A Modular Framework for Privacy-Enhancing Signatures: Generalizations, Extensions and Novel Building Blocks


A Modular Framework for Privacy-Enhancing Signatures: Generalizations, Extensions and Novel Building Blocks


David Derler


New computing paradigms such as cloud computing attracted significant attention in recent years and meanwhile numerous enterprises outsource data, computations or services to cloud computing providers for flexibility and/or cost efficiency reasons. In addition, we carry computing devices in our pockets, which allow to ubiquitously access the distributed infrastructures and services emerging from this trend. While these novel paradigms have many advantages, they raise open questions in various directions. Many of those questions are related to security and privacy and arise from processing data at (typically not fully trusted) third parties.

We address the question as how to cryptographically ensure the authenticity of processed data, while at the same time maintaining the privacy of the involved parties. Thereby, we focus on two (partially intersecting) aspects of privacy. First, we target privacy with respect to the party who authenticates the data, i.e., the signer. There are many scenarios where signers reveal more personal information than actually required upon authentication. Our work in this direction aims to counter this by cryptographic schemes and protocols, which inherently ensure that - beyond the fact that the authenticated data stems from a member of some authorized group - nothing is revealed about the signer. Second, we target privacy with respect to the authenticated data itself. In particular, authenticated data often contains sensitive information and disclosing this information to unauthorized parties may pose severe privacy issues. To this end, it is important to have means to remove or replace the privacy sensitive parts before disclosure. At the same time, it is important to guarantee authenticity of the parts of the data which were not removed or replaced. Our work in this direction thus covers cryptographic schemes and protocols which allow modifications of authenticated data in a controlled, signer-defined manner, while still upholding the authenticity guarantees.

From a technical point of view, this thesis improves upon the state of the art of provably secure cryptographic schemes and protocols within the areas of interest outlined above. More precisely, we present novel paradigms and constructions, as well as generalizations and extensions of existing paradigms and constructions. Thereby we follow a modular approach which makes our constructions conceptually simple and easy to understand. While modularity often comes at the cost of reduced efficiency, we stress that the paradigms underlying our constructions are also appealing from a practical point of view.