Efficient Threshold Secret Sharing Schemes Secure against Rushing Cheaters

In this paper, we consider three very important issues namely detection, identification and robustness of $k$-out-of-$n$ secret sharing schemes against rushing cheaters who are allowed to submit (possibly forged) shares {\em after} observing shares of the honest users in the reconstruction phase. Towards this we present five different schemes. Among these, first we present two $k$-out-of-$n$ secret sharing schemes, the first one being capable of detecting $(k-1)/3$ cheaters such that $|V_i|=|S|/\epsilon^3$ and the second one being capable of detecting $n-1$ cheaters such that $|V_i|=|S|/\epsilon^{k+1}$, where $S$ denotes the set of all possible secrets, $\epsilon$ denotes the successful cheating probability of cheaters and $V_i$ denotes set all possible shares. Next we present two $k$-out-of-$n$ secret sharing schemes, the first one being capable of identifying $(k-1)/3$ rushing cheaters with share size $|V_i|$ that satisfies $|V_i|=|S|/\epsilon^k$. This is the first scheme whose size of shares does not grow linearly with $n$ but only with $k$, where $n$ is the number of participants. For the second one, in the setting of public cheater identification, we present an efficient optimal cheater resilient $k$-out-of-$n$ secret sharing scheme against rushing cheaters having the share size $|V_i|= (n-t)^{n+2t}|S|/\epsilon^{n+2t}$. The proposed scheme achieves {\em flexibility} in the sense that the security level (i.e. the cheater(s) success probability) is independent of the secret size. Finally, we design an efficient $(k, \delta)$ robust secret sharing secure against rushing adversary with optimal cheater resiliency. Each of the five proposed schemes has the smallest share size having the mentioned properties among the existing schemes in the respective fields

Broadcast-Optimal Two Round MPC with Asynchronous Peer-to-Peer Channels

In this paper we continue the study of two-round broadcast-optimal MPC, where broadcast is used in one of the two rounds, but not in both. We consider the realistic scenario where the round that does not use broadcast is asynchronous. Since a first asynchronous round (even when followed by a round of broadcast) does not admit any secure computation, we introduce a new notion of asynchrony which we call $(t_d, t_m)$-asynchrony. In this new notion of asynchrony, an adversary can delay or drop up to $t_d$ of a given party\u27s incoming messages; we refer to $t_d$ as the deafness threshold. Similarly, the adversary can delay or drop up to $t_m$ of a given party\u27s outgoing messages; we refer to $t_m$ as the muteness threshold. We determine which notions of secure two-round computation are achievable when the first round is $(t_d, t_m)$-asynchronous, and the second round is over broadcast. Similarly, we determine which notions of secure two-round computation are achievable when the first round is over broadcast, and the second round is (fully) asynchronous. We consider the cases where a PKI is available, when only a CRS is available but private communication in the first round is possible, and the case when only a CRS is available and no private communication is possible before the parties have had a chance to exchange public keys

Round-Optimal Multi-party Computation with Identifiable Abort

Secure multi-party computation (MPC) protocols that are resilient to a dishonest majority allow the adversary to get the output of the computation while, at the same time, forcing the honest parties to abort. Aumann and Lindell introduced the enhanced notion of security with identifiable abort, which still allows the adversary to trigger an abort but, at the same time, it enables the honest parties to agree on the identity of the party that led to the abort. More recently, in Eurocrypt 2016, Garg et al. showed that, assuming access to a simultaneous message exchange channel for all the parties, at least four rounds of communication are required to securely realize non-trivial functionalities in the plain model. Following Garg et al., a sequence of works has matched this lower bound, but none of them achieved security with identifiable abort. In this work, we close this gap and show that four rounds of communication are also sufficient to securely realize any functionality with identifiable abort using standard and generic polynomial-time assumptions. To achieve this result we introduce the new notion of bounded-rewind secure MPC that guarantees security even against an adversary that performs a mild form of reset attacks. We show how to instantiate this primitive starting from any MPC protocol and by assuming trapdoor-permutations. The notion of bounded-rewind secure MPC allows for easier parallel composition of MPC protocols with other (interactive) cryptographic primitives. Therefore, we believe that this primitive can be useful in other contexts in which it is crucial to combine multiple primitives with MPC protocols while keeping the round complexity of the final protocol low

Your Reputation\u27s Safe with Me: Framing-Free Distributed Zero-Knowledge Proofs

