Offline e-cash system

The e-cash scheme and the digital content transactions are the need of the hour. In the coming years, all these digital transactions will grow tremendously. So, a secure e-cash scheme is of utmost requirement. e-cash scheme, which is untraceable and maintains the security features, make it possible for the customers and the merchants to exchange the e-cash and the merchandise with privacy. So, there is a need to design an e-cash scheme with strong cryptosystem and algorithms in order to facilitate efficient digital transactions. There are two types of e-cash systems: Offine e-cash systems and online e-cash systems. Offine e-cash systems make it possible for the customer to pay the e-coin to the merchant without any involvement of bank. In online schemes, we require the involvement of the bank. The two most fundamental security features associated with offine scheme is the anonymity and the double spending detection. The proposed scheme maintains both the above features along with unforgeability. Besides, the E-coins have their expiration date so that the bank faces no hassles and can manage its database efficiently. This feature also ensures portability as the coins can be transferred to storage devices through the networks

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

ROYALE: A Framework for Universally Composable Card Games with Financial Rewards and Penalties Enforcement

While many tailor made card game protocols are known, the vast majority of those suffer from three main issues: lack of mechanisms for distributing financial rewards and punishing cheaters, lack of composability guarantees and little flexibility, focusing on the specific game of poker. Even though folklore holds that poker protocols can be used to play any card game, this conjecture remains unproven and, in fact, does not hold for a number of protocols (including recent results). We both tackle the problem of constructing protocols for general card games and initiate a treatment of such protocols in the Universal Composability (UC) framework, introducing an ideal functionality that captures general card games constructed from a set of core card operations. Based on this formalism, we introduce Royale, the first UC-secure general card games which supports financial rewards/penalties enforcement. We remark that Royale also yields the first UC-secure poker protocol. Interestingly, Royale performs better than most previous works (that do not have composability guarantees), which we highlight through a detailed concrete complexity analysis and benchmarks from a prototype implementation

A CCA2 Secure Variant of the McEliece Cryptosystem

The McEliece public-key encryption scheme has become an interesting alternative to cryptosystems based on number-theoretical problems. Differently from RSA and ElGa- mal, McEliece PKC is not known to be broken by a quantum computer. Moreover, even tough McEliece PKC has a relatively big key size, encryption and decryption operations are rather efficient. In spite of all the recent results in coding theory based cryptosystems, to the date, there are no constructions secure against chosen ciphertext attacks in the standard model - the de facto security notion for public-key cryptosystems. In this work, we show the first construction of a McEliece based public-key cryptosystem secure against chosen ciphertext attacks in the standard model. Our construction is inspired by a recently proposed technique by Rosen and Segev

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

A Secure and Efficient Off-line Electronic Payment System forWireless Networks

An electronic cash system allows the exchange of digital coins with value assured by the bank’s signature and with concealed user identity. In an electronic cash system, a user can withdraw coins from the bank and then spends each coin anonymously and unlinkably. In this paper we propose a secure and efficient off-line electronic payment system based on bilinear pairings and group signature schemes. The anonymity of the customer is revocable by a trustee in case of a dispute. Because the amount of communication in the payment protocol is about 480 bits, the proposed off-line electronic payment system can be used in wireless networks with limited bandwidth