# Watermarking Protocols: A Short Guide for Beginners
###### Abstract  
Watermarking protocols, in conjunction with digital watermarking technologies, make it possible to trace back digital copyright infringers by identifying who has legitimately purchased digital content and then illegally shared it on the Internet. Although they can act as an effective deterrent against copyright violations, their adoption in the current web context is made difficult due to unresolved usability and performance issues. This paper aims at providing researchers with the basics needed to design watermarking protocols suited to the web context. It is focused on two important aspects. The first concerns the basic requirements that make a protocol usable by both web users and content providers, whereas the second concerns the security primitives and how they have been used to implement the most relevant examples of watermarking protocols documented in the literature. In this way, researchers can rely on a quick guide to getting started in the field of watermarking protocols.  
watermarking protocols; digital copyright protection; digital rights management 1  
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example, financial incentives have reduced the earning capacity of content providers. ISPs have often been accused of privacy violation in monitoring the data traffic of web users with the aim of preventing illegal content sharing. DRM systems, for their part, cannot counter the illegal sharing of contents legitimately bought by web users. They cannot legally prove the ownership of content that has been downloaded and tampered as well as identify those responsible for copyright infringement. Moreover, DRM systems have been often accused of having too much control over the protected contents compared to what the copyright law intends.  
A different approach to protecting digital copyright is represented by solutions based on digital watermarking [4]. They make it possible to embed hidden \"fingerprints\", in the form of \"watermarks\", into the contents sold on the Internet [5]. In this way, if watermarks are generated so as to identify buyers, and if the watermarked contents are released according to interaction schemes specified by \"watermarking protocols\", the embedded watermarks can be used as a proof of ownership to identify the web users who have legitimately purchased contents and then illegally shared them on the Internet [6].  
The capacity to identify initial copyright \"infringers\" by examining one of the copies of content illegally shared on the Internet makes watermarking protocols an effective solution to the digital copyright problem. The basic idea consists of a simple consideration: the more a web user illegally shares a legitimately purchased content, the greater the likelihood that the copyright infringement will be picked up and punished. This represents an effective deterrent against copyright violations.  
A consequence of what is reported above is that the scientific literature on the watermarking protocols of the last two decades is characterized by a wide variety of proposals. The proposed protocols mainly differ in the requirements they meet and in the security primitives they exploit, thus achieving very different results in terms of security and practicability for the web context.  
This paper proposes a systematization of preliminary knowledge needed to design innovative watermarking protocols, particularly those based on the popular \"buyer-seller\" scheme [7; 8]. Therefore, the paper firstly specifies and discusses the basic requirements that make a protocol usable by both web users and content providers. Then, it lists the security primitives used to implement the most relevant examples of watermarking protocols documented in the literature. In this way, researchers can rely on a short guide to design watermarking protocols without making the classic errors characterizing many of the previous protocols proposed by the scientific community.