Rational Multiparty Computation

The field of rational cryptography considers the design of cryptographic protocols in the presence of rational agents seeking to maximize local utility functions. This departs from the standard secure multiparty computation setting, where players are assumed to be either honest or malicious. ^ We detail the construction of both a two-party and a multiparty game theoretic framework for constructing rational cryptographic protocols. Our framework specifies the utility function assumptions necessary to realize the privacy, correctness, and fairness guarantees for protocols. We demonstrate that our framework correctly models cryptographic protocols, such as rational secret sharing, where existing work considers equilibrium concepts that yield unreasonable equilibria. Similarly, we demonstrate that cryptography may be applied to the game theoretic domain, constructing an auction market not realizable in the original formulation. Additionally, we demonstrate that modeling players as rational agents allows us to design a protocol that destabilizes coalitions. Thus, we establish a mutual benefit from combining the two fields, while demonstrating the applicability of our framework to real-world market environments.^ We also give an application of game theory to adversarial interactions where cryptography is not necessary. Specifically, we consider adversarial machine learning, where the adversary is rational and reacts to the presence of a data miner. We give a general extension to classification algorithms that returns greater expected utility for the data miner than existing classification methods

Novel Secret Sharing and Commitment Schemes for Cryptographic Applications

In the second chapter, the notion of a social secret sharing (SSS) scheme is introduced in which shares are allocated based on a player's reputation and the way she interacts with other parties. In other words, this scheme renews shares at each cycle without changing the secret, and it allows the trusted parties to gain more authority. Our motivation is that, in real-world applications, components of a secure scheme have different levels of importance (i.e., the number of shares a player has) and reputation (i.e., cooperation with other parties). Therefore, a good construction should balance these two factors accordingly. In the third chapter, a novel socio-rational secret sharing (SRS) scheme is introduced in which rational foresighted players have long-term interactions in a social context, i.e., players run secret sharing while founding and sustaining a public trust network. To motivate this, consider a repeated secret sharing game such as sealed-bid auctions. If we assume each party has a reputation value, we can then penalize (or reward) the players who are selfish (or unselfish) from game to game. This social reinforcement stimulates the players to be cooperative in the secret recovery phase. Unlike the existing protocols in the literature, the proposed solution is stable and it only has a single reconstruction round. In the fourth chapter, a comprehensive analysis of the existing dynamic secret sharing (DSS) schemes is first provided. In a threshold scheme, the sensitivity of the secret and the number of players may fluctuate due to various reasons. Moreover, a common problem with almost all secret sharing schemes is that they are ``one-time'', meaning that the secret and shares are known to everyone after secret recovery. We therefore provide new techniques where the threshold and/or the secret can be changed multiple times to arbitrary values after the initialization. In addition, we introduce a new application of dynamic threshold schemes, named sequential secret sharing (SQS), in which several secrets with increasing thresholds are shared among the players who have different levels of authority. In the fifth chapter, a cryptographic primitive, named multicomponent commitment scheme (MCS) is proposed where we have multiple committers and verifiers. This new scheme is used to construct different sealed-bid auction protocols (SAP) where the auction outcomes are defined without revealing the losing bids. The main reason for constructing secure auctions is the fact that the values of the losing bids can be exploited in future auctions and negotiations if they are not kept private. In our auctioneer-free protocols, bidders first commit to their bids before the auction starts. They then apply a decreasing price mechanism to define the winner and selling price in an unconditionally secure setting

