2025 Volume 33 Pages 276-283
Card-based cryptographic protocols are useful for performing secure computations using physical cards instead of digital systems and are well-suited for educational purposes, especially for those new to studying multi-party computation (MPC). In this paper, we investigate using cards (such as business cards or mahjong tiles) with the same design on the back, but the front sides can face different directions. These cards are defined as those whose backs are indistinguishable and whose fronts can be differentiated based on the top and bottom. Mahjong tiles, painted the same color on the back, cannot be differentiated from the back even when swapped. Thus, tiles whose fronts show different designs when swapped can be used as top and bottom cards. Here, we examine the practical feasibility of implementing such protocols, focusing on whether the shuffle is practical. We present a realistic method to determine if protocols using up-down cards can be implemented by replacing cards with mahjong tiles. Additionally, we introduce the construction of a new protocol specifically for shuffling mahjong tiles. This study aims to provide a practical approach to utilizing up-down cards in secure and efficient card protocols, demonstrating their versatility and applicability in real-world scenarios.
