37 research outputs found
Secure Multiparty Computation with Partial Fairness
A protocol for computing a functionality is secure if an adversary in this
protocol cannot cause more harm than in an ideal computation where parties give
their inputs to a trusted party which returns the output of the functionality
to all parties. In particular, in the ideal model such computation is fair --
all parties get the output. Cleve (STOC 1986) proved that, in general, fairness
is not possible without an honest majority. To overcome this impossibility,
Gordon and Katz (Eurocrypt 2010) suggested a relaxed definition -- 1/p-secure
computation -- which guarantees partial fairness. For two parties, they
construct 1/p-secure protocols for functionalities for which the size of either
their domain or their range is polynomial (in the security parameter). Gordon
and Katz ask whether their results can be extended to multiparty protocols.
We study 1/p-secure protocols in the multiparty setting for general
functionalities. Our main result is constructions of 1/p-secure protocols when
the number of parties is constant provided that less than 2/3 of the parties
are corrupt. Our protocols require that either (1) the functionality is
deterministic and the size of the domain is polynomial (in the security
parameter), or (2) the functionality can be randomized and the size of the
range is polynomial. If the size of the domain is constant and the
functionality is deterministic, then our protocol is efficient even when the
number of parties is O(log log n) (where n is the security parameter). On the
negative side, we show that when the number of parties is super-constant,
1/p-secure protocols are not possible when the size of the domain is
polynomial
Classical Cryptographic Protocols in a Quantum World
Cryptographic protocols, such as protocols for secure function evaluation
(SFE), have played a crucial role in the development of modern cryptography.
The extensive theory of these protocols, however, deals almost exclusively with
classical attackers. If we accept that quantum information processing is the
most realistic model of physically feasible computation, then we must ask: what
classical protocols remain secure against quantum attackers?
Our main contribution is showing the existence of classical two-party
protocols for the secure evaluation of any polynomial-time function under
reasonable computational assumptions (for example, it suffices that the
learning with errors problem be hard for quantum polynomial time). Our result
shows that the basic two-party feasibility picture from classical cryptography
remains unchanged in a quantum world.Comment: Full version of an old paper in Crypto'11. Invited to IJQI. This is
authors' copy with different formattin
A Unified Security Perspective on Legally Fair Contract Signing Protocols
Inspired by Maurer\u27s universal zero knowledge (UZK) abstract perspective and building on legally fair contract signing protocols without keystones, we propose and analyze the security of the first UZK class of co-signing protocols. We construct our main idea considering the stringent issue of scheme compatibility which characterizes communication systems. Typical examples are the cases of certificates in a public key infrastructure and the general issue of upgrading the version of a system. Thus, working in a general framework may reduce implementation errors and save application development and maintenance time
Unconditionally Secure Rational Secret Sharing in Standard Communication Networks
Rational secret sharing protocols in both the two-party and
multi-party settings are proposed. These protocols are built in
standard communication networks and with unconditional security.
Namely, the protocols run over standard point-to-point networks
without requiring physical assumptions or simultaneous channels, and
even a computationally unbounded player cannot gain more than
by deviating from the protocol. More precisely, for the
-out-of- protocol the is a negligible function in
the size of the secret, which is caused by the information-theoretic
MACs used for authentication. The -out-of- protocol is
-resilient and the is exponentially small in the
number of participants. Although secret recovery cannot be
guaranteed in this setting, a participant can at least reduce the
Shannon entropy of the secret to less than after the protocol.
When the secret-domain is large, every rational player has great
incentive to participate in the protocol
Fair Computation with Rational Players
We consider the problem of fair multiparty computation, where fairness means (informally) that all parties should learn the correct output. A seminal result of Cleve (STOC 1986) shows that fairness is, in general, impossible to achieve if a majority of the parties is malicious. Here, we treat all parties as rational and seek to understand what can be done.
Asharov et al. (Eurocrypt 2011) showed impossibility of rational fair computation in the two-party setting, for a particular function and a particular choice of utilities. We observe, however, that in their setting the parties have no strict incentive to compute the function even in an ideal world where fairness is guaranteed. Revisiting the problem, we show that rational fair computation is possible, for arbitrary functions, as long as the parties have a strict incentive to compute the function in an ideal world where fairness is guaranteed. Our results extend to more general utility functions that do not directly correspond to fairness, as well as to the multi-party setting. Our work thus shows a new setting in which game-theoretic considerations can be used to circumvent a cryptographic impossibility result
Legally Fair Contract Signing Without Keystones
International audienceIn two-party computation, achieving both fairness and guaranteed output delivery is well known to be impossible. Despite this limitation , many approaches provide solutions of practical interest by weakening somewhat the fairness requirement. Such approaches fall roughly in three categories: " gradual release " schemes assume that the aggrieved party can eventually reconstruct the missing information; " optimistic schemes " assume a trusted third party arbitrator that can restore fairness in case of litigation; and " concurrent " or " legally fair " schemes in which a breach of fairness is compensated by the aggrieved party having a digitally signed cheque from the other party (called the keystone). In this paper we describe and analyse a new contract signing paradigm that doesn't require keystones to achieve legal fairness, and give a concrete construction based on Schnorr signatures which is compatible with standard Schnorr signatures and provably secure
Mobile Commerce: Secure Multi-party Computation & Financial Cryptography
Abstract: The basic objective of this work is to construct an efficient and secure mechanism for mobile commerce applying the concept of financial cryptography and secure multi-party computation. The mechanism (MCM) is defined by various types of elements: a group of agents or players, actions, a finite set of inputs of each agent, a finite set of outcomes as defined by output function, a set of objective functions and constraints, payment function, a strategy profile, dominant strategy and revelation principle. The mechanism adopts a set of intelligent moves as dominant strategies: (a) flexible use of hybrid payment system which supports cash, e-payment and m-payment, (b) secure multi-party computation to ensure information security and privacy and (c) call intelligent analytics to assess and mitigate possible threats on m-commerce service. The mechanism supports three different types of transaction processing protocols (P1, P2 and P3) and calls a cryptographic protocol (Pc). The cryptographic protocol performs a set of functions sequentially such as authentication, authorization, correct identification, privacy verification and audit of correctness, fairness, rationality, accountability and transparency of secure multi-party computation on each m-transaction. The basic building blocks of the cryptographic protocol are signcryption, proofs of knowledge, commitments and secret sharing. This work also presents the complexity analysis of the mechanism in terms of computational cost, communication cost, security and business intelligence.
Keywords: Secure multi-party computation, Financial cryptography, Mobile commerce mechanism, Threat analytics, Digital econom