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We present a simple and efficient compiler for transforming secure multi-party computation (MPC) protocols that enjoy security only with an honest majority into MPC protocols that guarantee security with no honest majority, in the oblivious-transfer (OT) hybrid model. Our technique works by combining a secure protocol in the honest majority setting with a protocol achieving only security against semi-honest parties in the setting of no honest majority. Applying our compiler to variants of protocols from the literature, we get several applications for secure two-party computation and for MPC with no honest majority. These include:-Constant-rate two-party computation in the OT-hybrid model. We obtain a statistically UC-secure two-party protocol in the OT-hybrid model that can evaluate a general circuit C of size s and depth d with a total communication complexity of O(s) + poly(k, d, log s) and O(d) rounds. The above result generalizes to a constant number of parties.-Extending OTs in the malicious model. We obtain a computationally efficient protocol for generating many string OTs from few string OTs with only a constant amortized communication overhead compared to the total length of the string OTs.-Black-box constructions for constant-round MPC with no honest majority. We obtain general computationally UC-secure MPC protocols in the OT-hybrid model that use only a constant number of rounds, and only make a black-box access to a pseudorandom generator. This gives the first constant-round protocols for three or more parties that only make a black-box use of cryptographic primitives (and avoid expensive zero-knowledge proofs).

We introduce Attribute-Based Signatures (ABS), a versatile primitive that allows a party to sign a message with fine-grained control over identifying information. In ABS, a signer, who possesses a set of attributes from the authority, can sign a message with a predicate that is satisfied by his attributes. The signature reveals no more than the fact that a single user with some set of attributes satisfying the predicate has attested to the message. In particular, the signature hides the attributes used to satisfy the predicate and any identifying information about the signer (that could link multiple signatures as being from the same signer). Furthermore, users cannot collude to pool their attributes together.We give a general framework for constructing ABS schemes, and then show several practical instantiations based on groups with bilinear pairing operations, under standard assumptions. Further, we give a construction which is secure even against a malicious attribute authority, but the security for this scheme is proven in the generic group model. We describe several practical problems that motivated this work, and how ABS can be used to solve them. Also, we show how our techniques allow us to extend Groth-Sahai NIZK proofs to be simulation-extractable and identity-based with low overhead.

We show that every language in N P has a (black-box) concurrent zero-knowledge proof system usingÕ(log n) rounds of interaction. The number of rounds in our protocol is optimal, in the sense that any language outside BPP requires at leastΩ(log n) rounds of interaction in order to be proved in black-box concurrent zero-knowledge. The zeroknowledge property of our main protocol is proved under the assumption that there exists a collection of claw-free functions. Assuming only the existence of one-way functions, we show the existence ofÕ(log n)-round concurrent zero-knowledge arguments for all languages in N P.

Abstract. Motivated by the problem of protecting cryptographic hardware, we continue the investigation of private circuits initiated in [16]. In this work, our aim is to construct circuits that should protect the secrecy of their internal state against an adversary who may modify the values of an unbounded number of wires, anywhere in the circuit. In contrast, all previous works on protecting cryptographic hardware relied on an assumption that some portion of the circuit must remain completely free from tampering. We obtain the first feasibility results for such private circuits. Our main result is an efficient transformation of a circuit C, realizing an arbitrary (reactive) functionality, into a private circuit C realizing the same functionality. The transformed circuit can successfully detect any serious tampering and erase all data in the memory. In terms of the information available to the adversary, even in the presence of an unbounded number of adaptive wire faults, the circuit C emulates a black-box access to C.