Distributed Zero-Knowledge (dZK) proofs, recently introduced by Boneh et al. (CYPTO`19), allow a prover $P$ to prove NP statements on an input $x$ which is distributed between $k$ verifiers $V_1,\ldots,V_k$, where each $V_i$ holds only a piece of $x$. As in standard ZK proofs, dZK proofs guarantee Completeness when all parties are honest; Soundness against a malicious prover colluding with $t$ verifiers; and Zero Knowledge against a subset of $t$ malicious verifiers, in the sense that they learn nothing about the NP witness and the input pieces of the honest verifiers. Unfortunately, dZK proofs provide no correctness guarantee for an honest prover against a subset of maliciously corrupted verifiers. In particular, such verifiers might be able to ``frame\u27\u27 the prover, causing honest verifiers to reject a true claim. This is a significant limitation, since such scenarios arise naturally in dZK applications, e.g., for proving honest behavior, and such attacks are indeed possible in existing dZKs. We put forth and study the notion of strong completeness for dZKs, guaranteeing that true claims are accepted even when $t$ verifiers are maliciously corrupted. We then design strongly-complete dZK proofs using the ``MPC-in-the-head\u27\u27 paradigm of Ishai et al. (STOC`07), providing a novel analysis that exploits the unique properties of the distributed setting. To demonstrate the usefulness of strong completeness, we present several applications in which it is instrumental in obtaining security. First, we construct a certifiable version of Verifiable Secret Sharing (VSS), which is a VSS in which the dealer additionally proves that the shared secret satisfies a given NP relation. Our construction withstands a constant fraction of corruptions, whereas a previous construction of Ishat et al. (TCC`14) could only handle $k^{\varepsilon}$ corruptions for a small $\varepsilon<1$. We also design a reusable version of certifiable VSS that we introduce, in which the dealer can prove an unlimited number of predicates on the same shared secret. Finally, we extend a compiler of Boneh et al. (CRYPTO`19), who used dZKs to transform a class of ``natural\u27\u27 semi-honest protocols in the honest-majority setting into maliciously secure ones with abort. Our compiler uses strongly-complete dZKs to obtain identifiable abort

How Can the Use of Human Enhancement (HE) Technologies in the Military Be Ethically Assessed?

War is a terrible price to pay for the prospect of peace. Yet every nation has a moral obligation to protect its citizenry from unjust aggression and threats to security. To be sure, war is always a failure. It is a failure of mankind to come together in mutual respect for the inherent dignity of the human person. The issue of the use of HE in the military is relevant today because the Global War on Terror (GWOT) and the rapid rise of emerging technologies have led to a never-before-seen type of asymmetrical warfare. The rise of these technologies can threaten the inherent dignity of the human person. In turn, the value that a nation places on human dignity in many ways is a gauge of what sorts of rights it will guarantee to its citizens, which impacts their ability to pursue basic human goods and contribute to the common good. Military culture seeks to instill virtues, such as courage and justice, in soldiers and also uphold particular military values, such as honor and selfless service. These virtues and values can be threatened if the use of HE in the military are used for immoral purposes. Paternalism, coercion, undue influence, and limited autonomy are all factors that can undermine the dignity of soldiers. Yet these threats can be overcome through a moral framework for how to ethically assess the use of HE in the military. The moral criteria of reversibility, upholding moral agency and military values, voluntary informed consent, and the use of non-HE technologies first (last resort) presented in this dissertation allows one to approach different HE technologies for use in the military and determine if they are compatible with human flourishing. It will be imperative that HE technologies in the military, if morally permissible, are used on a small-scale and only for necessity, not convenience. This approach is valuable because it can overcome demands put forth from the civilian realm that these HE technologies should be available to them as well; based upon philosophical claims of autonomy and individual rights. This dissertation is distinct insofar that it provides a comprehensive approach to current and future ethical issues related to HE in the military. To strengthen and compliment this moral framework, some recommendations are put forth in this dissertation. These include greater transparency in HE research and use, the designation of soldiers as a vulnerable population, greater ethics education for military health care professionals, the codification of international principles and guidelines for the use of HE technologies in the military, and finally a recommendation to balance the overarching principles of autonomy and individualism with a communitarian ethic and common good approach as a beneficial way to assess the use of HE in the military

Places, People and Health: A socio-geographical perspective on wellbeing of mothers and their children in deprived neighbourhoods of Malta

This study explores socio-geographical processes influencing health and wellbeing in deprived Maltese neighbourhoods, contributing new knowledge on how these compare with those reported in research on Anglo-American cultures. This qualitative research obtained data from three deprived neighbourhoods in Malta through in-depth interviews with 31 mothers (of diverse marital status) and their children. Some of these families were followed across a period of time thus obtaining longitudinal data. The research employed a grounded theory approach, and constant comparative approach was used to explore how social processes differed across neighbourhoods. Familial and neighbourhood ties, networks and other aspects of social capital emerge as highly significant, and often beneficial for health and wellbeing. However, divisive processes in these social networks also had negative impacts, less often reported in other research. This thesis emphasises that there is a strong connection between material neighbourhood factors and social relations, as the physical built environment, housing conditions, service provision, welfare benefits, and employment opportunities influence social processes and impact on health and wellbeing in diverse ways. The history of the place, as well as individual life histories, together with a cross-generational and longitudinal approach the significance of the ‘time’ dimension, thus contributing to the complexities of health and wellbeing in neighbourhoods. This study adds to literature on social determinants of health operating in a Maltese, Mediterranean context. It emphasises that there are traditional norms that still determine the health and wellbeing of inhabitants in their neighbourhoods, however, social and economic changes are also transforming these neighbourhoods. It further reveals how individual agency interacts with the social and material environment to affect wellbeing outcomes, albeit within limits on individuals’ power and resources. The findings therefore highlight the importance of a relational approach in order to understand the connection between people, place and health