Simple and Efficient Single Round Almost Perfectly Secure Message Transmission Tolerating Generalized Adversary

Patra et al. gave a necessary and sufficient condition for the possibility of almost perfectly secure message transmission protocols tolerating general, non-threshold Q^2 adversary structure. However, their protocol requires at least three rounds and performs exponential (exponential in the size of the adversary structure) computation and communication. Moreover, they have left it as an open problem to design efficient protocol for almost perfectly secure message transmission, tolerating Q^2 adversary structure. In this paper, we show the first single round almost perfectly secure message transmission protocol tolerating Q^2 adversary structure. The computation and communication complexities of the protocol are both polynomial} in the size of underlying linear secret sharing scheme (LSSS) and adversary structure. This solves the open problem raised by Patra et al.. When we restrict our general protocol to threshold adversary with n=2t+1, we obtain a single round, communication optimal almost secure message transmission protocol tolerating threshold adversary, which is much more computationally efficient and relatively simpler than the previous communication optimal protocol of Srinathan et al

Unconditionally Reliable and Secure Message Transmission in Undirected Synchronous Networks: Possibility, Feasibility and Optimality

We study the interplay of network connectivity and the issues related to the ‘possibility’, ‘feasibility’ and ‘optimality’ for unconditionally reliable message transmission (URMT) and unconditionally secure message transmission (USMT) in an undirected synchronous network, under the influence of an adaptive mixed adversary having unbounded computing power, who can corrupt some of the nodes in the network in Byzantine, omission, fail-stop and passive fashion respectively. We consider two types of adversary, namely threshold and non-threshold. One of the important conclusions we arrive at from our study is that allowing a negligible error probability significantly helps in the ‘possibility’, ‘feasibility’ and ‘optimality’ of both reliable and secure message transmission protocols. To design our protocols, we propose several new techniques which are of independent interest

Unconditionally Reliable Message Transmission in Directed Neighbour Networks

The problem of unconditionally reliable message transmission (URMT) is to design a protocol which when run by players in a network enables a sender S to deliver a message to a receiver R with high probability, even when some players in the network are under the control of an unbounded adversary. Renault and Tomala [JoC2008] gave a characterization of undirected neighbour networks over which URMT tolerating Byzantine adversary is possible. In this paper, we generalize their result to the case of directed networks

Interplay between (Im)perfectness, Synchrony and Connectivity: The Case of Reliable Message Transmission

For unconditionally reliable message transmission (URMT) in synchronous directed networks of n nodes, a subset of which may be Byzantine faulty, it is well-known that the minimum connectivity requirements for zero-error (perfect) protocols to exist is strictly higher than those where a negligible yet non-zero error probability is allowed (Monte Carlo protocols). In this work, we study the minimum connectivity requirements for the existence of (a) synchronous Las Vegas protocols, (b) asynchronous Monte Carlo protocols, and (c) asynchronous Las Vegas protocols for URMT. Interestingly, we prove that in any network, synchronous Las Vegas URMT protocol exists if and only if asynchronous Monte Carlo URMT protocol exists too. We further show that asynchronous Las Vegas URMT protocols exist if and only if synchronous perfect protocols exist. We conclude with another interesting result: there exists networks where the number of critical edges for the ‘easier’ randomized variants are asymptotically higher than that for the perfect variant. Thus, our results establish an interesting interplay between (im)perfectness, synchrony and connectivity for the case of URMT

On The Communication Complexity of Perfectly Secure Message Transmission in Directed Networks

In this paper, we re-visit the problem of perfectly secure message transmission (PSMT) in a directed network under the presence of a threshold adaptive Byzantine adversary, having unbounded computing power. Desmedt et.al have given the characterization for three or more phase PSMT protocols over directed networks. Recently, Patra et. al. have given the characterization of two phase PSMT over directed networks. Even though the issue of tradeoff between phase complexity and communication complexity of PSMT protocols has been resolved in undirected networks, nothing is known in the literature regarding directed networks. In this paper, we completely settle down this issue. Specifically, we derive the lower bounds on communication complexity of (a) two phase PSMT protocols and (b) three or more phase PSMT protocols in directed networks. Moreover, we show that our lower bounds are asymptotically tight, by designing communication optimal PSMT protocols in directed networks, which are first of their kind. We re-visit the problem of perfectly reliable message transmission (PRMT) as well. Any PRMT protocol that sends a message containing $\ell$ field elements, has a trivial lower bound of ­O($\ell$) field elements on its communication complexity. Thus any PRMT protocol that sends a message of $\ell$ eld elements by communicating O(\ell) field elements, is referred as communication optimal PRMT or PRMT with constant factor overhead. Here, we characterize the class of directed networks over which communication optimal PRMT or PRMT with constant factor overhead is possible. Moreover, we design a communication optimal PRMT over a directed network that satisfies the conditions stated in our characterization. Our communication optimal PRMT/PSMT protocols employ several new techniques based on coding theory, which are of independent interest

Unconditionally Secure Message Transmission in Arbitrary Directed Synchronous Networks Tolerating Generalized Mixed Adversary

In this paper, we re-visit the problem of {\it unconditionally secure message transmission} (USMT) from a sender {\bf S} to a receiver {\bf R}, who are part of a distributed synchronous network, modeled as an {\it arbitrary} directed graph. Some of the intermediate nodes between {\bf S} and {\bf R} can be under the control of the adversary having {\it unbounded} computing power. Desmedt and Wang \cite{Desmedt} have given the characterization of USMT in directed networks. However, in their model, the underlying network is abstracted as directed node disjoint paths (also called as wires/channels) between {\bf S} and {\bf R}, where the intermediate nodes are oblivious, message passing nodes and perform no other computation. In this work, we first show that the characterization of USMT given by Desmedt et.al \cite{Desmedt} does not hold good for {\it arbitrary} directed networks, where the intermediate nodes perform some computation, beside acting as message forwarding nodes. We then give the {\it true} characterization of USMT in arbitrary directed networks. As far our knowledge is concerned, this is the first ever {\it true} characterization of USMT in arbitrary directed networks

Statistically Reliable and Secure Message Transmission in Directed Networks

Consider the following problem: a sender S and a receiver R are part of a directed synchronous network and connected through intermediate nodes. Specifically, there exists n node disjoint paths, also called as wires, which are directed from S to R and u wires, which are directed from R to S. Moreover, the wires from S to R are disjoint from the wires directed from R to S. There exists a centralized, static adversary who has unbounded computing power and who can control at most t wires between S and R in Byzantine fashion. S has a message m^S, which we wants to send to R. The challenge is to design a protocol, such that after interacting in phases as per the protocol, R should correctly output m^R = m^S, except with error probability 2^{-\Omega(\kappa)}, where \kappa is the error parameter. This problem is called as statistically reliable message transmission (SRMT). The problem of statistically secure message transmission (SSMT) has an additional requirement that at the end of the protocol, m^S should be information theoretically secure. Desmedt et.al have given the necessary and sufficient condition for the existence of SRMT and SSMT protocols in the above settings. They also presented an SSMT protocol, satisfying their characterization. Desmedt et.al claimed that their protocol is efficient and has polynomial computational and communication complexity. However, we show that it is not so. That is, we specify an adversary strategy, which may cause the protocol to have exponential computational and communication complexity. We then present new and efficient SRMT and SSMT protocols, satisfying the characterization of Desmedt et.al Finally we show that the our proposed protocols are communication optimal by deriving lower bound on the communication complexity of SRMT and SSMT protocols. As far our knowledge is concerned, our protocols are the first communication optimal SRMT and SSMT protocols in directed networks