# Blockchain and Smart Contracts for Digital Copyright Protection
## 1 Introduction and Motivations  
The rapid and continuous spread of social networks and advances in multimedia technologies have turned ordinary web users into producers of multimedia digital content. Such users wish to control the dissemination of their digital content, which adds relevance to the problem of digital copyright protection. More precisely, problems arise because current multimedia and network technologies enable web users to obtain digital content legitimately and then illicitly exchange it through file-sharing applications. Duplication, modification, and re-distribution of contents do not reduce their perceived quality. Therefore, the inadequate digital copyright protection of content financially damages professional content providers and leads to serious privacy issues for ordinary internet users. Even though the scientific community is actively studying the problem of digital copyright protection, the proposed solutions still fail to reconcile the conflicting interests of those who provide content and those who want to enjoy it. The former demand a high remuneration for their contents and low protection and distribution costs. The latter mainly want to preserve their privacy. Consequently, they are inclined to refuse protection mechanisms that could identify them. However, they also wish to achieve further goals, such as having complete control, according to the \"fair use\" doctrine, over the reproduction and distribution of the purchased contents and paying as little as possible for the contents available on the Internet. Among the different solutions proposed over the years, \"watermarking protocols\" represent a convincing alternative [1]. Such protocols define the transaction schemes by which digital watermarked contents have to be traded through the Internet [2]. They exploit digital watermarking techniques to insert hidden information, called \"watermarks\", into the contents before they are commercialized. Watermarks represent actual \"fingerprints\" that can identify both the providers of the contents and their buyers. As a result, thewatermarks embedded into the contents released according to watermarking protocols make it possible to identify the copyright \"infringers\", that is, the legitimate buyers of contents who have illegally shared them on the Internet: it is sufficient to discover copies of such content and extract the watermarks from them. Therefore, the more content is illegally shared, the greater the chances of discovering and punishing copyright infringements. The current watermarking protocols use different types of protection schemes based on a wide variety of security primitives. The result is that they achieve different results in terms of security and practicability for the web context. In particular, the protocols based on the well-known \"buyer-seller\" scheme [1; 3] are characterized by high levels of security. However, they often fail to meet relevant design requirements, and this condition makes them impracticable in the current web context [1]. In this regard, a questionable issue of the implementation of buyer-seller watermarking protocols is represented by the involvement of \"watermark certification authorities\" (WCAs) acting as trusted third parties (TTPs) in the protection schemes [1; 3; 4]. In such schemes, the WCAs guarantee the transactions by which buyers get digitally watermarked contents from sellers. However, WCAs can be a source of collusion with buyers or sellers [2], and this condition represents a severe security problem for the protocols. In order to remedy such a problem, several watermarking protocols do not employ WCAs [5; 6; 7; 8]. These protocols improve security, but the absence of WCAs makes the participation of buyers in the purchase transactions difficult because of complex security actions. Consequently, such protocols are inadequate for the current web context [1]. A compromise solution is obtained by adopting the \"buyer-friendly\" and \"mediated\" design approach described in [9]. It reintroduces the WCA but with a limited role.