Published papers

The main service provided by the coming quantum Internet will be creating entanglement between any two quantum nodes. We discuss and classify attacks on quantum repeaters, which will serve roles similar to those of classical Internet routers. We have modeled the components for and structure of quantum repeater network nodes. With this model, we point out attack vectors, then analyze attacks in terms of confidentiality, integrity, and availability. While we are reassured about the promises of quantum networks from the confidentiality point of view, integrity and availability present new vulnerabilities not present in classical networks and require care to handle properly. We observe that the requirements on the classical computing/networking elements affect the systems' overall security risks. This component-based analysis establishes a framework for further investigation of network-wide vulnerabilities.

The Public Key Infrastructure (PKI) plays an essential role in securing Internet communications by enabling Certificate Authorities (CAs) to issue TLS certificates to web servers. These certificates enable browsers to authenticate web servers, though their expiration can disrupt website availability. To mitigate the problem, the Automated Certificate Management Environment (ACME) protocol automates certificate issuance and domain certification. However, Organization Validation (OV) certificate management is not automated because organization validation involves validating an organization's legal status and intent that are manually conducted, and are challenging to automate. This paper proposes an extended ACME protocol using Verifiable Credentials (VCs) to automate organization validation. VCs are digitally signed credentials issued by trusted authorities containing verifiable information, such as identity or educational qualifications. Reusing a previously issued VC for organization validation can pose significant risks by potentially enabling malicious actors to bypass the validation process. The proposed protocol solves this challenge by forcing a client requesting an OV certificate to obtain a short-lived VC after initiating the organization validation process, thereby limiting the reuse of VC and preventing unauthorized use. Our experimental results demonstrate that this protocol enhances security by preventing misuse of invalid VC while minimizing delays in certificate issuance.

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.

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.

We present an open-source event-driven simulation package called QuISP built on top of the OMNeT++ discrete event simulation framework. Behavior of quantum devices in small quantum networks has been the focus of recent studies, it is still an open question how they will work in networks of practical size. QuISP is designed to simulate large-scale quantum networks to investigate their behavior under realistic, noisy and heterogeneous configurations. Our confidence in the simulator is supported by comparing its output to analytic results for a small network. QuISP can handle thousands of qubits in hundreds of nodes on a laptop computer, preparing for full Quantum Internet simulation. This simulator promotes the development of protocols for larger and complex quantum networks.

Fair exchange protocols on blockchain enable an atomic exchange of digital goods and cryptocurrency between untrusted parties. Two prominent protocols, Zero-Knowledge Contingent Payment (ZKCP) and FairSwap, guarantee fairness: either both parties receive the expected items (digital goods and cryptocurrency) or neither does. However, both protocols lack accountability: when an exchange terminates abnormally, the protocol cannot identify which party caused the failure. This lack of accountability undermines an applicability of fair exchange protocols in decentralized settings. To fill this gap, we identify a common accountability issue in both protocols and revise them by adding signature exchange and chaining. We measure the computational overheads of the revised protocols in our implementation and experiment.

In this paper, we propose an architecture of 'Media Analysis Platform (MAP)' that enables multi-perspective analysis of posts on social networking services that may contain potentially misleading or suspicious information. Typically, a post consists of multiple elements such as text and images. Existing methods against false information focus on analyzing each element, for example, detecting AI-generated images or examining claims in a text. An analyst, who analyzes the post, has their own intent and may combine multiple methods for each element. However, a platform that integrates these heterogeneous analysis methods has not yet been established. To address this gap, we propose MAP, a framework for applying and combining diverse analysis methods to each element. MAP stores the original post, decomposed elements, and each analysis result. The architecture allows analysis methods to be connected as modular components. To demonstrate the feasibility of MAP, we implement the prototype system that can analyze posts on X.com and dummy posts. To consistently represent each analysis result in MAP, we also design the SPOTL (Subject, Predicate, Object, Time, Location) data model for the prototype system. With the prototype system, we demonstrate analysis of a hypothetical dummy post and an actual post on X.com and discuss future research directions. We anticipate that MAP will serve as a basis for integrating various analysis methods for posts.

In this paper, we propose a scheme that allows verifying whether a certifier, as the certificate issuer, has validated the claim indicated in the certificate. A certificate represents that the certifier has validated the claim indicated in the certificate. Typically, a validator assumes that the certifier issued the certificate after validating the claim and regards the claim as valid upon verifying the certificate's integrity. However, when the assumption does not hold, transparency of the process leading up to certificate issuance is required. The existing scheme, Certificate Transparency, guarantees the transparency of the certificate issuance itself, but does not guarantee whether the certifier has validated the claim before issuance. Therefore, in this paper, we propose a scheme that embeds proof that the claim satisfies the certifier's validation criteria into the certificate. Considering the evidence examined by the certifier may contain the certificate subject's private information, we apply a zero-knowledge proof that only proves that specific criteria have been met. Under the proposed scheme, the validator can verify that the criteria have been met without disclosing the evidence. To demonstrate the feasibility of the proposed scheme, we implemented a prototype and confirmed that it works as expected. With the proposed scheme, it is possible to validate the claim in a certificate without assuming the certifier's validation process.

Decentralized data spaces are transforming how organizations, services, and autonomous agents interact, fostering collaboration while presenting new challenges for trust and interoperability. Existing frameworks often rely on centralized governance, which limits seamless operation across diverse ecosystems, especially with the increasing presence of AI agents and IoT devices. This paper introduces a Verifiable Autonomous Identity Architecture, enabling real-time, protocol-level trust formation and verification based on cryptographic proofs and open standards. The approach distinctly separates identity, data verification, and authorization, utilizing mechanisms such as OpenID Federation, Client ID Metadata Document, and Originator Profile to achieve scalable and interoperable trust. Rather than replacing existing governance models, the architecture overlays mechanisms for accountability, exception handling, and policy definition, delegating routine trust decisions to automated, runtime processes. While empirical validation is reserved for future work, this conceptual foundation supports the evolution of resilient, agent-oriented data spaces and next-generation decentralized digital ecosystems.