5 research outputs found
Maximizing Utility Among Selfish Users in Social Groups
We consider the problem of a social group of users trying to obtain a
"universe" of files, first from a server and then via exchange amongst
themselves. We consider the selfish file-exchange paradigm of give-and-take,
whereby two users can exchange files only if each has something unique to offer
the other. We are interested in maximizing the number of users who can obtain
the universe through a schedule of file-exchanges. We first present a practical
paradigm of file acquisition. We then present an algorithm which ensures that
at least half the users obtain the universe with high probability for files
and users when , thereby showing an
approximation ratio of 2. Extending these ideas, we show a -
approximation algorithm for , and a - approximation algorithm for , , .
Finally, we show that for any , there exists a schedule of file
exchanges which ensures that at least half the users obtain the universe.Comment: 11 pages, 3 figures; submitted for review to the National Conference
on Communications (NCC) 201
The Online Disjoint Set Cover Problem and its Applications
Given a universe of elements and a collection of subsets
of , the maximum disjoint set cover problem (DSCP) is to
partition into as many set covers as possible, where a set cover
is defined as a collection of subsets whose union is . We consider the
online DSCP, in which the subsets arrive one by one (possibly in an order
chosen by an adversary), and must be irrevocably assigned to some partition on
arrival with the objective of minimizing the competitive ratio. The competitive
ratio of an online DSCP algorithm is defined as the maximum ratio of the
number of disjoint set covers obtained by the optimal offline algorithm to the
number of disjoint set covers obtained by across all inputs. We propose an
online algorithm for solving the DSCP with competitive ratio . We then
show a lower bound of on the competitive ratio for any
online DSCP algorithm. The online disjoint set cover problem has wide ranging
applications in practice, including the online crowd-sourcing problem, the
online coverage lifetime maximization problem in wireless sensor networks, and
in online resource allocation problems.Comment: To appear in IEEE INFOCOM 201
Securing Smart Contract On The Fly
We present Solythesis, a source to source Solidity compiler which takes a
smart contract code and a user specified invariant as the input and produces an
instrumented contract that rejects all transactions that violate the invariant.
The design of Solythesis is driven by our observation that the consensus
protocol and the storage layer are the primary and the secondary performance
bottlenecks of Ethereum, respectively. Solythesis operates with our novel delta
update and delta check techniques to minimize the overhead caused by the
instrumented storage access statements. Our experimental results validate our
hypothesis that the overhead of runtime validation, which is often too
expensive for other domains, is in fact negligible for smart contracts. The CPU
overhead of Solythesis is only 0.12% on average for our 23 benchmark contracts
Verifiable Timed Signatures Made Practical
A verifiable timed signature (VTS) scheme allows one to time-lock a signature on a known message for a given amount of time T such that after performing a sequential computation for time T anyone can extract s from the time-lock. Verifiability ensures that anyone can publicly check if a time-lock contains a valid signature on m without solving it first, and that the signature can be obtained by solving the same for time T.
This work formalizes VTS, presents efficient constructions compatible with BLS, Schnorr, and ECDSA signatures, and experimentally demonstrates that (unlike the predecessors) our constructions can be employed in practice. On a technical level, we design an efficient cut-and-choose protocol based on the recently proposed homomorphic time-lock puzzles to prove the validity of a signature encapsulated in a time-lock puzzle. We also present a new efficient range proof protocol that significantly improves upon existing proposals in terms of the proof size, and is of independent interest.
VTS is a versatile tool with numerous existing applications. In this work, we demonstrate VTS’s applicability to resolve three challenging issues in the space of cryptocurrencies. Specifically, we show how VTS is the cryptographic cornerstone to construct:
(i) Payment channel networks with improved on-chain unlinkability of users involved in a transaction,
(ii) multi-party signing of transactions
for cryptocurrencies without any on-chain notion of time and
(iii) cryptocurrency-enabled fair multi-party computation protocol