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

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

Weakening the Isolation Assumption of Tamper-proof Hardware Tokens

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

Basing Obfuscation on Simple Tamper-Proof Hardware Assumptions

Code obfuscation is one of the most powerful concepts in cryptography. It could yield functional encryption, digital rights management, and maybe even secure cloud computing. However, general code obfuscation has been proven impossible and the research then focused on obfuscating very specific functions, studying weaker security definitions for obfuscation, and using tamper-proof hardware tokens to achieve general code obfuscation. Following this last line this work presents the first scheme which bases general code obfuscation of multiple programs on one single stateless hardware token. Our construction is proven secure in the UC-framework and proceeds in three steps: 1. We construct an obfuscation scheme based on fully homomorphic encryption (FHE) and a hybrid functionality conditional decrypt, which decrypts the result of a homomorphic computation given a proof that the computation was performed as intended. One difficulty of the first step are possible decryptions errors in the FHE. These decryption errors can occur whenever the randomness for the encryption is chosen maliciously by the receiver of the obfuscated code. Such decryption errors then could make a real obfuscated computation distinguishable from a black box use of the non-obfuscated program. 2. Given two common reference strings (CRS) we construct a UC-protocol realizing the functionality conditional decrypt with a stateless hardware token. As the token is stateless it is resettable by a dishonest receiver and the proofs given to the token must be resettably sound. One additional difficulty occurs when the issuer of the token can be corrupted. A malicious token can be stateful and it cannot be prevented that it aborts after a hardwired number of invocations. To prevent adaptive behavior of a malicious token the data of the receiver has to be hidden from the token and the proofs given to the token must even hide the size of the program and the length of the computation. 3. Last we construct a protocol constructing a CRS with a stateless hardware token. Care has to be taken here to not let the token learn anything about the resulting CRS which could not be simulated, because the very same token will later be used in a protocol based on the security of this CRS

Constant Round Adaptively Secure Protocols in the Tamper-Proof Hardware Model

Achieving constant-round adaptively secure protocols (where all parties can be corrupted) in the plain model is a notoriously hard problem. Very recently, three works published in TCC 2015 (Dachman-Soled et al., Garg and Polychroniadou, Canetti et al.), solved the problem in the Common Reference String (CRS) model. In this work, we present a constant-round adaptive UC-secure computation protocol for all well-formed functionalities in the tamper-proof hardware model using stateless tokens from only one-way functions. In contrast, all prior works in the CRS model require very strong assumptions, in particular, the existence of indistinguishability obfuscation. As a corollary to our techniques, we present the first adaptively secure protocols in the Random Oracle Model (ROM) with round complexity proportional to the depth of circuit implementing the functionality. Our protocols are secure in the Global Random Oracle Model introduced recently by Canetti, Jain and Scafuro in CCS 2014 that provides strong compositional guarantees. More precisely, we obtain an adaptively secure UC-commitment scheme in the global ROM assuming only one-way functions. In comparison, the protocol of Canetti, Jain and Scafuro achieves only static security and relies on the specific assumption of Discrete Diffie-Hellman assumption (DDH)

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

Composable Security in the Tamper Proof Hardware Model under Minimal Complexity

We put forth a new formulation of tamper-proof hardware in the Global Universal Composable (GUC) framework introduced by Canetti et al. in TCC 2007. Almost all of the previous works rely on the formulation by Katz in Eurocrypt 2007 and this formulation does not fully capture tokens in a concurrent setting. We address these shortcomings by relying on the GUC framework where we make the following contributions: (1) We construct secure Two-Party Computation (2PC) protocols for general functionalities with optimal round complexity and computational assumptions using stateless tokens. More precisely, we show how to realize arbitrary functionalities with GUC security in two rounds under the minimal assumption of One-Way Functions (OWFs). Moreover, our construction relies on the underlying function in a black-box way. As a corollary, we obtain feasibility of Multi-Party Computation (MPC) with GUC-security under the minimal assumption of OWFs. As an independent contribution, we identify an issue with a claim in a previous work by Goyal, Ishai, Sahai, Venkatesan and Wadia in TCC 2010 regarding the feasibility of UC-secure computation with stateless tokens assuming collision-resistant hash-functions (and the extension based only on one-way functions). (2) We then construct a 3-round MPC protocol to securely realize arbitrary functionalities with GUC-security starting from any semi-honest secure MPC protocol. For this construction, we require the so-called one-many commit-and-prove primitive introduced in the original work of Canetti, Lindell, Ostrovsky and Sahai in STOC 2002 that is round-efficient and black-box in the underlying commitment. Using specially designed ``input-delayed\u27\u27 protocols we realize this primitive (with a 3-round protocol in our framework) using stateless tokens and one-way functions (where the underlying one-way function is used in a black-box way)

(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