6 research outputs found
Improving Round and Communication Metrics in Consensus
No full textByzantine Consensus protocols, aka Byzantine Agreement (BA), Byzantine Broadcast (BB), allow a set of n mutually distrusting parties to share input values and agree on the same output value. Notable practical applications of consensus in blockchains and MPC require efficient, practical implementations of consensus protocols. The rise of blockchain systems and the general trend towards decentralized services, which inherently utilize consensus, as well as the increased demand of consensus as a primitive for implementing other cryptographic protocols (e.g. in MPC), brought once again this topic in the forefront of research.
A recurring trilemma of consensus when utilized in practice is striking the balance in order to construct protocols that are: i) efficient, ii) resilient under harsh adversarial conditions, and iii) with minimal assumptions for the corresponding setting. These properties are usually negatively associated; efficient protocols often either require stronger assumptions, or are less resilient to strong adversaries.
In this dissertation we aim to improve the efficiency of consensus protocols under harsh adversarial conditions and weak assumptions. We explore directions towards improving the two major metrics of efficiency of such protocols; i.e. their communication and round complexities. We focus on protocols operating in a synchronous communication network, and show how to achieve efficiency of either rounds or communication under different assumptions, against weakly adaptive adversaries with high corruption threshold. We will construct i) (Parallel) Broadcast protocols with improved state of the art communication, ii) the first Broadcast protocol with sublinear rounds without trusted setup, and iii) the first Deterministic Byzantine Agreement protocol with adaptive O(n·f) communication
Gossiping for Communication-Efficient Broadcast
No full textByzantine Broadcast is crucial for many cryptographic pro- tocols such as secret sharing, multiparty computation and blockchain consensus. In this paper we apply gossiping (propagating a message by sending to a few random parties who in turn do the same, until the mes- sage is delivered) and propose new communication-efficient protocols, under dishonest majority, for Single-Sender Broadcast (BC) and Parallel Broadcast (PBC), improving the state-of-the-art in several ways.
As our first warm-up result, we give a randomized protocol for BC which achieves O(n^2κ^2) communication complexity from plain public key setup assumptions. This is the first protocol with subcubic communication in this setting, but does so only against static adversaries.
Using some ideas from our BC protocol, we then move to our central con- tribution and present two protocols for PBC that are secure against adap- tive adversaries. To the best of our knowledge we are the first to study PBC specifically: All previous approaches for parallel BC (PBC) naively run n instances of single-sender Broadcast, increasing the communication complexity by an undesirable factor of n. Our insight of avoiding black- box invocations of BC is particularly crucial for achieving our asymptotic improvements. In particular:
1. Our first PBC protocol achieves O(n^3κ^2) communication complexity and relies only on plain public key setup assumptions.
2. Our second PBC protocol uses trusted setup and achieves nearly optimal communication complexity O(n^2κ^4).
Both PBC protocols yield an almost linear improvement over the best known solutions involving n parallel invocations of the respective BC protocols such as those of Dolev and Strong (SIAM Journal on Comput- ing, 1983) and Chan et al. (Public Key Cryptography, 2020). Central to our PBC protocols is a new problem that we define and solve, that we call “Converge”. In Converge, parties must run an adaptively-secure and efficient protocol such that by the end of the protocol, the honest parties that remain possess a superset of the union of the inputs of the initial honest parties
Gossiping for Communication-Efficient Broadcast
No full textByzantine Broadcast is crucial for many cryptographic pro- tocols such as secret sharing, multiparty computation and blockchain consensus. In this paper we apply gossiping (propagating a message by sending to a few random parties who in turn do the same, until the mes- sage is delivered) and propose new communication-efficient protocols, under dishonest majority, for Single-Sender Broadcast (BC) and Parallel Broadcast (PBC), improving the state-of-the-art in several ways.
As our first warm-up result, we give a randomized protocol for BC which achieves O(n^2κ^2) communication complexity from plain public key setup assumptions. This is the first protocol with subcubic communication in this setting, but does so only against static adversaries.
Using some ideas from our BC protocol, we then move to our central con- tribution and present two protocols for PBC that are secure against adap- tive adversaries. To the best of our knowledge we are the first to study PBC specifically: All previous approaches for parallel BC (PBC) naively run n instances of single-sender Broadcast, increasing the communication complexity by an undesirable factor of n. Our insight of avoiding black- box invocations of BC is particularly crucial for achieving our asymptotic improvements. In particular:
1. Our first PBC protocol achieves O(n^3κ^2) communication complexity and relies only on plain public key setup assumptions.