For the intelligibility of the language of the secret services

A modern nyelvfilozófia megalapítója, Ludwig Wittgenstein egyik fiatalkori értekezésében (Notes on Logic, 1913) a nyelv egyetlen, hiánytalanul értelmes használatát a tények hiánytalan leképezésében látta. Mindenféle más jellegű használatára irányuló próbálkozás szükségképpen értelmetlen. Későbbi munkáiban azonban (Blue Book, 1933-1934) a korai gondolatokkal szemben már kételkedik a nyelvnek a világot kizárólagosan leíró feladatában és a nyelvet inkább olyan halmaznak tekinti, amelyben minden tevékenység más és más célt szolgál, amelyek a világ leírásán túl ún. „leképezési” feladatokat töltenek be. Sőt, ez utóbbi funkciónak a Filozófiai vizsgálódásokban (Philosophical Investigations, 1953) már elsőbbséget biztosít: „Mivel minden nyíltan előttünk van, nincs is mit magyarázni…” Az évek során a meghatározó logikai értelmezéstől kissé távolodva nem csak a nyelv mindennapisága, „társadalmisága” került egyre inkább előtérbe, de annak hangsúlyozása is, hogy a nyelvi struktúrában a nyelvi jel és a jelölt csak feltételesen kapcsolódik egymáshoz, amely felismerés már a későbbi nyelvi antropológiai kutatások irányába is mutat. A szociolingvisztika már kifejezetten olyan társadalmi tényezőket vizsgál, amelyek alapján a különböző beszédformák különböző csoportokhoz, társadalmi rétegekhez kapcsolhatók. A nyelv ennek alapján különféle változatok, területi és társadalmi indikátorok mentén kialakult dialektusok, stílusok összessége. A közösség egészére jellemző, a változatok összességéből álló heterogén nyelvi készlet pedig már valóban távol került a nyelvtől a tények kizárólagos leképezését elváró Wittgensteini kiindulóponttól. A nyelvfilozófus gondolatmenetében bekövetkezett változásnak általánosabb következményei majd az XX. század második felétől láthatók igazán, ugyanis ekkortól lesz egyre erőteljesebben jelen a világ nyelvtudományában a nyelvhasználat és a társadalom összefüggéseinek vizsgálatára fókuszáló szociolingvisztika. A tanulmány módszertana ez utóbbi változatot, az érett Wittgenstein gondolatmenetét kívánja alkalmazni a titkosszolgálatok nyelvének történeti vizsgálata során

Revisiting Secure Two-Party Computation with Rational Players

A seminal result of Cleve (STOC 1986) showed that fairness, in general, is impossible to achieve in case of two-party computation if one of them is malicious. Later, Gordon et al. (STOC 2008, JACM 2011) observed that there exist two distinct classes of functions for which fairness can be achieved. One is any function without an embedded XOR, and the other one is a particular function containing an embedded XOR. In this paper, we revisit both classes of functions in two-party computation under rational players for the first time. We identify that the protocols proposed by Gordon et al. achieve fairness in non-rational setting only. In this direction, we design two protocols, one for the millionares\u27 problem or the greater-than function (any function without embedded XOR can be converted to this function) and the other for the particular embedded XOR function of Gordon et al., and show that with rational players, our protocols achieve fairness, correctness and strict Nash equilibrium under suitable choice of parameters in complete information game setting. The dealer is offline in both of our protocols and this is in contrast with the work of Groce et al. (Eurocrypt 2012) which shows fairness and Bayesian Nash equilibrium in two party computation with rational players for arbitrary function in an incomplete information game setting

Privacy protocols

Security protocols enable secure communication over insecure channels. Privacy protocols enable private interactions over secure channels. Security protocols set up secure channels using cryptographic primitives. Privacy protocols set up private channels using secure channels. But just like some security protocols can be broken without breaking the underlying cryptography, some privacy protocols can be broken without breaking the underlying security. Such privacy attacks have been used to leverage e-commerce against targeted advertising from the outset; but their depth and scope became apparent only with the overwhelming advent of influence campaigns in politics. The blurred boundaries between privacy protocols and privacy attacks present a new challenge for protocol analysis. Covert channels turn out to be concealed not only below overt channels, but also above: subversions, and the level-below attacks are supplemented by sublimations and the level-above attacks.Comment: 38 pages, 6 figure

Cyber-crime Science = Crime Science + Information Security

Cyber-crime Science is an emerging area of study aiming to prevent cyber-crime by combining security protection techniques from Information Security with empirical research methods used in Crime Science. Information security research has developed techniques for protecting the confidentiality, integrity, and availability of information assets but is less strong on the empirical study of the effectiveness of these techniques. Crime Science studies the effect of crime prevention techniques empirically in the real world, and proposes improvements to these techniques based on this. Combining both approaches, Cyber-crime Science transfers and further develops Information Security techniques to prevent cyber-crime, and empirically studies the effectiveness of these techniques in the real world. In this paper we review the main contributions of Crime Science as of today, illustrate its application to a typical Information Security problem, namely phishing, explore the interdisciplinary structure of Cyber-crime Science, and present an agenda for research in Cyber-crime Science in the form of a set of suggested research questions