27 research outputs found

    Permissionless Consensus in the Resource Model

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    In the permissionless regime of distributed computing, participants may join and leave an internet-scale protocol execution at will. The permissionless regime poses challenges to the classical techniques used for consensus protocols, in which participants attempt to agree on a function of their inputs. For example, classical consensus techniques require bounding the numbers of honest and corrupt participants, and for honest participants to remain online throughout. Bitcoin\u27s introduction of Proof of Work enabled dynamic participation by shifting focus from the number of parties to the number of hash puzzles that parties collectively solve, and in turn, enforcing constraints on the blocks sent by honest parties. Other Bitcoin-inspired works have developed Proof of X (PoX) variants to remediate the shortcomings of Proof of Work. We propose a new abstraction called resources and argue that in practice, several PoX variants appear to implement resources. For every resource that a party obtains, it is permitted to send a special protocol message. We show that given few additional assumptions, resources are sufficient to achieve consensus in the permissionless regime, even in the presence of a full-information adversary that can choose which parties get resources and when they get them. In particular, it is not necessary to know a bound on the network delay, participants do not need clocks, and participants can join and leave the execution arbitrarily, even after sending only a single message. We require only a known upperbound on the rate at which resources enter the system, relative to the maximum network delay (without needing to know the network delay), and that over the long term, a majority of resources are acquired by honest participants. Our protocol for consensus in the permissionless model follows from a protocol for graph consensus, which we define as a generalization of blockchains. Our graph consensus works even when resources enter the system at high rates, but the required honest majority increases with the rate. We show how to modify the protocol slightly to achieve one-bit consensus. Finally, we show that for every graph consensus protocol that outputs a majority of honest vertices there exists a one-bit consensus protocol

    Composing Timed Cryptographic Protocols: Foundations and Applications

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    Time-lock puzzles are unique cryptographic primitives that use computational complexity to keep information secret for some period of time, after which security expires. Unfortunately, current analysis techniques of time-lock primitives provide no sound mechanism to build multi-party cryptographic protocols which use expiring security as a building block. We explain in this paper that all other attempts at this subtle problem lack either composability, a fully consistent analysis, or functionality. The subtle flaws in the existing frameworks reduce to an impossibility by Mahmoody et al., who showed that time-lock puzzles with super-polynomial gaps (between committer and solver) cannot be constructed from random oracles alone; yet still the analyses of algebraic puzzles today treat the solving process as if each step is a generic or random oracle. This paper presents a new complexity theoretic based framework and new structural theorems to analyze timed primitives with full generality and in composition (which is the central modular protocol design tool). The framework includes a model of security based on fine-grained circuit complexity which we call residual complexity, which accounts for possible leakage on timed primitives as they expire. Our definitions for multi-party computation protocols generalize the literature standards by accounting for fine-grained polynomial circuit depth to model computational hardness which expires in feasible time. Our composition theorems incur degradation of (fine-grained) security as items are composed. In our framework, simulators are given a polynomial “budget” for how much time they spend, and in composition these polynomials interact. Finally, we demonstrate via a prototypical auction application how to apply our framework and theorems. For the first time, we show that it is possible to prove – in a way that is fully consistent, with falsifiable assumptions – properties of multi-party applications based on leaky, temporarily secure components

    Techniques for Almost-Asynchronous Distributed Cryptography

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    Prenatal Isolated Ventricular Septal Defect May Not Be Associated with Trisomy 21

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    The aim of this study was to examine if isolated fetal ventricular septal defect (VSD) is associated with trisomy 21. One hundred twenty six cases with prenatal VSD diagnosed by a pediatric cardiologist were reviewed. Cases with known risk factors for congenital heart disease, the presence of other major anomalies, soft signs for trisomy 21 or a positive screen test for trisomy 21 were excluded. Ninety two cases formed the study group. None of the cases in the study group had trisomy 21. The upper limit of prevalence for trisomy 21 in isolated VSD is 3%. When prenatal VSD is not associated with other major anomalies, soft markers for trisomy 21 or a positive nuchal translucency or biochemical screen, a decision whether to perform genetic amniocentesis should be individualized. The currently unknown association between isolated VSD and microdeletions and microduplications should be considered when discussing this option

    IPF patients are limited by mechanical and not pulmonary-vascular factors – results of a derivation-validation cohort study

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    BACKGROUND: During cardiopulmonary exercise testing (CPET), Idiopathic Pulmonary Fibrosis (IPF) patients do not reach their direct maximum voluntary ventilation (MVV) and have deranged gas exchange. Their exercise limitation is therefore attributed to a pulmonary vascular mechanism. METHODS: We studied two cohorts (derivation and validation) of IPF patients with lung function testing and CPET. Maximal ventilation at exercise (VEpeak) was compared to direct MVV by Bland-Altman analysis. RESULTS: In the derivation cohort (n = 101), direct MVV over-estimated VEpeak by a factor of 1.51, driven by respiratory rate during MVV that was 1.99 times higher at rest as compared to VEpeak at exercise. The formula (FEV1 × 20.1) + 15.4 was shown to predict VEpeak (r2 = 0.56) in the derivation cohort. In the validation cohort of 78 patients, VEpeak was within a factor of 1.27 (6.8 l/min) of predicted according to the novel formula. According to the novel prediction formula the majority of patients (58%) in the entire cohort have VEpeak within 85% of their predicted MVV, which would indicate a mechanical respiratory limitation to exercise. CONCLUSION: Estimation of direct MVV performed at rest leads to significant over-estimation of the breathing reserve in IPF patients. This may lead to over-diagnosis of pulmonary vascular limitation in these patients. Expected maximal ventilation at exercise may be accurately predicted indirectly by an IPF-specific formula

    Function and Structure of Polytene Chromosomes During Insect Development

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    Störungen des Kaliumstoffwechsels und ihre klinische Bedeutung

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