23 research outputs found
Rational Behavior in Committee-Based Blockchains
We study the rational behaviors of participants in committee-based blockchains. Committee-based blockchains rely on specific blockchain consensus that must be guaranteed in presence of rational participants. We consider a simplified blockchain consensus algorithm based on existing or proposed committee-based blockchains that encapsulates the main actions of the participants: voting for a block, and checking its validity. Knowing that those actions have costs, and achieving the consensus gives rewards to committee members, we study using game theory how strategic players behave while trying to maximizing their gains. We consider different reward schemes, and found that in each setting, there exist equilibria where blockchain consensus is guaranteed; in some settings however, there can be coordination failures hindering consensus. Moreover, we study equilibria with trembling participants, which is a novelty in the context of committee-based blockchains. Trembling participants are rational that can do unintended actions with a low probability. We found that in presence of trembling participants, there exist equilibria where blockchain consensus is guaranteed; however, when only voters are rewarded, there also exist equilibria where validity can be violated
Liveness Checking of the HotStuff Protocol Family
Byzantine consensus protocols aim at maintaining safety guarantees under any
network synchrony model and at providing liveness in partially or fully
synchronous networks. However, several Byzantine consensus protocols have been
shown to violate liveness properties under certain scenarios. Existing testing
methods for checking the liveness of consensus protocols check for time-bounded
liveness violations, which generate a large number of false positives. In this
work, for the first time, we check the liveness of Byzantine consensus
protocols using the temperature and lasso detection methods, which require the
definition of ad-hoc system state abstractions. We focus on the HotStuff
protocol family that has been recently developed for blockchain consensus. In
this family, the HotStuff protocol is both safe and live under the partial
synchrony assumption, while the 2-Phase Hotstuff and Sync HotStuff protocols
are known to violate liveness in subtle fault scenarios. We implemented our
liveness checking methods on top of the Twins automated unit test generator to
test the HotStuff protocol family. Our results indicate that our methods
successfully detect all known liveness violations and produce fewer false
positives than the traditional time-bounded liveness checks.Comment: Preprint of a paper accepted at IEEE PRDC 202
From Symmetric to Asymmetric Asynchronous Byzantine Consensus
Consensus is arguably one of the most important notions in distributed
computing. Among asynchronous, randomized, and signature-free implementations,
the protocols of Most\'efaoui et al. (PODC 2014 and JACM 2015) represent a
landmark result, which has been extended later and taken up in practical
systems. The protocols achieve optimal resilience and takes, in expectation,
only a constant expected number of rounds of quadratic message complexity.
Randomization is provided through a common-coin primitive. In traditional
consensus protocols, all involved processes adhere to a global, symmetric
failure model, typically only defined by bounds on the number of faulty
processes. Motivated by applications to blockchains, however, more flexible
trust assumptions have recently been considered. In particular, with asymmetric
trust, a process is free to choose which other processes it trusts and which
ones might collude against it. This paper revisits the optimal asynchronous
protocol of Most\'efaoui et al. and shows how to realize it with asymmetric
trust. The paper starts by pointing out in detail why some versions of this
protocol may violate liveness. Then it proposes a fix for the protocol that
does not affect its properties, but lets it regain the simplicity of its
original version (PODC 2014). At the same time, the paper shows how to realize
randomized signature-free asynchronous Byzantine consensus with asymmetric
quorums. This results in an optimal consensus protocol with subjective,
asymmetric trust and constant expected running time. It is suitable for
applications to blockchains, for instance
Remove-Win: a Design Framework for Conflict-free Replicated Data Collections
Internet-scale distributed systems often replicate data within and across
data centers to provide low latency and high availability despite node and
network failures. Replicas are required to accept updates without coordination
with each other, and the updates are then propagated asynchronously. This
brings the issue of conflict resolution among concurrent updates, which is
often challenging and error-prone. The Conflict-free Replicated Data Type
(CRDT) framework provides a principled approach to address this challenge.
This work focuses on a special type of CRDT, namely the Conflict-free
Replicated Data Collection (CRDC), e.g. list and queue. The CRDC can have
complex and compound data items, which are organized in structures of rich
semantics. Complex CRDCs can greatly ease the development of upper-layer
applications, but also makes the conflict resolution notoriously difficult.
This explains why existing CRDC designs are tricky, and hard to be generalized
to other data types. A design framework is in great need to guide the
systematic design of new CRDCs.
To address the challenges above, we propose the Remove-Win Design Framework.
