With the development of modern Internet and ad hoc networks, there is an increasing need for autonomous entities cooperating together to perform knowledge-and information-based decision making. Data in its original form, however, typically contains sensitive information about individuals and unmanaged data exchange may violate individual privacy. Secure multi-party computation is proposed to solve this problem and has become a hot topic. Yao's garbled circuit approach is a major method in designing protocols for secure two-party computation. However, only few papers dealt with the fundamental property of fairness for Yao's garbled circuit protocol, in which one introduced a third party and others were based on cut-and-choose techniques that are inherently expensive. In this paper, we propose an efficient fair UC-secure Yao's garbled circuit protocol in the malicious model based on Jarecki et al.'s construction and Ou Ruan et al.'s gradual release commitment. The protocol avoids involving any third party or using cut-and-choose techniques, and it is more bandwidth-efficient than other Yao-based protocols. Furthermore, its security could be proved within the UC framework.
With the development of modern Internet and ad hoc networks, there is an increasing need for autonomous entities cooperating together to perform knowledge-and information-based decision making. Data in its original form, however, typically contains sensitive information about individuals and unmanaged data exchange may violate individual privacy. Secure multi-party computation is proposed to solve this problem and has become a hot topic. Yao's garbled circuit approach is a major method in designing protocols for secure two-party computation. However, only few papers dealt with the fundamental property of fairness for Yao's garbled circuit protocol, in which one introduced a third party and others were based on cut-and-choose techniques that are inherently expensive. In this paper, we propose an efficient fair UC-secure Yao's garbled circuit protocol in the malicious model based on Jarecki et al.'s construction and Ou Ruan et al.'s gradual release commitment. The protocol avoids involving any third party or using cut-and-choose techniques, and it is more bandwidth-efficient than other Yao-based protocols. Furthermore, its security could be proved within the UC framework.
Yao first introduced a constant-round protocol for secure two-party computation (2PC) withstanding semi-honest adversaries by using a tool called "garbled circuit". Later, many protocols based on garbled circuit approach have been presented, most of which discussed malicious adversaries and efficiency about 2PC. However, there only have a few protocols dealing with the fundamental property of fairness for Yao's garbled circuit approach, in which one involved a trusted third party and the others are very expensive. In the paper, we propose' an efficient Yao's garbled circuit protocol for fair secure 2PC based on ElGamal encryption, Pedersen commitment, Cachin et al.'s verifiable oblivious transfer and Ou-Ruan et al.'s gradual release homomorphic timed commitment. The protocol achieves two advantages: it doesn't need the third party and it is more efficient than other fair secure Yao's protocols.
Consider several parties that do not trust each other, yet they wish to correctly compute some common function of their local inputs while keeping these inputs private. This problem is known as "Secure Multi-Party Computation", and was introduced by Andrew Yao in 1982. Secure multi-party computations have some real world examples like electronic auctions, electronic voting or fingerprinting. In this thesis we consider the case where there are only two parties involved. This is known as "Secure Two-Party Computation". If there is a trusted third party called Carol, then the problem is pretty straightforward. The participating parties could hand their inputs in Carol who can compute the common function correctly and could return the outputs to the corresponding parties. The goal is to achieve (almost) the same result when there is no trusted third party. Cryptographic protocols are designed in order to solve these kinds of problems. These protocols are analyzed within an appropriate model in which the behavior of parties is structured. The basic level is called the Semi-Honest Model where parties are assumed to follow the protocol specification, but later can derive additional information based on the messages which have been received so far. A more realistic model is the so-called Malicious Model. The common approach is to first analyze a protocol in the semi-honest model and then later extend it into the malicious model. Any cryptographic protocol for secure two-party computation must satisfy the following security requirements: correctness, privacy and fairness. It must guarantee the correctness of the result while preserving the privacy of the parties’ inputs, even if one of the parties is malicious and behaves arbitrarily throughout the protocol. It must also guarantee fairness. This roughly means that whenever a party aborts the protocol prematurely, he or she should not have any advantage over the other party in discovering the output. The main question for researchers is to construct new protocols that achieve the above mentioned goals for secure multi-party computation. Of course, such protocols must be secure in a given model, as well as be as efficient as possible. In 1986, Yao presented the first general protocol for secure two-party computation which was applicable only to the semi-honest model. He uses a tool called "Garbled Circuit". Yao’s protocol uses the underlying primitives ("Pseudorandom Generator" and "Oblivious Transfer") as blackboxes which lead to efficient results. After Yao’s work many variants and improvements have been proposed for the malicious model. In this thesis, we design several new protocols for secure two-party computation based on Yao’s garbled circuit. Before we present the details of our new designs, we first show several weaknesses, security flaws or problems with the existing protocols in the literature. We first work in the semi-honest model and then extend it into the malicious model by presenting new protocols. Finally we add fairness to our protocol. Oblivious transfer (OT) is a fundamental primitive in modern cryptography which is useful for implementing protocols for secure multi-party computation. We study several variants of oblivious transfer in this thesis. We present a new protocol for the so-called "Committed OT". This protocol is very efficient in the sense that it is quite good in comparison to the most efficient committed OT protocols in the literature. The abovementioned flaw with the use of OT can be fixed with our committed oblivious transfer protocol. Furthermore, it is more general than all previous protocols, and, therefore, it is of independent interest. We also deal with fairness in this thesis. For protocols based on garbled circuit, so far only Benny Pinkas has presented a protocol in the literature for achieving fairness. We show a subtle problem with this protocol where the privacy of the inputs of one party can be compromised. We also describe this problem in detail which is in fact related to the fairness, and finally propose a more efficient scheme that does achieve fairness.
