On Efficient Non-Interactive Oblivious Transfer with Tamper-Proof Hardware

Oblivious transfer (OT, for short) [RAB81] is a fundamental primitive in the foundations of Cryptography. While in the standard model OT constructions rely on public-key cryptography, only very recently Kolesnikov in [KOL10] showed a truly efficient string OT protocol by using tamper-proof hardware tokens. His construction only needs few evaluations of a block cipher and requires stateless (therefore resettable) tokens that is very efficient for practical applications. However, the protocol needs to be interactive, that can be an hassle for many client-server setting and the security against malicious sender is achieved in a covert sense, meaning that a malicious sender can actually obtain the private input of the receiver while the receiver can detect this malicious behavior with probability 1/2. Furthermore the protocol does not enjoy forward security (by breaking a token one violates the security of all previously played OTs). In this work, we propose new techniques to achieve efficient non-interactive string OT using tamper-proof hardware tokens. While from one side our tokens need to be stateful, our protocol enjoys several appealing features: 1) it is secure against malicious receivers and the input privacy of honest receivers is guaranteed unconditionally against malicious senders, 2) it is forward secure, 3) it enjoys adaptive input security, therefore tokens can be sent before parties know their private inputs. This gracefully fits a large number of client-server settings (digital TV, e-banking) and thus many practical applications. On the bad side, the output privacy of honest receivers is not satisfied when tokens are reused for more than one execution

Computing on Encrypted Data

Abstract. Encryption secures our stored data but seems to make it in-ert. Can we process encrypted data without having to decrypt it first? Answers to this fundamental question give rise to a wide variety of appli-cations. Here, we explore this question in a number of settings, focusing on how interaction and secure hardware can help us compute on en-crypted data, and what can be done if we have neither interaction nor secure hardware at our disposal.

The Cryptographic Strength of Tamper-Proof Hardware

Tamper-proof hardware has found its way into our everyday life in various forms, be it SIM cards, credit cards or passports. Usually, a cryptographic key is embedded in these hardware tokens that allows the execution of simple cryptographic operations, such as encryption or digital signing. The inherent security guarantees of tamper-proof hardware, however, allow more complex and diverse applications

(Efficient) Universally Composable Oblivious Transfer Using a Minimal Number of Stateless Tokens

We continue the line of work initiated by Katz (Eurocrypt 2007) on using tamper-proof hardware tokens for universally composable secure computation. As our main result, we show an oblivious-transfer (OT) protocol in which two parties each create and exchange a single, stateless token and can then run an unbounded number of OTs. We also show a more efficient protocol, based only on standard symmetric-key primitives (block ciphers and collision-resistant hash functions), that can be used if a bounded number of OTs suffice. Motivated by this result, we investigate the number of stateless tokens needed for universally composable OT. We prove that our protocol is optimal in this regard for constructions making black-box use of the tokens (in a sense we define). We also show that nonblack-box techniques can be used to obtain a construction using only a single stateless token

Lower Bounds in the Hardware Token Model

We study the complexity of secure computation in the tamper-proof hardware token model. Our main focus is on non-interactive unconditional two-party computation using bit-OT tokens, but we also study computational security with stateless tokens that have more complex functionality. Our results can be summarized as follows: - There exists a class of functions such that the number of bit-OT tokens required to securely implement them is at least the size of the sender\u27s input. The same applies for receiver\u27s input size (with a different class of functionalities). - Non-adaptive protocols in the hardware token model imply efficient (decomposable) randomized encodings. This can be interpreted as evidence to the impossibility of non-adaptive protocols for a large class of functions. - There exists a functionality for which there is no protocol in the stateless hardware token model accessing the tokens at most a constant number of times, even when the adversary is computationally bounded. En route to proving our results, we make interesting connections between the hardware token model and well studied notions such as OT hybrid model, randomized encodings, and obfuscation

A Unified Approach to Constructing Black-box UC Protocols in Trusted Setup Models

We present a unified framework for obtaining black-box constructions of Universal Composable (UC) protocol in trusted setup models. Our result is analogous to the unified framework of Lin, Pass, and Venkitasubramaniam [STOC\u2709, Asiacrypt\u2712] that, however, only yields non-black-box constructions of UC protocols. Our unified framework shows that to obtain black-box constructions of UC protocols, it suffices to implement a special purpose commitment scheme that is, in particular, concurrently extractable using a given trusted setup. Using our framework, we improve black-box constructions in the common reference string and tamper-proof hardware token models by weakening the underlying computational and setup assumptions

On the Composability of Statistically Secure Random Oblivious Transfer

We show that random oblivious transfer protocols that are statistically secure according to a definition based on a list of information-theoretical properties are also statistically universally composable. That is, they are simulatable secure with an unlimited adversary, an unlimited simulator, and an unlimited environment machine. Our result implies that several previous oblivious transfer protocols in the literature that were proven secure under weaker, non-composable definitions of security can actually be used in arbitrary statistically secure applications without lowering the security