This doctoral dissertation presents contributions advancing the state-of-the-art of secure two-party computation (S2PC) — a cryptographic primitive that allows two mutually distrustful parties, with respective private inputs, to evaluate a function of their combined input, while ensuring privacy of inputs and outputs and integrity of the computation, externally indistinguishable from an interaction mediated by a trusted party. The dissertation shows that S2PC can be made more practical by means of innovative cryptographic techniques, namely by engineered use of commitment schemes with special properties, enabling more efficient protocols, with provable security and applicable to make systems more dependable. This is one further step toward establishing S2PC as a practical tool for privacy-preserving applications. The main technical contribution is a new protocol for S2PC of Boolean circuits, based on an innovative technique called forge-and-lose.1 Building on top of a traditional cut-and-choose of garbled circuits (cryptographic versions of Boolean circuits), the protocol improves efficiency by reducing by a factor of approximately 3 the needed number of garbled circuits. This significantly reduces a major communication component of S2PC with malicious parties, for circuits of practical size. The protocol achieves simulatable S2PC-with-commitments, producing random commitments of the circuit input and output bits of both parties. The commitments also enable direct linkage of several S2PCs in a malicious adversarial setting. As second result, the dissertation describes an improvement to the efficiency of one of the needed sub-protocols: simulatable two-party coin-flipping.1 The sub-protocol is based on a new universally composable commitment scheme that for bit-strings of increasing size can achieve an asymptotic communication-complexity rate arbitrarily close to 1. The dissertation then discusses how S2PC-with-commitments can enable in brokered identification systems a difficult-to-achieve privacy property — a kind of unlinkability.1 This mitigates a vector of potential mass surveillance by an online central entity (a hub), which is otherwise empowered in systems being developed at nation scale for authentication of citizens. When the hub mediates between identity providers and service providers the authentication of users, an adequate S2PC (e.g., of a block-cipher) can prevent the hub from learning user pseudonyms that would allow linking transactions of the same user across different services providers. 1 Parts of these contributions were previously presented at ASIACRYPT 2013, PETS 2015 and PKC 2016.

Jan 1, 20170 citations

Faster Maliciously Secure Two-Party Computation Using the GPU Full version

Tore Kasper Frederiksen, Thomas Pelle Jakobsen, Jesper Buus Nielsen

Abstract We present a new protocol for maliciously secure two-party computation based on cut-and-choose of garbled circuits using the recent idea of “forge-and-loose”, which eliminates around a factor 3 of garbled circuits that needs to be constructed and evaluated. Our protocol introduces a new way to realize the “forge-and-loose” approach, which avoids an auxiliary secure two-party computation protocol, does not rely on any number theoretic assumptions and parallelizes well in a same instruction, multiple data (SIMD) framework. With this approach we prove our protocol universally composable-secure against a malicious adversary assuming access to oblivious transfer, commitment and coin-tossing functionalities in the random oracle model. Finally, we construct, and benchmark, a SIMD implementation of this protocol using a GPU as a massive SIMD device. The findings compare favorably with all previous implementations of maliciously secure, two-party com-putation. 1

Jan 1, 20140 citations