Contributing Partners

TUGRAZ, UNI PASSAU, AIT

Executive Summary

PRISMACLOUD aims at bringing novel cryptographic concepts and methods to practical application to improve the security and privacy of cloud based services and make them usable for providers and users.
The purpose of this report is to document the progress on research activities within the Task 4.2 Functional and malleable signatures up to M30 of the PRISMACLOUD project. Task 4.2 focuses on advancements with respect to novel constructions, features and interconnections between various types of signature schemes, as well as improvements of their efficiency for selected (restricted) functionalities required by practical applications. Furthermore, the application of functional and malleable signatures for verifiable computing and authenticity preserving computations on signed data are studied.
More precisely, this task conducts research in the following fields.
Task 4.2.2 Research on suitability of functional and malleable signatures for verifiable computing and authenticity preserving computations on signed data. Functional signatures allow for constructing non-interactive arguments and delegation schemes. This makes this primitive very interesting for verifiable computing and integrity preserving modification of data, both important topics in the research and deployment of cloud computing. Thus, in this subtask the suitability of malleable/functional signatures for both applications will be studied. More precisely, it will be analyzed how signatures can be used to perform modifications and computations on data stored in the cloud, such that as a result not only integrity but also authenticity can be provided. In addition, the connection of functional signatures to the general paradigm of verifiable computing will be studied.
Task 4.2.3 Research on the design of functional and malleable signatures. Most of the existing generic constructions of functional signatures as well as P-homomorphic signatures are too inefficient for practical purposes as they are generally designed to work for arbitrary functions or predicates. One aim in this task is to study and develop schemes for relevant (restricted) functionalities and predicates based on building blocks, which allow for efficient instantiations and thus use in practice. Furthermore, we will study signature schemes that combine several features provided by other schemes in isolation, for instance, malleable, threshold signatures, multi-signatures as well as proxy-signature schemes. Moreover, we will study the relationships between various types of such signature schemes. We will also analyse how to construct signature schemes that provide new homomorphic properties that are of particular interest for verifiable computing. For instance, properties of multikey fully homomorphic encryption schemes [LTV12] open up new interesting opportunities especially in the direction of verifiable computing. Therefore, we will analyze and further elaborate this recent approach. In particular, we will investigate how to construct a signature analogue, i.e., a multikey homomorphic signature scheme, and how both, such encryption and signature schemes, can be combined with (existing) verifiable computing techniques.