307 research outputs found
Instantaneous Decentralized Poker
We present efficient protocols for amortized secure multiparty computation
with penalties and secure cash distribution, of which poker is a prime example.
Our protocols have an initial phase where the parties interact with a
cryptocurrency network, that then enables them to interact only among
themselves over the course of playing many poker games in which money changes
hands.
The high efficiency of our protocols is achieved by harnessing the power of
stateful contracts. Compared to the limited expressive power of Bitcoin
scripts, stateful contracts enable richer forms of interaction between standard
secure computation and a cryptocurrency.
We formalize the stateful contract model and the security notions that our
protocols accomplish, and provide proofs using the simulation paradigm.
Moreover, we provide a reference implementation in Ethereum/Solidity for the
stateful contracts that our protocols are based on.
We also adopt our off-chain cash distribution protocols to the special case
of stateful duplex micropayment channels, which are of independent interest. In
comparison to Bitcoin based payment channels, our duplex channel implementation
is more efficient and has additional features
Bitcoin Transaction Malleability and MtGox
In Bitcoin, transaction malleability describes the fact that the signatures
that prove the ownership of bitcoins being transferred in a transaction do not
provide any integrity guarantee for the signatures themselves. This allows an
attacker to mount a malleability attack in which it intercepts, modifies, and
rebroadcasts a transaction, causing the transaction issuer to believe that the
original transaction was not confirmed. In February 2014 MtGox, once the
largest Bitcoin exchange, closed and filed for bankruptcy claiming that
attackers used malleability attacks to drain its accounts. In this work we use
traces of the Bitcoin network for over a year preceding the filing to show
that, while the problem is real, there was no widespread use of malleability
attacks before the closure of MtGox
Improvements to Secure Computation with Penalties
Motivated by the impossibility of achieving fairness in secure computation
[Cleve, STOC 1986], recent works study a model of fairness in which an adversarial party that aborts on receiving output is forced to pay a mutually predefined monetary penalty to every other party that did not receive the output. These works show how to design protocols for secure computation with penalties that tolerate an arbitrary number of corruptions. In this work, we improve the efficiency of protocols for secure computation with penalties in a hybrid model where parties have access to the âclaim-or-refundâ transaction functionality. Our first improvement is for the ladder protocol of Bentov and Kumaresan (Crypto 2014) where we improve the dependence of the script complexity of the protocol (which corresponds to miner verification
load and also space on the blockchain) on the number of parties from quadratic to linear (and in particular, is completely independent of the underlying function). Our second improvement is for the see-saw protocol of Kumaresan et al. (CCS 2015) where we reduce the total number of claim-or-refund transactions and also the script complexity from quadratic to linear in the number of parties. We also present a âdual-modeâ protocol that offers different guarantees depending on the number of corrupt parties: (1) when s n/2 parties are corrupt, this protocol guarantees fairness with penalties (i.e., if the adversary gets the output, then either the honest parties get output as well or they get compensation via penalizing the adversary). The above protocol works as long as t+s < n, matching the bound obtained for secure computation protocols in the standard model (i.e., replacing âfairness with penaltiesâ with âsecuritywith-abortâ (full security except fairness)) by Ishai et al. (SICOMP
2011). Keywords: Bitcoin, secure computation, fairness.National Science Foundation (U.S.) (Grant CNS-1350619)National Science Foundation (U.S.) (Grant CNS1414119)Alfred P. Sloan Foundation (Research Fellowship)Microsoft (Faculty Fellowship
TKSE: Trustworthy keyword search over encrypted data with two-side verifiability via blockchain
AXA Research Fund Singapor
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