2. Our second PBC protocol uses trusted setup and achieves nearly optimal communication complexity O(n^2κ^4).
Both PBC protocols yield an almost linear improvement over the best known solutions involving n parallel invocations of the respective BC protocols such as those of Dolev and Strong (SIAM Journal on Comput- ing, 1983) and Chan et al. (Public Key Cryptography, 2020). Central to our PBC protocols is a new problem that we define and solve, that we call “Converge”. In Converge, parties must run an adaptively-secure and efficient protocol such that by the end of the protocol, the honest parties that remain possess a superset of the union of the inputs of the initial honest parties
Risk assessment of post‐myocardial infarction patients with preserved ejection fraction using 45‐min short resting Holter electrocardiographic recordings
No full textAbstract Background Risk stratification for sudden cardiac death in post‐myocardial infarction (post‐MI) patients remains a challenging task. Several electrocardiographic noninvasive risk factors (NIRFs) have been associated with adverse outcomes and were used to refine risk assessment. This study aimed to evaluate the performance of NIRFs extracted from 45‐min short resting Holter ECG recordings (SHR), in predicting ventricular tachycardia inducibility with programmed ventricular stimulation (PVS) in post‐MI patients with preserved left ventricular ejection fraction (LVEF). Methods We studied 99 post‐MI ischemia‐free patients (mean age: 60.5 ± 9.5 years, 86.9% men) with LVEF ≥40%, at least 40 days after revascularization. All the patients underwent PVS and a high‐resolution SHR. The following parameters were evaluated: mean heart rate, ventricular arrhythmias (premature ventricular complexes, couplets, tachycardias), QTc duration, heart rate variability (HRV), deceleration capacity, heart rate turbulence, late potentials, and T‐wave alternans. Results PVS was positive in 24 patients (24.2%). HRV, assessed by the standard deviation of normal‐to‐normal R–R intervals (SDNN), was significantly decreased in the positive PVS group (42 ms vs. 51 ms, p = .039). SDNN values <50 ms were also associated with PVS inducibility (OR 3.081, p = .032 in univariate analysis, and 4.588, p = .013 in multivariate analysis). No significant differences were identified for the other NIRFs. The presence of diabetes, history of ST‐elevation MI (STEMI) and LVEF <50% were also important predictors of positive PVS. Conclusions HRV assessed from SHR, combined with other noninvasive clinical and echocardiographic variables (diabetes, STEMI history, LVEF), can provide an initial, practical, and rapid screening tool for arrhythmic risk assessment in post‐MI patients with preserved LVEF
Risk assessment of post-myocardial infarction patients with preserved ejection fraction using 45-min short resting Holter electrocardiographic recordings
No full textBackground: Risk stratification for sudden cardiac death in post-myocardial infarction (post-MI) patients remains a challenging task. Several electrocardiographic noninvasive risk factors (NIRFs) have been associated with adverse outcomes and were used to refine risk assessment. This study aimed to evaluate the performance of NIRFs extracted from 45-min short resting Holter ECG recordings (SHR), in predicting ventricular tachycardia inducibility with programmed ventricular stimulation (PVS) in post-MI patients with preserved left ventricular ejection fraction (LVEF). Methods: We studied 99 post-MI ischemia-free patients (mean age: 60.5 ± 9.5 years, 86.9% men) with LVEF ≥40%, at least 40 days after revascularization. All the patients underwent PVS and a high-resolution SHR. The following parameters were evaluated: mean heart rate, ventricular arrhythmias (premature ventricular complexes, couplets, tachycardias), QTc duration, heart rate variability (HRV), deceleration capacity, heart rate turbulence, late potentials, and T-wave alternans. Results: PVS was positive in 24 patients (24.2%). HRV, assessed by the standard deviation of normal-to-normal R–R intervals (SDNN), was significantly decreased in the positive PVS group (42 ms vs. 51 ms, p =.039). SDNN values <50 ms were also associated with PVS inducibility (OR 3.081, p =.032 in univariate analysis, and 4.588, p =.013 in multivariate analysis). No significant differences were identified for the other NIRFs. The presence of diabetes, history of ST-elevation MI (STEMI) and LVEF <50% were also important predictors of positive PVS. Conclusions: HRV assessed from SHR, combined with other noninvasive clinical and echocardiographic variables (diabetes, STEMI history, LVEF), can provide an initial, practical, and rapid screening tool for arrhythmic risk assessment in post-MI patients with preserved LVEF. © 2023 The Authors. Annals of Noninvasive Electrocardiology published by Wiley Periodicals LLC
Arrhythmic risk stratification in post-myocardial infarction patients with preserved ejection fraction: The PRESERVE EF study
No full textAims: Sudden cardiac death (SCD) annual incidence is 0.6-1% in post-myocardial infarction (MI) patients with left ventricular ejection fraction (LVEF)≥40%. No recommendations for implantable cardioverter-defibrillator (ICD) use exist in this population. Methods and results: We introduced a combined non-invasive/invasive risk stratification approach in post-MI ischaemia-free patients, with LVEF ≥ 40%, in a multicentre, prospective, observational cohort study. Patients with at least one positive electrocardiographic non-invasive risk factor (NIRF): premature ventricular complexes, non-sustained ventricular tachycardia, late potentials, prolonged QTc, increased T-wave alternans, reduced heart rate variability, abnormal deceleration capacity with abnormal turbulence, were referred for programmed ventricular stimulation (PVS), with ICDs offered to those inducible. The primary endpoint was the occurrence of a major arrhythmic event (MAE), namely sustained ventricular tachycardia/fibrillation, appropriate ICD activation or SCD. We screened and included 575 consecutive patients (mean age 57 years, LVEF 50.8%). Of them, 204 (35.5%) had at least one positive NIRF. Forty-one of 152 patients undergoing PVS (27-7.1% of total sample) were inducible. Thirty-seven (90.2%) of them received an ICD. Mean follow-up was 32 months and no SCDs were observed, while 9 ICDs (1.57% of total screened population) were appropriately activated. None patient without NIRFs or with NIRFs but negative PVS met the primary endpoint. The algorithm yielded the following: sensitivity 100%, specificity 93.8%, positive predictive value 22%, and negative predictive value 100%. Conclusion: The two-step approach of the PRESERVE EF study detects a subpopulation of post-MI patients with preserved LVEF at risk for MAEs that can be effectively addressed with an ICD. Clinicaltrials.gov identifier: NCT02124018 © 2019 The Author(s)
