2,360 research outputs found
New Efficient Error-Free Multi-Valued Consensus with Byzantine Failures
In this report, we investigate the multi-valued Byzantine consensus problem.
We introduce two algorithms: the first one achieves traditional validity
requirement for consensus, and the second one achieves a stronger "q-validity"
requirement. Both algorithms are more efficient than the ones introduces in our
recent PODC 2011 paper titled "Error-Free Multi-Valued Consensus with Byzantine
Failures"
Complexity of Multi-Value Byzantine Agreement
In this paper, we consider the problem of maximizing the throughput of
Byzantine agreement, given that the sum capacity of all links in between nodes
in the system is finite. We have proposed a highly efficient Byzantine
agreement algorithm on values of length l>1 bits. This algorithm uses error
detecting network codes to ensure that fault-free nodes will never disagree,
and routing scheme that is adaptive to the result of error detection. Our
algorithm has a bit complexity of n(n-1)l/(n-t), which leads to a linear cost
(O(n)) per bit agreed upon, and overcomes the quadratic lower bound
(Omega(n^2)) in the literature. Such linear per bit complexity has only been
achieved in the literature by allowing a positive probability of error. Our
algorithm achieves the linear per bit complexity while guaranteeing agreement
is achieved correctly even in the worst case. We also conjecture that our
algorithm can be used to achieve agreement throughput arbitrarily close to the
agreement capacity of a network, when the sum capacity is given
FairLedger: A Fair Blockchain Protocol for Financial Institutions
Financial institutions are currently looking into technologies for
permissioned blockchains. A major effort in this direction is Hyperledger, an
open source project hosted by the Linux Foundation and backed by a consortium
of over a hundred companies. A key component in permissioned blockchain
protocols is a byzantine fault tolerant (BFT) consensus engine that orders
transactions. However, currently available BFT solutions in Hyperledger (as
well as in the literature at large) are inadequate for financial settings; they
are not designed to ensure fairness or to tolerate selfish behavior that arises
when financial institutions strive to maximize their own profit.
We present FairLedger, a permissioned blockchain BFT protocol, which is fair,
designed to deal with rational behavior, and, no less important, easy to
understand and implement. The secret sauce of our protocol is a new
communication abstraction, called detectable all-to-all (DA2A), which allows us
to detect participants (byzantine or rational) that deviate from the protocol,
and punish them. We implement FairLedger in the Hyperledger open source
project, using Iroha framework, one of the biggest projects therein. To
evaluate FairLegder's performance, we also implement it in the PBFT framework
and compare the two protocols. Our results show that in failure-free scenarios
FairLedger achieves better throughput than both Iroha's implementation and PBFT
in wide-area settings
Optimal Error-Free Multi-Valued Byzantine Agreement
Byzantine agreement (BA) is a distributed consensus problem where n processors want to reach agreement on an ?-bit message or value, but up to t of the processors are dishonest or faulty. The challenge of this BA problem lies in achieving agreement despite the presence of dishonest processors who may arbitrarily deviate from the designed protocol. In this work by using coding theory, together with graph theory and linear algebra, we design a coded BA protocol (termed as COOL) that achieves consensus on an ?-bit message with optimal resilience, asymptotically optimal round complexity, and asymptotically optimal communication complexity when ? ? t log t, simultaneously. The proposed COOL is a deterministic BA protocol that is guaranteed to be correct in all executions (error free) and does not rely on cryptographic technique such as signatures, hashing, authentication and secret sharing (signature free). It is secure against computationally unbounded adversary who takes full control over the dishonest processors (information-theoretic secure). The main idea of the proposed COOL is to use a carefully-crafted error correction code that provides an efficient way of exchanging "compressed" information among distributed nodes, while keeping the ability of detecting errors, masking errors, and making a consistent and validated agreement at honest distributed nodes. We show that our results can also be extended to the setting of Byzantine broadcast, aka Byzantine generals problem, where the honest processors want to agree on the message sent by a leader who is potentially dishonest. The results reveal that coding is an effective approach for achieving the fundamental limits of Byzantine agreement and its variants. Our protocol analysis borrows tools from coding theory, graph theory and linear algebra
BRIDGE: Byzantine-resilient Decentralized Gradient Descent
Decentralized optimization techniques are increasingly being used to learn
machine learning models from data distributed over multiple locations without
gathering the data at any one location. Unfortunately, methods that are
designed for faultless networks typically fail in the presence of node
failures. In particular, Byzantine failures---corresponding to the scenario in
which faulty/compromised nodes are allowed to arbitrarily deviate from an
agreed-upon protocol---are the hardest to safeguard against in decentralized
settings. This paper introduces a Byzantine-resilient decentralized gradient
descent (BRIDGE) method for decentralized learning that, when compared to
existing works, is more efficient and scalable in higher-dimensional settings
and that is deployable in networks having topologies that go beyond the star
topology. The main contributions of this work include theoretical analysis of
BRIDGE for strongly convex learning objectives and numerical experiments
demonstrating the efficacy of BRIDGE for both convex and nonconvex learning
tasks.Comment: 18 pages, 1 figure, 1 table; preprint of a conference pape
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