Paper 2016/201

From Stateful Hardware to Resettable Hardware Using Symmetric Assumptions

Nico Doettling, Daniel Kraschewski, Joern Mueller-Quade, and Tobias Nilges

Abstract

Universally composable multi-party computation is impossible without setup assumptions. Motivated by the ubiquitous use of secure hardware in many real world security applications, Katz (EUROCRYPT 2007) proposed a model of tamper-proof hardware as a UC-setup assumption. An important aspect of this model is whether the hardware token is allowed to hold a state or not. Real world examples of tamper-proof hardware that can hold a state are expensive hardware security modules commonly used in mainframes. Stateless, or resettable hardware tokens model cheaper devices such as smartcards, where an adversarial user can cut off the power supply, thus resetting the card's internal state. A natural question is how the stateful and the resettable hardware model compare in their cryptographic power, given that either the receiver or the sender of the token (and thus the token itself) might be malicious. In this work we show that any UC-functionality that can be implemented by a protocol using a single untrusted stateful hardware token can likewise be implemented using a single untrusted resettable hardware token, assuming only the existence of one-way functions. We present two compilers that transform UC-secure protocols in the stateful hardware model into UC-secure protocols in the resettable hardware model. The first compiler can be proven secure assuming merely the existence of one-way functions. However, it (necessarily) makes use of computationally rather expensive non-black-box techniques. We provide an alternative second compiler that replaces the expensive non-black-box component of the first compiler by few additional seed OTs. While this second compiler introduces the seed OTs as additional setup assumptions, it is computationally very efficient.