Secret sharing and multiparty computation (also called "secure function evaluation") are fundamental primitives in modern cryptography, allowing a group of mutually distrustful players to perform correct, distributed computations under the sole assumption that some number of them will follow the protocol honestly. This paper investigates how much trust is necessary -- that is, how many players must remain honest -- in order for distributed quantum computations to be possible. We present a verifiable quantum secret sharing (VQSS) protocol, and a general secure multiparty quantum computation (MPQC) protocol, which can tolerate any \left[ {\frac{{n - 1}} {2}} \right] cheaters among n players. Previous protocols for these tasks tolerated \left[ {\frac{{n - 1}} {4}} \right] and \left[ {\frac{{n - 1}} {6}} \right] cheaters, respectively. The threshold we achieve is tight -- even in the classical case, "fair" multiparty computation is not possible if any set of n/2 players can cheat. Our protocols rely on approximate quantum errorcorrecting codes, which can tolerate a larger fraction of errors than traditional, exact codes. We introduce new families of authentication schemes and approximate codes tailored to the needs of our protocols, as well as new state purification techniques along the lines of those used in faulttolerant quantum circuits.
Secret sharing and multiparty computation (also called “secure function evaluation”) are fundamental primitives in modern cryptography, allowing a group of mutually distrustful players to perform correct, distributed computations under the sole assumption that some number of them will follow the protocol honestly. This paper investigates how much trust is necessary – that is, how many players must remain honest – in order for distributed quantum computations to be possible. We present a verifiable quantum secret sharing (VQSS) protocol, and a general secure multiparty quantum computation (MPQC) protocol, which can tolerate any ⌊ n−1 2 ⌋ cheaters among n players. Previous protocols for these tasks tolerated ⌊ n−1 n−1 4 ⌋ and ⌊ 6 ⌋ cheaters, respectively. The threshold we achieve is tight — even in the classical case, “fair ” multiparty computation is not possible if any set of n/2 players can cheat. Our protocols rely on approximate quantum errorcorrecting codes, which can tolerate a larger fraction of errors than traditional, exact codes. We introduce new families of authentication schemes and approximate codes tailored to the needs of our protocols, as well as new state purification techniques along the lines of those used in fault-tolerant quantum circuits.
This paper introduces quantum multiparty protocols which allow the use of temporary assumptions. We prove that secure quantum multiparty computations are possible if and only if classical multi party computations work. But these strict assumptions are necessary only during the execution of the protocol and can be loosened after termination of the protocol. We consider two settings: 1. A collusion of players tries to learn the secret inputs of honest players or tries to modify the result of the computation. 2. A collusion of players cheats in the above way or tries to disrupt the protocol, i.e., the collusion tries to abort the computation or leaks information to honest players. We give bounds on the collusions tolerable after a protocol has terminated and we state protocols reaching these bounds.
One of the key characteristics of secure quantum communication is quantum secure multiparty computation. In this paper, we propose a quantum secure multiparty summation (QSMS) protocol that can be applied to many complex quantum operations. It is based on the
(t,n) threshold approach. We combine the classical and quantum phenomena to make this protocol realistic and secure. Because the current protocols employ the
(n,n) threshold approach, which requires all honest players to execute the quantum multiparty summation protocol, they have certain security and efficiency problems. However, we employ a
(t,n) threshold approach, which requires the quantum summation protocol to be computed only by
t honest players. Our suggested protocol is more economical, practical, and secure than alternative protocols.
We show that any multiparty functionality can be evaluated using a 2-round protocol with perfect correctness and perfect semihonest security, provided that the majority of parties are honest. This settles the round complexity of information-theoretic semihonest multiparty computation, resolving a longstanding open question [Y. Ishai and E. Kushilevitz, Randomizing polynomials: A new representation with applications to round-efficient secure computation, in Proceedings of the 41st Annual Symposium on Foundations of Computer Science FOCS 2000, IEEE Computer Society, 2000, pp. 294--304]. The protocol is efficient for
NC1 functionalities. Furthermore, given black-box access to a one-way function, the protocol can be made efficient for any polynomial functionality, at the cost of only guaranteeing computational security. Our results are based on a new notion of multiparty randomized encoding which extends and relaxes the standard notion of randomized encoding of functions [Y. Ishai and E. Kushilevitz, Randomizing polynomials: A new representation with applications to round-efficient secure computation, in Proceedings of the 41st Annual Symposium on Foundations of Computer Science FOCS 2000, IEEE Computer Society, 2000, pp. 294--304]. The property of a multiparty randomized encoding (MPRE) is that if the functionality
g is an encoding of the functionality
f, then for any (permitted) coalition of players, their respective outputs and inputs in
g allow them to simulate their respective inputs and outputs in
f, without learning anything else, including the other outputs of
f. We further introduce a new notion of effective degree, and show that the round complexity of a functionality
f is characterized by the degree of its MPRE. We construct degree-2 MPREs for general functionalities in several settings under different assumptions, and use these constructions to obtain 2-round protocols. Our constructions also give rise to new protocols in the client-server model with optimal round complexity.
We introduce a scheme for secure multiparty computation utilizing the quantum correlations of entangled states. First we present a scheme for two-party computation, exploiting the correlations of a Greenberger-Horne-Zeilinger state to provide, with the help of a third party, a near-private computation scheme. We then present a variation of this scheme which is passively secure with threshold
t=2, in other words, remaining secure when pairs of players conspire together provided they faithfully follow the protocol. Furthermore, we show that the passively secure variant can be modified to be secure when cheating parties are allowed to deviate from the protocol. We show that this can be generalized to computations of
n-party polynomials of degree 2 with a threshold of $n\ensuremath{-}1$. The threshold achieved is significantly higher than the best known classical threshold, which satisfies the bound
t<n/2. Our schemes, each complying with a different definition of security, shed light on which physical assumptions are necessary in order to achieve quantum secure multiparty computation.