Welcome to the resource topic for 2008/534
Title:
Somewhat Non-Committing Encryption and Efficient Adaptively Secure Oblivious Transfer
Authors: Juan A. Garay, Daniel Wichs, Hong-Sheng Zhou
Abstract:Designing efficient cryptographic protocols tolerating adaptive adversaries, who are able to corrupt parties on the fly as the computation proceeds, has been an elusive task. Indeed, thus far no \emph{efficient} protocols achieve adaptive security for general multi-party computation, or even for many specific two-party tasks such as oblivious transfer (OT). In fact, it is difficult and expensive to achieve adaptive security even for the task of \emph{secure communication}, which is arguably the most basic task in cryptography. In this paper we make progress in this area. First, we introduce a new notion called \emph{semi-adaptive} security which is slightly stronger than static security but \emph{significantly weaker than fully adaptive security}. The main difference between adaptive and semi-adaptive security is that, for semi-adaptive security, the simulator is not required to handle the case where \emph{both} parties start out honest and one becomes corrupted later on during the protocol execution. As such, semi-adaptive security is much easier to achieve than fully adaptive security. We then give a simple, generic protocol compiler which transforms any semi-adaptively secure protocol into a fully adaptively secure one. The compilation effectively decomposes the problem of adaptive security into two (simpler) problems which can be tackled separately: the problem of semi-adaptive security and the problem of realizing a weaker variant of secure channels. We solve the latter problem by means of a new primitive that we call {\em somewhat non-committing encryption} resulting in significant efficiency improvements over the standard method for realizing (fully) secure channels using (fully) non-committing encryption. Somewhat non-committing encryption has two parameters: an equivocality parameter \ell (measuring the number of ways that a ciphertext can be ``opened’') and the message sizes k. Our implementation is very efficient for small values \ell, \emph{even} when k is large. This translates into a very efficient compilation of many semi-adaptively secure protocols (in particular, for a task with small input/output domains such as bit-OT) into a fully adaptively secure protocol. Finally, we showcase our methodology by applying it to the recent Oblivious Transfer protocol by Peikert \etal\ [Crypto 2008], which is only secure against static corruptions, to obtain the first efficient, adaptively secure and composable OT protocol. In particular, to transfer an n-bit message, we use a constant number of rounds and O(n) public key operations.
ePrint: https://eprint.iacr.org/2008/534
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