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.
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.
The Universal Composability framework was introduced by Canetti to study the security of protocols which are concurrently executed with other protocols in a network environment. Unfortunately it was shown that in the so called plain model, a large class of functionalities cannot be securely realized. These severe impossibility results motivated the study of other models involving some sort of setup assumptions, where general positive results can be obtained. Until recently, all the setup assumptions which were proposed required some trusted third party (or parties). Katz recently proposed using a physical setup to avoid such trusted setup assumptions. In his model, the physical setup phase includes the parties exchanging tamper proof hardware tokens implementing some functionality. The tamper proof hardware is modeled so as to assume that the receiver of the token can do nothing more than observe its input/output characteristics. It is further assumed that the sender knows the program code of the hardware token which it distributed. Based on the DDH assumption, Katz gave general positive results for universally composable multi-party computation tolerating any number of dishonest
Secure computation in the presence of tamper-proof hardware tokens is proven under the assumption that the holder of the token is only given black-box access to the functionality of the token. Starting with the work of Goldreich and Ostrovsky [GO96], a long series of works studied tamper-proof hardware for realizing two-party functionalities in a variety of settings. In this work we focus our attention on two important complexity measures of stateless token-based secure computation: round complexity and hardness assumptions and present the following results in the two-party setting: • A round optimal generic secure protocol in the plain model assuming one-way functions, where the tokens are created by a single party. • A round optimal generic UC secure protocol assuming one-way functions. Our constructions are proven in the real/ideal paradigm with security in the presence of static malicious adversaries. As a side contribution, we identify a flaw in one of the feasibility results regarding UC secure protocols in the tamper proof model proved in the work of Goyal, Ishai, Sahai, Venkatesan and Wadia (TCC 2010) and correct history by attributing the work of Choi, Katz, Schroöder, Yerukhimovic and Zhou (TCC 2014) to establishing the (same) feasibility result.
Designing secure protocols in the Universal Composability (UC) framework confers many advantages. In particular, it allows the protocols to be securely used as building blocks in more complex protocols, and assists in understanding their security properties. Unfortunately, most existing models in which universally composable computation is possible (for useful functionalities) require a trusted setup stage. Recently, Katz [Eurocrypt ’07] proposed an alternative to the trusted setup assumption: tamper-proof hardware. Instead of trusting a third party to correctly generate the setup information, each party can create its own hardware tokens, which it sends to the other parties. Each party is only required to trust that its own tokens are tamper-proof. Katz designed a UC commitment protocol that requires both parties to generate hardware tokens. In addition, his protocol relies on a specific number-theoretic assumption. In this paper, we construct UC commitment protocols for “David ” and “Goliath”: we only require a single party (Goliath) to be capable of generating tokens. We construct a version of the protocol that is secure for computationally unbounded parties, and a more efficient version that makes computational assumptions only about David (we require only the existence of a one-way function). Our protocols are simple enough to be performed by hand on David’s side. These properties may allow such protocols to be used in situations which are inherently asymmetric in real-life, especially those involving individuals versus large organizations. Classic examples include voting protocols (voters versus “the government”) and protocols involving private medical data (patients versus insurance-agencies or hospitals).
A number of works have investigated using tamper-proof hardware tokens as tools to achieve a variety of cryptographic tasks. In particular, Goldreich and Ostrovsky considered the goal of software protection via oblivious RAM. Goldwasser, Kalai, and Rothblum introduced the concept of one-time programs: in a one-time program, an honest sender sends a set of simple hardware tokens to a (potentially malicious) receiver. The hardware tokens allow the receiver to execute a secret program specified by the sender’s tokens exactly once (or, more generally, up to a fixed t times). A recent line of work initiated by Katz examined the problem of achieving UC-secure computation using hardware tokens. Motivated by the goal of unifying and strengthening these previous notions, we consider the general question of basing secure computation on hardware tokens. We show that the following tasks, which cannot be realized in the “plain ” model, become feasible if the parties are allowed to generate and exchange tamper-proof hardware tokens. • Unconditional non-interactive secure computation. We show that by exchanging simple stateful hardware tokens, any functionality can be realized with unconditional security against malicious parties. In the case of two-party functionalities f(x, y) which take their inputs from a sender and a receiver and deliver
Abstract Everlasting security models the setting where hardness assumptions hold during the execution of a protocol but may get broken in the future. Due to the strength of this adversarial model, achieving any meaningful security guarantees for composable protocols is impossible without relying on hardware assumptions (Müller-Quade and Unruh, JoC’10). For this reason, a rich line of research has tried to leverage physical assumptions to construct well-known everlasting cryptographic primitives, such as commitment schemes. The only known everlastingly UC secure commitment scheme, due to Müller-Quade and Unruh (JoC’10), assumes honestly generated hardware tokens. The authors leave the possibility of constructing everlastingly UC secure commitments from malicious hardware tokens as an open problem. Goyal et al. (Crypto’10) constructs unconditionally UC-secure commitments and secure computation from malicious hardware tokens, with the caveat that the honest tokens must encapsulate other tokens. This extra restriction rules out interesting classes of hardware tokens, such as physically uncloneable functions (PUFs). In this work, we present the first construction of an everlastingly UC-secure commitment scheme in the fully malicious token modelwithout requiringhonest token encapsulation. Our scheme assumes the existence of PUFs and is secure in the common reference string model. We also show that our results are tight by giving an impossibility proof for everlasting UC-securecomputationfrom non-erasable tokens (such as PUFs), even with trusted setup.
Recent results have shown the usefulness of tamper-proof hardware tokens as a setup assumption for building UC-secure two-party computation protocols, thus providing broad security guarantees and allowing the use of such protocols as buildings blocks in the modular design of complex cryptography protocols. All these works have in common that they assume the tokens to be completely isolated from their creator, but this is a strong assumption. In this work we investigate the feasibility of cryptographic protocols in the setting where the isolation of the hardware token is weakened. We consider two cases: (1) the token can relay messages to its creator, or (2) the creator can send messages to the token after it is sent to the receiver. We provide a detailed characterization for both settings, presenting both impossibilities and information-theoretically secure solutions.