Published papers

To receive effective treatment during emergency response due to seizures or unforeseen accidents, a patient with intractable diseases must disclose information about their disease to an emergency physician. If the patient loses consciousness while traveling, the patient should disclose this information to a companion in advance. However, disclosing this information to a companion is undesirable because the information is confidential. Thus, we propose a system that discloses specific information on intractable diseases only when an emergency physician has verified they possess a medical license. Otherwise, the proposed system only discloses appropriate first aid information. We implemented a prototype of the proposed under the assumption that a physician has a digital medical license based on verifiable credentials (i.e., a standard for digital credentials). With this system, the patient does not disclose confidential information to the patient's companion but does disclose necessary information to the emergency physician.

In this paper, to estimate the risk of economic loss incurred by both parties in production order transactions, we propose a scheme that enables escrow and confirmation of the results without relying on a third party. In such transactions, both parties risk incurring economic losses if the other party behaves dishonestly. Generally, the risk can be reduced with an escrow service provided by a trusted third party. However, there is a risk of fraud by the third party; in some cases, the third party may not be available for the buyer or seller. Several existing schemes utilize fair exchange and blockchain to disburse the deposited payment upon the delivery of specific data. However, in production order transactions, some cases cannot be handled only by completion of delivery, such as disputes that arise when the data does not meet the quality expected by the buyer. In such cases, before the transaction starts, a party would confirm the counterparty's behavior in past transactions to estimate the risk of a dispute occurring. In this paper, we propose a scheme that records the history of past transaction processes while utilizing blockchain-based escrow and allows future counterparties to confirm the history as a reference for estimating risk. By the opportunity loss that a history of dishonest behavior causes and applying blockchain-based escrow, the scheme motivates sellers and buyers to behave in good faith. We implemented a prototype system on top of Ethereum and verified its feasibility. By expanding the scope of transactions, we expect that it will be possible to determine whether transactions between individuals over the Internet are feasible without relying on a specific escrow service.

Improvements in fabrication devices such as 3D printers are becoming possible for personal fabrication to freely fabricate any products. To clarify who is liable for the product, the fabricator should keep the fabrication history in an immutable and sustainably accessible manner. In this paper, we propose a new scheme, Fabchain, that can record the fabrication history in such a manner. By utilizing a scheme that employs a blockchain as an auditable communication channel, Fabchain manages print jobs for the fabricator's 3D printer over the blockchain, while maintaining a history of a print job. We implemented Fabchain on Ethereum and evaluated the performance for recording a print job. Our results demonstrate that Fabchain can complete communication of a print job sequence in less than 1 minute on the Ethereum test network. We conclude that Fabchain can manage a print job in a reasonable duration for 3D printing, while satisfying the requirements for immutability and sustainability.

© 2018 ACM. A type of Bitcoin node called “Full Node” has to hold the entire of historical transaction data called “Blockchain” to verify that new transactions are correct or not. To operate nodes as Full Nodes, the required storage size will be too large for resource-constrained devices. In this paper, to mitigate storage size, we propose a storage load balancing scheme by distributed storage based on Distributed Hash Table (DHT). By our scheme, nodes in a DHT cluster can behave like Full Nodes without holding the entire of the blockchain.

© 2018 IEEE. There are several studies that propose identity management that utilizes blockchain technology. In Ethereum, the main programmable blockchain system, data on a blockchain are saved as encoded binaries for executing automatic verification. To decode a binary, users need to know the application binary interfaces (ABI) that describes the data structure of the registered information. However, the manner by which ABI are shared in the current blockchain protocol is opaque. To resolve this problem, we describe a new protocol for embedding the ABI on a blockchain transaction when a registrant registers information. Our method enables users to read registered data with information on the blockchain alone, and it guarantees transparency without requiring users to trust third parties.

© 2018 Copyright held by the owner/author(s). In a consensus algorithm based on Proof-of-Work, miners are motivated by crypto rewards. Furthermore, security is guaranteed because a cost of a 50% attack chance is higher than the potential rewards. However, because of the sudden price jump of cryptocurrencies and cheap prices of mining machines like ASICs, the cost and profit were on equilibrium for Bitcoin in 2017. In this situation, attackers are motivated by the balance between hash power and profits. In this paper, we describe that there is relevance between mining power on the network and price of tokens that can be taken securely on a blockchain. Users who exchange tokens on the PoW blockchain should monitor mining power and exchange tokens cheaper than the attack cost so that profit and cost of the attacker are not in equilibrium.