102,134 research outputs found
On the Round Complexity of Randomized Byzantine Agreement
We prove lower bounds on the round complexity of randomized Byzantine agreement (BA) protocols, bounding the halting probability of such protocols after one and two rounds. In particular, we prove that:
1) BA protocols resilient against n/3 [resp., n/4] corruptions terminate (under attack) at the end of the first round with probability at most o(1) [resp., 1/2+ o(1)].
2) BA protocols resilient against n/4 corruptions terminate at the end of the second round with probability at most 1-Theta(1).
3) For a large class of protocols (including all BA protocols used in practice) and under a plausible combinatorial conjecture, BA protocols resilient against n/3 [resp., n/4] corruptions terminate at the end of the second round with probability at most o(1) [resp., 1/2 + o(1)].
The above bounds hold even when the parties use a trusted setup phase, e.g., a public-key infrastructure (PKI).
The third bound essentially matches the recent protocol of Micali (ITCS\u2717) that tolerates up to n/3 corruptions and terminates at the end of the third round with constant probability
Characterization of phase-averaged coherent states
We present the full characterization of phase-randomized or phase-averaged
coherent states, a class of states exploited in communication channels and in
decoy state-based quantum key distribution protocols. In particular, we report
on the suitable formalism to analytically describe the main features of this
class of states and on their experimental investigation, that results in
agreement with theory. We also show the results we obtained by manipulating the
phase-averaged coherent states with linear optical elements and testify their
good quality by employing some non-Gaussianity measures and the concept of
mutual information.Comment: 15 pages, 11 figure
Randomized protocols for asynchronous consensus
The famous Fischer, Lynch, and Paterson impossibility proof shows that it is
impossible to solve the consensus problem in a natural model of an asynchronous
distributed system if even a single process can fail. Since its publication,
two decades of work on fault-tolerant asynchronous consensus algorithms have
evaded this impossibility result by using extended models that provide (a)
randomization, (b) additional timing assumptions, (c) failure detectors, or (d)
stronger synchronization mechanisms than are available in the basic model.
Concentrating on the first of these approaches, we illustrate the history and
structure of randomized asynchronous consensus protocols by giving detailed
descriptions of several such protocols.Comment: 29 pages; survey paper written for PODC 20th anniversary issue of
Distributed Computin
Agreements between Industry and Academia on Publication Rights: A Retrospective Study of Protocols and Publications of Randomized Clinical Trials.
BACKGROUND: Little is known about publication agreements between industry and academic investigators in trial protocols and the consistency of these agreements with corresponding statements in publications. We aimed to investigate (i) the existence and types of publication agreements in trial protocols, (ii) the completeness and consistency of the reporting of these agreements in subsequent publications, and (iii) the frequency of co-authorship by industry employees.
METHODS AND FINDINGS: We used a retrospective cohort of randomized clinical trials (RCTs) based on archived protocols approved by six research ethics committees between 13 January 2000 and 25 November 2003. Only RCTs with industry involvement were eligible. We investigated the documentation of publication agreements in RCT protocols and statements in corresponding journal publications. Of 647 eligible RCT protocols, 456 (70.5%) mentioned an agreement regarding publication of results. Of these 456, 393 (86.2%) documented an industry partner's right to disapprove or at least review proposed manuscripts; 39 (8.6%) agreements were without constraints of publication. The remaining 24 (5.3%) protocols referred to separate agreement documents not accessible to us. Of those 432 protocols with an accessible publication agreement, 268 (62.0%) trials were published. Most agreements documented in the protocol were not reported in the subsequent publication (197/268 [73.5%]). Of 71 agreements reported in publications, 52 (73.2%) were concordant with those documented in the protocol. In 14 of 37 (37.8%) publications in which statements suggested unrestricted publication rights, at least one co-author was an industry employee. In 25 protocol-publication pairs, author statements in publications suggested no constraints, but 18 corresponding protocols documented restricting agreements.
CONCLUSIONS: Publication agreements constraining academic authors' independence are common. Journal articles seldom report on publication agreements, and, if they do, statements can be discrepant with the trial protocol
The Contest Between Simplicity and Efficiency in Asynchronous Byzantine Agreement
In the wake of the decisive impossibility result of Fischer, Lynch, and
Paterson for deterministic consensus protocols in the aynchronous model with
just one failure, Ben-Or and Bracha demonstrated that the problem could be
solved with randomness, even for Byzantine failures. Both protocols are natural
and intuitive to verify, and Bracha's achieves optimal resilience. However, the
expected running time of these protocols is exponential in general. Recently,
Kapron, Kempe, King, Saia, and Sanwalani presented the first efficient
Byzantine agreement algorithm in the asynchronous, full information model,
running in polylogarithmic time. Their algorithm is Monte Carlo and drastically
departs from the simple structure of Ben-Or and Bracha's Las Vegas algorithms.
In this paper, we begin an investigation of the question: to what extent is
this departure necessary? Might there be a much simpler and intuitive Las Vegas
protocol that runs in expected polynomial time? We will show that the
exponential running time of Ben-Or and Bracha's algorithms is no mere accident
of their specific details, but rather an unavoidable consequence of their
general symmetry and round structure. We define a natural class of "fully
symmetric round protocols" for solving Byzantine agreement in an asynchronous
setting and show that any such protocol can be forced to run in expected
exponential time by an adversary in the full information model. We assume the
adversary controls Byzantine processors for , where is an
arbitrary positive constant . We view our result as a step toward
identifying the level of complexity required for a polynomial-time algorithm in
this setting, and also as a guide in the search for new efficient algorithms.Comment: 21 page
Consensus with Max Registers
We consider the problem of implementing randomized wait-free consensus from max registers under the assumption of an oblivious adversary. We show that max registers solve m-valued consensus for arbitrary m in expected O(log^* n) steps per process, beating the Omega(log m/log log m) lower bound for ordinary registers when m is large and the best previously known O(log log n) upper bound when m is small. A simple max-register implementation based on double-collect snapshots translates this result into an O(n log n) expected step implementation of m-valued consensus from n single-writer registers, improving on the best previously-known bound of O(n log^2 n) for single-writer registers
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