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arXiv:1805.06227

On the Significance of Quiescent Protocols for Asynchronous Perfectly Secure Message Transmission

Anupriya Inumella

Published May 16, 2018• 0 citations
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Abstract

We consider the problem of perfect (information-theoretically) secure message transmission (PSMT) from a sender $S$ to a receiver $R$ in asynchronous directed networks tolerating dual adversary. The adversary can control at most $t_p$ nodes in a passive fashion and at most $t_f$ nodes in a fail-stop fashion. We achieve PSMT in this setting with a new class of message transmission protocols called the quiescent protocols. Quiescent protocols ensure that the players eventually stop sending messages but in contrast to the usual terminating protocols, these protocols do not require the players to go into the halt state, from which they cannot take any further action. To show the significance of quiescent protocols, we first identify conditions in which it is impossible for any terminating protocol to achieve PSMT. To circumvent this impossibility, we transform the existing synchronous PSMT protocol into a quiescent protocol which accomplishes PSMT. By focusing on designing quiescent protocols rather than terminating protocols, we show that asynchrony in the network does not demand any extra connectivity over its state-of-the-art synchronous counterpart. This shows that quiescent protocols are befitted to solve PSMT in asynchronous networks with fail-stop faults.

DOI
10.48550/arxiv.1805.06227

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We consider the problem of perfect (information-theoretically) secure message transmission (PSMT) from a sender SSS to a receiver RRR in asynchronous directed networks tolerating dual adversary. The adversary can control at most tpt_ptp​ nodes in a passive fashion and at most tft_ftf​ nodes in a fail-stop fashion. We achieve PSMT in this setting with a new class of message transmission protocols called the quiescent protocols. Quiescent protocols ensure that the players eventually stop sending messages but in contrast to the usual terminating protocols, these protocols do not require the players to go into the halt state, from which they cannot take any further action. To show the significance of quiescent protocols, we first identify conditions in which it is impossible for any terminating protocol to achieve PSMT. To circumvent this impossibility, we transform the existing synchronous PSMT protocol into a quiescent protocol which accomplishes PSMT. By focusing on designing quiescent protocols rather than terminating protocols, we show that asynchrony in the network does not demand any extra connectivity over its state-of-the-art synchronous counterpart. This shows that quiescent protocols are befitted to solve PSMT in asynchronous networks with fail-stop faults.
In a distributed network, we consider two special nodes called the sender S and the receiver R that are connected by n node-disjoint (except for S and R) bi-directional wires. Out of these n wires, the adversary can control at most t wires (of its choice) in Byzantine fashion. In this setting, our goal is to design a message transmission protocol Π that assures the following two conditions hold: (1) by the end of the protocol Π, R gets the correct message m transmitted by S without any error (perfect reliability), and (2) the adversary learns no information about m, whatsoever, in information theoretic sense (perfect secrecy). Protocols that satisfy these two conditions are known as the Perfectly Secure Message Transmission (PSMT) protocols.
We study the problem of information-theoretically secure message transmission (SMT) in asynchronous directed networks. In line with the literature, the distrust and failures of the network is captured via a computationally unbounded Byzantine adversary that may corrupt some subset of nodes. We give a characterization of networks over which SMT from sender S to receiver R is possible in both the well-known settings, namely perfect SMT (PSMT) and unconditional SMT (USMT). We distinguish between two variants of USMT: one in which R can output an incorrect message (with small probability) and another in which R never outputs a wrong message, but may choose to abort (with small probability). We also provide efficient protocols for an important class of networks.
We study the problem of perfectly reliable message transmission (PRMT) and perfectly secure message transmission (PSMT) in an undirected synchronous network tolerating an all powerful threshold mobile Byzantine adversary. Specifically, we show that the mobility of the threshold adversary does not affect the possibility and optimality of PRMT and PSMT protocols. We also characterise PSMT in directed networks tolerating mobile adversary. All existing PRMT and PSMT protocols abstract the paths between the sender and the receiver as wires, neglecting the intermediate nodes in the paths, thus causing significant over estimation in the communication and round complexity of protocols. Here, we consider the underlying paths as a whole instead of abstracting them as wires and derive a tight bound on the number of rounds required to achieve reliable communication tolerating a threshold mobile adversary with arbitrary roaming speed. Finally, we briefly study PRMT and PSMT protocols in the presence of non-threshold mobile Byzantine adversary. Even though the characterisation for PRMT/PSMT is shown to be same against both threshold static and threshold mobile adversary (in this article), we show that the characterisation for PRMT/PSMT against non-threshold static and non-threshold mobile adversary are not same.
In the perfectly secure message transmission (PSMT) problem, two synchronized non-faulty players (or processors), the Sender S and the Receiver R are connected by n wires (each of which facilitates 2-way communication); S has a message, represented by a sequence oft elements from a finite field, that he wishes to send to R; after exchanging messages in phases R should correctly obtain S 's message, while an adversary listening on and actively controlling any set of t (or less) wires should have no information about S 's message. Similarly, in the problem of perfect reliable message transmission (PRMT), the receiver R should correctly obtain S's message, in spite of the adversary actively controlling any set oft (or less) wires.