The remove-win strategy for conflict resolution is simple but powerful. The
remove operation just wipes out the data item, no matter how complex the value
is. The user of the CRDC only needs to specify conflict resolution for
non-remove operations. This resolution is destructed to three basic cases and
are left as open terms in the CRDC design skeleton. Stubs containing
user-specified conflict resolution logics are plugged into the skeleton to
obtain concrete CRDC designs. We demonstrate the effectiveness of our design
framework via a case study of designing a conflict-free replicated priority
queue. Performance measurements also show the efficiency of the design derived
from our design framework.Comment: revised after submissio
The Complexity of Symmetry Breaking in Massive Graphs
The goal of this paper is to understand the complexity of symmetry breaking problems, specifically maximal independent set (MIS) and the closely related beta-ruling set problem, in two computational models suited for large-scale graph processing, namely the k-machine model and the graph streaming model. We present a number of results. For MIS in the k-machine model, we improve the O~(m/k^2 + Delta/k)-round upper bound of Klauck et al. (SODA 2015) by presenting an O~(m/k^2)-round algorithm. We also present an Omega~(n/k^2) round lower bound for MIS, the first lower bound for a symmetry breaking problem in the k-machine model. For beta-ruling sets, we use hierarchical sampling to obtain more efficient algorithms in the k-machine model and also in the graph streaming model. More specifically, we obtain a k-machine algorithm that runs in O~(beta n Delta^{1/beta}/k^2) rounds and, by using a similar hierarchical sampling technique, we obtain one-pass algorithms for both insertion-only and insertion-deletion streams that use O(beta * n^{1+1/2^{beta-1}}) space. The latter result establishes a clear separation between MIS, which is known to require Omega(n^2) space (Cormode et al., ICALP 2019), and beta-ruling sets, even for beta = 2. Finally, we present an even faster 2-ruling set algorithm in the k-machine model, one that runs in O~(n/k^{2-epsilon} + k^{1-epsilon}) rounds for any epsilon, 0 <=epsilon <=1. For a wide range of values of k this round complexity simplifies to O~(n/k^2) rounds, which we conjecture is optimal.
Our results use a variety of techniques. For our upper bounds, we prove and use simulation theorems for beeping algorithms, hierarchical sampling, and L_0-sampling, whereas for our lower bounds we use information-theoretic arguments and reductions to 2-party communication complexity problems
Composable Computation in Leaderless, Discrete Chemical Reaction Networks
We classify the functions f:?^d ? ? that are stably computable by leaderless, output-oblivious discrete (stochastic) Chemical Reaction Networks (CRNs). CRNs that compute such functions are systems of reactions over species that include d designated input species, whose initial counts represent an input x ? ?^d, and one output species whose eventual count represents f(x). Chen et al. showed that the class of functions computable by CRNs is precisely the semilinear functions. In output-oblivious CRNs, the output species is never a reactant. Output-oblivious CRNs are easily composable since a downstream CRN can consume the output of an upstream CRN without affecting its correctness. Severson et al. showed that output-oblivious CRNs compute exactly the subclass of semilinear functions that are eventually the minimum of quilt-affine functions, i.e., affine functions with different intercepts in each of finitely many congruence classes. They call such functions the output-oblivious functions. A leaderless CRN can compute only superadditive functions, and so a leaderless output-oblivious CRN can compute only superadditive, output-oblivious functions. In this work we show that a function f:?^d ? ? is stably computable by a leaderless, output-oblivious CRN if and only if it is superadditive and output-oblivious
Asynchronous Byzantine Approximate Consensus in Directed Networks
In this work, we study the approximate consensus problem in asynchronous
message-passing networks where some nodes may become Byzantine faulty. We
answer an open problem raised by Tseng and Vaidya, 2012, proposing the first
algorithm of optimal resilience for directed networks. Interestingly, our
results show that the tight condition on the underlying communication networks
for asynchronous Byzantine approximate consensus coincides with the tight
condition for synchronous Byzantine exact consensus. Our results can be viewed
as a non-trivial generalization of the algorithm by Abraham et al., 2004, which
applies to the special case of complete networks. The tight condition and
techniques identified in the paper shed light on the fundamental properties for
solving approximate consensus in asynchronous directed networks.Comment: 25 pages, 2 figure
SoK: A Consensus Taxonomy in the Blockchain Era
Consensus (a.k.a. Byzantine agreement) is arguably one of the most fundamental problems in distributed systems, playing also an important role in the area of cryptographic protocols as the enabler of a (secure) broadcast functionality. While the problem has a long and rich history and has been analyzed from many different perspectives, recently, with the advent of blockchain protocols like Bitcoin, it has experienced renewed interest from a much wider community of researchers and has seen its application expand to various novel settings.
One of the main issues in consensus research is the many different variants of the problem that exist as well as the various ways the problem behaves when different setup, computational assumptions and network models are considered. In this work we perform a systematization of knowledge in the landscape of consensus research starting with the original formulation in the early 1980s up to the present
blockchain-based new class of consensus protocols. Our work is a roadmap for studying the consensus problem under its many guises, classifying the way it operates in many settings and highlighting the exciting new applications that have emerged in the blockchain era
TenderTee: Secure Tendermint
Blockchain and distributed ledger technologies have emerged as one of the most revolutionary distributed systems, with the goal of eliminating centralised intermediaries and installing distributed trusted services. They facilitate trustworthy trades and exchanges over the Internet, power cryptocurrencies, ensure transparency for documents, and much more.
Committee based-blockchains are considered today as a viable alternative to the original proof-of-work paradigm, since they offer strong consistency and are energy efficient. One of the most popular committee based-blockchain is Tendermint used as core by several popular blockchains such Tezos, Binance Smart Chain or Cosmos. Interestingly, Tendermint as many other committee based-blockchains is designed to tolerate one third of Byzantine nodes.
In this paper we propose TenderTee, an enhanced version of Tendermint, able to tolerate one half of Byzantine nodes. The resilience improvement is due to the use of a trusted abstraction, a light version of attested append-only memory, which makes the protocol immune to equivocation (i.e behavior of a faulty node when it sends different faulty messages to different nodes). Furthermore, we prove the correctness of TenderTee for both one-shot and repeated consensus specifications