(\mathcal{B}\) or \(\mathcal{CP}\) since it is forced to behave according to the protocol rules. This is a realistic assumption since the protocol forces \(\mathcal{BC}\) only to run two smart contracts, \(sm_{s}\) and \(sm_{p}\), whose code, once approved, is not allowed to be modified during the life of the blockchain [13; 14; 15]. * \(\mathcal{CP}\) and \(\mathcal{B}\) can be corrupted only \"statically\", i.e., deciding which of the two entities is corrupt is made before running the protocol and can no longer be changed [44].