In this thesis we focus on various aspects of secure message transmission protocols. Such protocols achieve the secure transmission of a message from a sender to a receiver - where the term “secure” encapsulates the notion of privacy and reliability of message transmission. These two parties are connected using an underlying network in which a static computationally unlimited active adversary able to corrupt up to t network nodes is assumed to be present. Such protocols are important to study as they are used extensively in various cryptographic protocols and are of interest to other research areas such as ad-hoc networks, military networks amongst others. Optimal bounds for the number of phases (communication from sender to receiver or vice versa), connectivity requirements (number of node disjoint network paths connecting sender and receiver - denoted by n), communication complexity (complexity of the number of field elements sent - where F is the finite field used and jFj = q) and transmission complexity (proportion of communication complexity to complexity of secrets transmitted) for secure message transmission protocols have been proven in previous work. In the one-phase model it has been shown that n 3t+1 node disjoint paths are required to achieve perfect communication. In the two phase model only n 2t + 1 node disjoint paths are necessary. This connectivity is also the required bound for almost perfectly secure one-phase protocols - protocols which achieve perfect privacy but with a negligible probability may fail to achieve reliability. In such cases the receiver accepts a different message to that transmitted by the sender or does not accept any message. The main focus of recent research in secure message transmission protocols has been to present new protocols which achieve optimal transmission complexity. This has been achieved through the transmission of multiple messages. If a protocol has a communication complexity of O(n3) field elements, to achieve optimal transmission complexity O(n2) secrets will have to be communicated. This has somewhat ignored the simplification and improvement of protocols which securely transmit a single secret. Such improvements include constructing more efficient protocols with regards to communication complexity, computational complexity and the number of field elements sent throughout the whole protocol. In the thesis we first consider one-phase almost perfectly secure message transmission and present two new protocols which improve on previous work. We present a polynomial time protocol of O(n2) communication complexity which at the time of writing this thesis, is computationally more efficient than any other protocol of similar communication complexity for the almost perfectly secure transmission of a single message. Even though our first almost perfectly secure transmission protocol is of polynomial time, it is important to study other protocols also and improve previous work presented by other researchers. This is the idea behind the second one-phase almost perfectly secure message transmission protocol we present which requires an exponential complexity of field operations but lower (O(n)) communication complexity. This protocol also improves on previous protocols of similar communication complexity, requiring in the order of O(log q) less computation to complete - where q denotes the size of the finite field used. Even though this protocol is of exponential time, for small values of n (e.g. when t = 1, t = 2 or t = 3) it may be beneficial to use this protocol for almost perfectly secure communication as opposed to using the polynomial time protocol. This is because less field elements need to be transmitted over the whole network which connects a sender and a receiver. Furthermore, an optimal almost perfectly secure transmission protocol will be one with O(n) communication complexity and with polynomial computational complexity. We hope that in the future, other researchers will be inspired by our proposed protocol, improve on our work and ideally achieve these optimal results. We also consider multi-phase protocols. By combining various cryptographic schemes, we present a new two-phase perfectly secure single message transmission protocol. At the time of writing this thesis, the protocol is the most efficient protocol when considering communication complexity. Our protocol has a communication complexity of O(n2) compared to O(n3) of previous work thus improving on the communication complexity by an order of O(n) for the perfectly secure message transmission of a single message. This protocol is then extended to a three phase protocol where a multi-recipient broadcast end channel network setting is considered. As opposed to point to point networks where a path from a sender reaches a single receiver, this network model is new in the field of message transmission protocols. In this model each path from a sender reaches multiple receivers, with all receivers receiving the same information from their common network communication channel. We show how the use of this protocol upon such a network can lead to great savings in the transmission and computation carried out by a single sender. We also discuss the importance and relevance of such a multi-recipient setting to practical applications. The first protocols in the field of perfectly secure message transmission with a human receiver are also presented. This is a topic proposed by my supervisor Professor Yvo Desmedt for which I constructed solutions. In such protocols, one of the communicating parties is considered to be a human who does not have access to a computational device. Because of this, solutions for such protocols need to be computationally efficient and computationally simple so that they can be executed by the human party. Experiments with human participants were carried out to assess how easily and accurately human parties used the proposed protocols. The experimental results are presented and these identify how well human participants used the protocols. In addition to the security of messages, we also consider how one can achieve anonymity of message transmission protocols. For such protocols, considering a single-receiver multi-sender scenario, the presence of a t-threshold bounded adversary and the transmission of multiple secrets (as many as the number of sender), once the protocols ends one should not be able to identify the sender of a received message. Considering a passive and active adversary new protocols are presented which achieve the secure and anonymous transmission of messages in the information-theoretic security model. Our proposed solutions can also be applied (with minor alterations) to the dual problem when a single-sender multi-recipient communication setting is considered. The contributions of the thesis are primarily theoretical - thus no implementation of the proposed protocols was carried out. Despite this, we reflect on practical aspects of secure message transmission protocols. We review the feasibility of implementing secure message transmission protocols in general upon various networks - focusing on the Internet which can be considered as the most important communication network at this time. We also describe in theory how concepts of secure message transmission protocols could possibly be used in practical implementations for secure communication on various existing communication networks. Open problems that remain unsolved in the research area of the proposed protocols are also discussed and we hope that these inspire research and future solutions for the design (and implementation) of better and more efficient secure message transmission protocols.