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**2017/433**

**Title:**

Two-Message Witness Indistinguishability and Secure Computation in the Plain Model from New Assumptions

**Authors:**
Saikrishna Badrinarayanan, Sanjam Garg, Yuval Ishai, Amit Sahai, Akshay Wadia

**Abstract:**

We study the feasibility of two-message protocols for secure two-party computation in the plain model, for functionalities that deliver output to one party, with security against malicious parties. Since known impossibility results rule out polynomial-time simulation in this setting, we consider the common relaxation of allowing super-polynomial simulation. We first address the case of zero-knowledge functionalities. We present a new construction of two-message zero-knowledge protocols with super-polynomial simulation from any (sub- exponentially hard) game-based two-message oblivious transfer protocol, which we call Weak OT. As a corollary, we get the first two-message WI arguments for NP from (sub-exponential) DDH. Prior to our work, such protocols could only be constructed from assumptions that are known to imply non-interactive zero-knowledge protocols (NIZK), which do not include DDH. We then extend the above result to the case of general single-output functionalities, showing how to construct two-message secure computation protocols with quasi-polynomial simulation from Weak OT. This implies protocols based on sub-exponential variants of several standard assumptions, including Decisional Diffie Hellman (DDH), Quadratic Residuosity Assumption, and Nth Residuosity Assumption. Prior works on two-message protocols either relied on some trusted setup (such as a common reference string) or were restricted to special functionalities such as blind signatures. As a corollary, we get three-message protocols for two-output functionalities, which include coin-tossing as an interesting special case. For both types of functionalities, the number of messages (two or three) is optimal. Finally, motivated by the above, we further study the Weak OT primitive. On the positive side, we show that Weak OT can be based on any semi-honest 2-message OT with a short second message. This simplifies a previous constructions of Weak OT from the Nth Residuosity Assumption. We also present a construction of Weak OT from Witness Encryption (WE) and injective one-way functions, implying the first construction of two-message WI arguments from WE. On the negative side, we show that previous constructions of Weak OT do not satisfy simulation-based security even if the simulator can be computationally unbounded.

**ePrint:**
https://eprint.iacr.org/2017/433

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