In the setting of secure two-party computation, two parties wish to securely compute a joint function of their private inputs, while revealing only the output. One of the primary techniques for achieving efficient secure two-party computation is that of Yao’s garbled circuits (FOCS 1986). In the semi-honest model, where just one garbled circuit is constructed and evaluated, Yao’s protocol has proven itself to be very efficient. However, a malicious adversary who constructs the garbled circuit may construct a garbling of a different circuit computing a different function, and this cannot be detected (due to the garbling). In order to solve this problem, many circuits are sent and some of them are opened to check that they are correct while the others are evaluated. This methodology, called cut-and-choose, introduces significant overhead, both in computation and in communication, and is mainly due to the number of circuits that must be used in order to prevent cheating. In this paper, we present a cut-and-choose protocol for secure computation based on garbled circuits, with security in the presence of malicious adversaries, that vastly improves on all previous protocols of this type. Concretely, for a cheating probability of at most 2−40, the best previous works send between 125 and 128 circuits. In contrast, in our protocol 40 circuits alone suffice (with some additional overhead). Asymptotically, we achieve a cheating probability of 2−s where s is the number of garbled circuits, in contrast to the previous best of 2−0.32s. We achieve this by introducing a new cut-and-choose methodology with the property that in order to cheat, all of the evaluated circuits must be incorrect, and not just the majority as in previous works. The security of our protocol relies on the Decisional Diffie-Hellman assumption.
Applying cut-and-choose techniques to Yao’s garbled circuit protocol has been a promising approach for designing efficient Two-Party Computation (2PC) with malicious and covert security, as is evident from various optimizations and software implementations in the recent years. We revisit the security and efficiency properties of this popular approach and propose alternative constructions and definitions that are more suitable for use in practice. • We design an efficient fully-secure malicious 2PC protocol for two-output functions that only requires O(t|C|) symmetric-key operations (with small constant factors) where |C | is the circuit size and t is a statistical security parameter. This is essentially the optimal complexity for protocols based on cut-and-choose, resolving a main question left open by the previous work on the subject. Our protocol utilizes novel techniques for enforcing garbler’s input consistency and handling twooutput functions that are more efficient than all prior solutions. • Motivated by the goal of eliminating the all-or-nothing nature of 2PC with covert security (that privacy and correctness are fully compromised if the adversary is not caught in the challenge phase), we propose a new security definition for 2PC that strengthens the guarantees provided by the standard
Protocols for secure two-party computation enable a pair of mistrusting parties to compute a joint function of their private inputs without revealing anything but the output. One of the fundamental techniques for obtaining secure computation is that of Yao’s garbled circuits. In the setting of malicious adversaries, where the corrupted party can follow any arbitrary (polynomial-time) strategy in an attempt to breach security, the cut-and-choose technique is used to ensure that the garbled circuit is constructed correctly. The cost of this technique is the construction and transmission of multiple circuits; specifically, s garbled circuits are used in order to obtain a maximum cheating probability of 2−s. In this paper, we show how to reduce the amortized cost of cut-and-choose based secure two-party computation to O s logN garbled circuits when N secure computations are run. We use this method to construct a secure protocol in the batch setting. Next, we show how the cut-