In card-based cryptography, a commitment to a Boolean value is usually represented by two face-down cards of different colors or numbers, whose order specifies the one-bit value (namely, 0 or 1). One of the most important primitives in card-based cryptography is a "copy protocol," which is supposed to make two identical copies of a given commitment. In the literature, there are several copy protocols, which can be categorized by kinds of shuffles they use; this paper focuses on those using only the so-called random cut, which is the simplest shuffle, and we propose two copy protocols that are more efficient than the existing ones. Specifically, we first work on a standard deck of cards and design a six-card copy protocol using three random cuts (on average). Since the previous protocol needs 5.5 random cuts, our protocol improves upon it. Next, we shift our attention to the case of a two-colored deck of cards, and construct a six-card copy protocol using three random cuts (on average). Because the previous protocol requires eight cards, our protocol uses two cards fewer than the previous one (although it uses one more shuffle). In addition, going back to the standard-deck setting, we provide a four-card XOR protocol using only one random cut for the first time.
In card-based cryptography, a commitment to a Boolean value is usually represented by two face-down cards of different colors or numbers, whose order specifies the one-bit value (namely, 0 or 1). One of the most important primitives in card-based cryptography is a "copy protocol," which is supposed to make two identical copies of a given commitment. In the literature, there are several copy protocols, which can be categorized by kinds of shuffles they use; this paper focuses on those using only the so-called random cut, which is the simplest shuffle, and we propose two copy protocols that are more efficient than the existing ones. Specifically, we first work on a standard deck of cards and design a six-card copy protocol using three random cuts (on average). Since the previous protocol needs 5.5 random cuts, our protocol improves upon it. Next, we shift our attention to the case of a two-colored deck of cards, and construct a six-card copy protocol using three random cuts (on average). Because the previous protocol requires eight cards, our protocol uses two cards fewer than the previous one (although it uses one more shuffle). In addition, going back to the standard-deck setting, we provide a four-card XOR protocol using only one random cut for the first time.
Executing a card-based cryptographic protocol is an attractive way to perform secure multiparty computation (MPC) with a deck of physical cards. Crèpeau and Kilian at CRYPTO 1993 proposed card-based AND and XOR protocols that can deal with a logical conjunction and exclusive-or of variables. Their protocols use a familiar shuffling action called a random cut that can be easily implemented by humans, while the numbers of required cards and shuffles are not small enough to be efficient and the protocols are Las Vegas algorithms, i.e., they are not finite-runtime. Several researchers have improved upon the protocols for a quarter of a century. Eventually, there are many AND protocols in the literature, some of which are quite efficient and practical to execute. By contrast, there are only three XOR protocols including the Crèpeau--Kilian protocol mentioned above. In this paper, we design an efficient XOR protocol using only random cuts; the numbers of required cards and shuffles are six and two (which is fixed), respectively. Our proposed XOR protocol is the first construction of a finite-runtime XOR protocol if we restrict ourselves to use only random cuts.
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.
Card-based protocols are cryptographic protocols that use a deck of physical cards. In this paper, we deal with card-based protocols using a standard deck of playing cards, which are commonly and commercially available. For finite-runtime committed-format protocols, Mizuki (CANS 2016) proposed an eight-card AND protocol with four random bisection cuts and a six-card COPY protocol with one random bisection cut. In this paper, we propose a partial-open action, which reveals any position of the face of cards, by generalizing the half-open action introduced by Miyahara and Mizuki (IJTCS-FAW 2022). Using the partial-open action, we propose four-card AND protocols with three random cuts, a four-card COPY protocol with three random cuts, and a four-card base conversion protocol with a random cut. We note that, without partial-open actions, these AND and COPY protocols with random cuts only are known to be impossible to construct using four cards. Therefore, the partial-open actions are inherently necessary to obtain our results.
In card-based cryptography, which uses physical cards for secure computation, proving lower bounds on the number of cards and shuffles is an important research problem. There are two different approaches to demonstrate the impossibility of protocol construction: formal verification and proof using pen and paper. In card-based protocols, shuffles are the most important operations to ensure the correctness and security of protocols. A random cut, which randomly shifts a sequence of cards, is considered the easiest shuffle to implement. For this reason, many protocols using only random cuts as shuffle operations have been proposed, and proving lower bounds on the number of cards and shuffles in this setting is also recognized as an important problem. As for XOR protocols using only random cuts, Toyoda et al. constructed a six-card XOR protocol, but so far it is not clear whether there are protocols with fewer cards. In this paper, we prove the impossibility of a four-card XOR protocol using only random cuts, employing both formal verification and proof using pen and paper. In formal verification, we apply the verification method for AND protocols developed by Koch et al. to XOR protocols, and show that the number of shuffles is at least three. In the proof using pen and paper, we show that a four-card XOR protocol is impossible to construct even when allowing any number of shuffles.
In card-based cryptography, designing AND protocols in committed format is a major topic of research. The state-of-the-art AND protocol proposed by Koch, Walzer, and Härtel in ASIACRYPT 2015 uses only four cards, which is the minimum permissible number. Their protocol»s minimality relies on somewhat complicated shuffles having non-uniform probabilities of possible outcomes. Restricting the allowed shuffles to "practical»» ones, namely uniform closed shuffles, to our knowledge, six cards are sufficient: The six-card AND protocol proposed by Mizuki and Sone in 2009 utilizes the random bisection cut, which is a uniform and cyclic (and hence, closed) shuffle. Thus, a question has arisen: Can we improve upon this six-card protocol using only practical shuffles? In other words, whether there exists a five-card AND protocol in committed format using only uniform closed shuffles has been one of the most important open questions in this field. In this paper, we answer the question affirmatively by designing a five-card committed-format AND protocol using only practical shuffles. The shuffles that our protocol uses are random cut and random bisection cut, both of which are uniform cyclic shuffles and can be easily implemented by humans.
Abstract In card-based cryptography, designing AND protocols in committed format is a major research topic. The state-of-the-art AND protocol proposed by Koch, Walzer, and Härtel in ASIACRYPT 2015 uses only four cards, which is the minimum permissible number. The minimality of their protocol relies on somewhat complicated shuffles having non-uniform probabilities of possible outcomes. Restricting the allowed shuffles to uniform closed ones entails that, to the best of our knowledge, six cards are sufficient: the six-card AND protocol proposed by Mizuki and Sone in 2009 utilizes the random bisection cut, which is a uniform and cyclic (and hence, closed) shuffle. Thus, a question has arisen: “Can we improve upon this six-card protocol using only uniform closed shuffles?” In other words, the existence or otherwise of a five-card AND protocol in committed format using only uniform closed shuffles has been one of the most important open questions in this field. In this paper, we answer the question affirmatively by designing five-card committed-format AND protocols using only uniform cyclic shuffles. The shuffles that our protocols use are the random cut and random bisection cut, both of which are uniform cyclic shuffles and can be easily implemented by humans.