Generic Fully Simulatable Adaptive Oblivious Transfer

We aim at constructing adaptive oblivious transfer protocols, enjoying fully simulatable security, from various well-known assumptions such as DDH, $d$-Linear, QR, DCR, and LWE. To this end, we present two generic constructions of adaptive OT, one of which utilizes verifiable shuffles together with threshold decryption schemes, while the other uses permutation networks together with what we call {\em loosely-homomorphic} key encapsulation schemes. We then show that specific choices of the building blocks lead to concrete adaptive OT protocols with fully simulatable security in the standard model under the targeted assumptions. Our generic methods can be extended to build universally composable (UC) secure, and leakage-resilient OT protocols

k Anonymous Private Query Based on Blind Signature and Oblivious Transfer

In this paper, we consider a scenario where there are a group of clients and a database server, and a client wishes to query the database, but does not want to reveal her or his query to the server. Current solutions for this problem are based on oblivious transfer, which usually requires high communication overhead. To reduce the communication overhead, we propose three k-anonymous private query protocols. Our first protocol is based on blind signature, where the server cannot determine the identity of the querying client from the group. Our second protocol is based on k-anonymous oblivious transfer, where the server cannot tell which record the querying client wants from k records. Our third protocol is a combination of the first and second protocols. Our protocols can achieve k-anonymity and are practical in many real-life applications

Privacy Enhancing Protocols using Pairing Based Cryptography

This thesis presents privacy enhanced cryptographic constructions, consisting of formal definitions, algorithms and motivating applications. The contributions are a step towards the development of cryptosystems which, from the design phase, incorporate privacy as a primary goal. Privacy offers a form of protection over personal and other sensitive data to individuals, and has been the subject of much study in recent years. Our constructions are based on a special type of algebraic group called bilinear groups. We present existing cryptographic constructions which use bilinear pairings, namely Identity-Based Encryption (IBE). We define a desirable property of digital signatures, blindness, and present new IBE constructions which incorporate this property. Blindness is a desirable feature from a privacy perspective as it allows an individual to obscure elements such as personal details in the data it presents to a third party. In IBE, blinding focuses on obscuring elements of the identity string which an individual presents to the key generation centre. This protects an individual's privacy in a direct manner by allowing her to blind sensitive elements of the identity string and also prevents a key generation centre from subsequently producing decryption keys using her full identity string. Using blinding techniques, the key generation centre does not learn the full identity string. In this thesis, we study selected provably-secure cryptographic constructions. Our contribution is to reconsider the design of such constructions with a view to incorporating privacy. We present the new, privacy-enhanced cryptographic protocols using these constructions as primitives. We refine useful existing security notions and present feasible security definitions and proofs for these constructions

Secure Blind Decryption

Abstract. In this work we construct public key encryption schemes that admit a protocol for blindly decrypting ciphertexts. In a blind decryp-tion protocol, a user with a ciphertext interacts with a secret keyholder such that the user obtains the decryption of the ciphertext and the key-holder learns nothing about what it decrypted. While we are not the first to consider this problem, previous works provided only weak secu-rity guarantees against malicious users. We provide, to our knowledge, the first practical blind decryption schemes that are secure under a strong CCA security definition. We prove our construction secure in the stan-dard model under simple, well-studied assumptions in bilinear groups. To motivate the usefulness of this primitive we discuss several applica-tions including privacy-preserving distributed file systems and Oblivious Transfer schemes that admit public contribution.