AbstractsComputer Science

In this thesis, we study encryption schemes with various “high functionalities” including one specific focus on broadcast encryption. As for the main contributions, we propose a framework for constructing practical broadcast encryption schemes and a unified framework for public-key encryption with various functionalities. The first focus of the thesis is on a special but important kind of encryption schemes, namely broadcast encryption. Such a scheme has many useful applications; the most important one to be mentioned is the digital right management. More precisely, broadcast encryption enables the protection of digital contents such as copyrighted DVD. Such a technology is “inevitable” nowadays as modern advancements in communication infrastructure and digital storage technologies have, on one hand, enabled pervasive digital media distribution, but on the other hand, also allowed the spread of “pirate” contents to be done easier than ever before. There are some broadcast encryption schemes available in the literature; however, as the number of all users in the system tends to be increased, these existing solutions tend to be quite inefficient, and eventually cannot be used in the real-world application. Our focus is then to construct practical broadcast encryption schemes, which can be “scalable”, in the sense that the efficiency of scheme will not be affected by the increasing number of users. As a result of the research, we achieve this goal by constructing the first schemes whose the main two parameters, namely the ciphertext size and the private key size, are independent of the number of all users, while the computational cost is semi-scalable (namely, the cost is increasing but slowly as logarithmically). Behind this scheme, we proposed a theoretical framework that can be used to construct efficient schemes in a systematical way. The second topic shifts the research focus from the practical point of views to more theoretical ones and looked beyond to more general encryption schemes with “high functionalities”. The motivation came from the fact that in recent years, there have been many cryptographic primitives which extend the normal public-key encryption to achieve useful functionalities such as ID-based encryption, Key-insulated encryption, Forward-secure encryption, Certificate-less encryption, and many more. Each functionality is proved to be useful in different scenarios and applications thereof. Although being seemingly related primitives, there was no unified framework for defining or constructing them. In this work, we proposed a unified framework called Directed Acyclic Graph Encryption (DAGE) that unifies these highly-functional encryption primitives into a unified syntax, a unified security notion, and unified generic/specific constructions. More precisely, we reduce a specification of such a primitive to its necessary and sufficient information, which is turned out to be its underlying graph: by specifying a graph, the definition and constructions will be automatically induced by the framework. We…