Year

2016

Degree Name

Doctor of Philosophy

Department

School of Computing and Information Technology

Abstract

We introduce novel techniques to achieve a wide range of functional encryption schemes. For our work, we explore a number of functional encryption schemes and observe their techniques to achieve adaptive security. Based on our observations, we develop several techniques to enable functional encryption schemes. Particularly, we utilize encoding frameworks to generalize our contribution which is applicable to a number of functional encryption schemes. We utilize the pair encoding framework (Eurocrypt'14) for our work, but we also introduce a new encoding framework to achieve efficient functional encryption schemes.

Firstly, using the pair encoding framework, we show that functional encryption schemes can be converted into their dual conversion without any efficiency loss. The dual conversion means the conversion of any scheme into the symmetric scheme in which the roles are swapped between private keys and ciphertexts (e.g ciphertext policy attribute based encryption and key policy attribute based encryption). Additionally, we provide a new construction in prime order groups for pair encodings. This construction improves the efficiency of previous constructions for pair encodings since it realizes functional encryption schemes which were only suggested in composite order groups into prime order groups.

As a new encoding framework, we introduce a tag based encoding which is more efficient than previous encodings when the size of predicate is large. It supports a number of functional encryptions. Also, generic constructions for tag based encodings are provided. In particular, they are adaptively secure in prime order groups under the standard assumption or static assumptions. Moreover, key policy attribute based encryption schemes are followed. These schemes share the technique of the tag based encoding but these schemes provide semi-adaptive security which is weaker than adaptive security. However, these schemes show many desirable properties such as multi-use of attribute, short ciphertexts and support large universe under the standard assumption in prime order groups. Therefore, those schemes complement tag based encoding.

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