# A Watermarking Protocol Based on Blockchain
## 1 Introduction  
Social networks and user-generated content platforms have turned common web users into actual producers of multimedia digital content. Such content can be easily duplicated without reducing their perceptual quality. They can be also maliciously modified and/or re-distributed, thus damaging the reputation of their legitimate owners, or revealing their private information, or causing economic loss. In addition, current mechanisms implemented to protect the copyright of multimedia digital content cannot adequately meet the protection requirements needed to solve piracy problems on the Internet. One of the technologies proposed to protect the users' copyrights on their multimedia digital content is \"digital watermarking\" [1, 2] used in conjunction with \"watermarking protocols\" [3, 4, 5]. Digital watermarking makes it possible to insert hidden information, such as, for example, a \"fingerprint\" [6, 7, 8], within any copy of content that has to be protected. Such information, called a \"watermark\", can be used to identify the user who possesses the content, and makes the copy of the content unique and personalized. However, to combat the unauthorized sharing of multimedia digital content on the Internet, it is necessary to distribute the watermarked content according to specific interaction schemes defined by watermarking protocols. Thus, whenever a copy of watermarked content is found in a suspicious location, such as in file repositories shared by peer-to-peer applications, 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. The most relevant watermarking protocols documented in the literature enable the implementation of mechanisms for copyright protection based on content tracking by fingerprinting [3; 4; 5; 8; 9]. They mainly involve two parties: the \"buyer\" and the \"seller\". The former wishes to get content from a web content provider, whereas the latter wishes to release it in a digitally protected form obtained by inserting a watermark. In particular, the early experiences also involve specific trusted third parties (TTPs), called \"watermark certification authorities\" (WCAs), whose main function is to guarantee the correct execution of the protocols [4; 10; 11; 12; 13; 14; 15]. However, the introduction of WCAs can reduce the security level of the protocols, since TTPs can give rise to potential collusive behaviors with buyers or sellers [2; 16]. As a consequence, a number of watermarking protocols are based on \"simplified\" interaction schemes that do not exploit WCAs [17; 18; 19; 20; 21]. Such approaches appear to be more secure, but they turn out to be impracticable in the current web context, since they are characterized by interaction schemes that force buyers to perform complex security actions to complete content purchase transactions [22]. The watermarking protocols described in [22; 23; 24] attempt to overcome the drawbacks affecting previous solutions existing in the literature by proposing a new \"buyer friendly\" and \"mediated\" design approach. Such an approach reintroduces the TTP, but its role is carefully limited to a restricted part of the protocol, so as to enable a simplified participation of buyers in the content purchase transactions without reducing the security level of the protocol. Although such experiences represent a good balance between security and easy participation of buyers in the protocol, further efforts are needed to simplify the interaction schemes of such watermarking protocols, so as to make them best suited to the current web context that does not like the presence of TTPs. In this regard, it is worth noting that blockchain technology has begun to be employed in the area of digital copyright protection [25; 26; 27; 28; 29].