# Blockchain and Cloud to Overcome the Problems of Buyer and Seller Watermarking Protocols
## 1 Introduction and Motivations  
One of the relevant problems of the current Internet is the copyright protection of digital contents distributed or published on the web. In fact, content providers expose economic losses caused by the failure to adequately protect their digital contents, which, if left unprotected, can be easily duplicated, modified, and re-distributed without reducing their perceptual quality, thus damaging their legitimate owners or revealing their private information. Such a problem also affects common web users, who are becoming producers of multimedia digital contents published on social networks or user generated content platforms. Digital watermarking [1, 2] promises to solve the problem of copyright protection. It enables the insertion of an hidden \"fingerprint\" [3], in the form of a \"watermark\", into any copy of content sold to a buyer. In this way, if the watermark is generated so as to specify copyright information and to identify the buyer, it can personalize each copy of content sold. Furthermore, if the watermarked copy is released according to the interaction scheme defined by a \"watermarking protocol\" [4, 5], the embedded watermark can be used as a proof of ownership to establish who has initially obtained the copy and then illegally shared it on the Internet. Even though digital watermarking and watermarking protocols can effectively support digital copyright protection, both are affected by documented problems. In particular, watermark insertion is a specialized and burdensome activity that requires huge computational resources, since it has to be carried out \"on-the-fly\", when content is sold orpublished [6; 7]. Such a condition represents an obstacle to both content providers and common web users wanting to exploit digital watermarking, since they usually lack the needed computing and storage resources in addition to specific security competence [5; 8]. Watermarking protocols, for their part, are characterized by two closely related problems: the presence of trusted third parties (TTPs) needed to enable the protocols, also called \"watermark certification authorities\" (WCAs) [9; 10], and the difficulty of the actions that have to be performed by the entities participating in the protocols [5; 11]. The former problem is caused by the collusive behaviours that the entities involved in the protocols could have towards TTPs [2; 3], which reduce the security level achieved by the protocols. In fact, a possible solution consists in designing protocols that do not employ TTPs [12; 13; 14; 15; 16]. However, the absence of TTPs often entails a complex involvement of buyers, who have to carry out difficult actions to participate in the protocols [5], and this consequence just aggravates the latter problem. To solve the problems reported above, this paper proposes a new watermarking protocol built around the experiences documented in [5; 8; 11; 17]. The protocol follows the \"buyer friendly\" and \"mediated\" approach by which it is possible to restrict the role of TTPs without complicating the involvement of buyers in the interaction scheme. Moreover, the protocol is expressly designed to exploit both blockchain technology [18; 19] and cloud computing platforms [20]. In particular, the blockchain technology is introduced to simplify the interaction scheme among the entities participating in the protocol without reducing security, whereas the ability to employ cloud computing platforms enables content providers to outsource a burdensome activity such as the on-the-fly insertion of personalized watermarks. The result is an innovative watermarking protocol that, unlike previous experiences documented in the literature, is characterized by an original protection scheme able to achieve a right balance between conflicting objectives: protocol security, limited role of the involved TTP, easy participation of buyers in the protocol, ability to resort to cloud computing to implement on-the-fly and personalized watermark insertions into the digital contents to protect.