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IEICE Transactions on Fundamentals of Electronics Communications and Computer Sciences

Securely Computing Three-Input Functions with Eight Cards

Takuya Nishida, Yu‐ichi Hayashi, Takaaki Mizuki, Hideaki Sone

Published Jan 1, 2015• 23 citations
Publisher

Abstract

Assume that Alice, Bob, and Carol, each of whom privately holds a one-bit input, want to learn the output of some Boolean function, say the majority function, of their inputs without revealing more of their own secret inputs than necessary. In this paper, we show that such a secure three-input function evaluation can be performed with a deck of real cards; specifically, the three players can learn only the output of the function using eight physical cards — four black and four red cards — with identical backs.

DOI
10.1587/transfun.e98.a.1145

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Assume that Alice, Bob, and Carol, each of whom privately holds a one-bit input, want to learn the output of some Boolean function, say the majority function, of their inputs without revealing more of their own secret inputs than necessary. In this paper, we show that such a secure three-input function evaluation can be performed with a deck of real cards; specifically, the three players can learn only the output of the function using eight physical cards — four black and four red cards — with identical backs.
Abstract Card-based cryptography is a variant of multi-party computation using physical cards like playing cards. There are two models on card-based cryptography, called public and private models. The public model assumes that all operations are executed publicly, while the private model allows the players private operations called private permutations (PP, for short). Much of the existing card-based protocols were developed under the public model. Under the public model, 2ncards are necessary for every protocol withn-bit input since at least two cards are required to express a bit. In this paper, we proposen-bit input protocols with fewer than 2ncards by utilizing PP, which shows the power of PP. In particular, we show that a protocol for (n-bit input) threshold function can be realized with only n+1n+1n+1 n+1 cards by reducing the threshold function to the majority voting. Toward this end, we first offer that two-bit input protocols for logic gates can be realized with fewer than four cards. Furthermore, we construct a new protocol for three-input majority voting with only four cards by observing the relationship between AND/OR operations. This protocol can be easily extended to more participants, and to the protocol for threshold functions.
Abstract Card-based cryptography is a secure computation protocol realized by using physical cards. There are two models on card-based cryptography: public and private models. We adopt private one that allows players to handle cards privately. While much of the existing works for card-based cryptography use two-colored cards, it is also a vital task to construct an efficient protocol with playing cards. In the public model, 2n cards are necessary for any n-bit input protocol since at least two cards are required to express a Boolean value. It holds true for both two-colored and playing-card settings. On the other hand, the private model enables us to construct a protocol with fewer than 2n cards. However, all existing protocols that achieve such properties are only in the two-colored setting. This paper shows that the private model enables us to construct a protocol with fewer than 2n cards using the playing cards. We first show two-bit input protocols with fewer than four cards for logical operations, AND, OR, and XOR. Furthermore, we show a three-input majority voting protocol using only three cards, which is constructed by combining our AND and OR protocols. Notably, our proposed protocols require no randomness. All operations are deterministic and depend only on players’ private inputs.
Consider three players Alice, Bob and Cath who hold a, b and c cards, respectively, from a deck of d=a+b+c cards. The cards are all different and players only know their own cards. Suppose Alice and Bob wish to communicate their cards to each other without Cath learning whether Alice or Bob holds a specific card. Considering the cards as consecutive natural numbers 0,1,..., we investigate general conditions for when Alice or Bob can safely announce the sum of the cards they hold modulo an appropriately chosen integer. We demonstrate that this holds whenever a,b>2 and c=1. Because Cath holds a single card, this also implies that Alice and Bob will learn the card deal from the other player's announcement.
Abstract Card-based cryptography started with the “five-card trick” designed by Den Boer (EUROCRYPT 1989); it enables Alice and Bob to securely evaluate the AND value of their private bits using a physical deck of five cards. It was then shown that the same task can be done with only four cards, i.e., Mizuki et al. proposed a four-card AND protocol (ASIACRYPT 2012). These two AND protocols are simple and easy even for non-experts, such as high school students, to execute. Their only common drawback is the need to prepare a customized deck consisting of red and black cards such that all cards of the same color must be identical. Fortunately, several existing protocols are based on a standard deck of playing cards (commercially available). Among them, the state-of-the-art AND protocol was constructed by Koch et al. (ASIACRYPT 2019); it uses four playing cards (such as ‘A, J, Q, K’) to securely evaluate the AND value. The protocol is elaborate, while its possible drawback is the need to repeat a shuffling operation six times (in expectation), which makes it less practical. This paper aims to provide the first practical protocol working on a standard deck of playing cards. We present an extremely simple AND protocol that terminates after only one shuffle using only four cards; our proposed protocol relies on a new operation, called the “half-open” action, whereby players can check only the suit of a face-down card without revealing the number on it. We believe that this new operation is easy-to-implement, and hence, our four-card AND protocol working on a standard deck is practical. We formalize the half-open action to present a formal description of our proposed protocol. Moreover, we discuss what is theoretically implied by introducing the half-open action and show that it can be applied to efficiently solving Yao’s Millionaires’ problem with a standard deck of cards.
Card-based cryptography is an area involving cryptographic techniques that use a deck of physical cards to implement secure computation. In this paper, we study card-based cryptographic protocols for symmetric functions f: {0, 1}n → R, where R is an arbitrary finite set. Any symmetric Boolean function has the property that its output value is determined only by the sum of input bits, and the class of them includes many interesting functions such as the AND and equality functions. In 2020, Ruangwises and Itoh proposed a protocol which uses 2n + 2 cards to securely compute any symmetric function f: {0, 1}n → R; reducing the number of cards to less than 2n + 2 was left as an open problem. While the Ruangwises–Itoh protocol uses a two-colored deck of cards, this paper considers using three-colored and four-colored decks: to give a partial answer to the open problem, we propose three protocols for symmetric functions using 2n + 1 cards or less. Specifically, we first propose a protocol using 2n + 1 cards from a three-colored deck. We then propose two protocols using 2n cards from a four-colored deck; one is a simpler protocol and the other is a more shuffle-efficient protocol.
Secure computation enables a number of players each holding a secret input value to compute a function of the inputs without revealing the inputs. It is known that secure computation is possible physically when the inputs are given as a sequence of physical cards. This research area is called card-based cryptography. One of the important problems in card-based cryptography is to minimize the number of cards and shuffles, where a shuffle is the most important (and somewhat heavy) operation in card-based protocols. In this paper, we determine the minimum number of shuffles for achieving general secure computation. Somewhat surprisingly, the answer is just one, i.e., we design a protocol which securely computes any Boolean circuit with only a single shuffle. The number of cards required for our protocol is proportional to the size of the circuit to be computed.