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Cryptography
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A Study on Security Notions of Anamorphic Signature
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Anamorphic signature is a cryptographic primitive that enables a secret communication channel even in the presence of a dictator who prohibits end-to-end encryption in order to censor all communications. Specifically, by embedding the intended message into the digital signatures attached to user communications, anamorphic signatures allow users to communicate secretly without being detected by the dictator, even under censorship. In other words, anamorphic signature can be seen as a technique that combines the properties of digital signatures with those of symmetric-key encryption. In this study, we examine the desirable security notions required for the use of anamorphic signatures.
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Formal Verification
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Construction and Formal Verification of TEE-based Secure File-sharing System Hiding Metadata
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When constructing secure file-sharing system among multiple participants without assuming trust in the cloud, protecting metadata and reducing the overhead caused by high-cost cryptographic operations are critical challenges. Trusted Execution Environments (TEEs) provide a useful approach to addressing these challenges.
In this study, to resolve the insufficient protection of access control information in prior works, we design a file-sharing system in which operations on confidential data are performed inside TEE deployed in each participant's machine. Furthermore, we formally model the protocol flow for new user to join the sharing, the threat model, and the required security properties, and rigorously guarantee the system's security through formal verification using ProVerif.
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Verifiable Credentials
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Condition-Based Verifiable Credential Issuance and Proofs Using Adaptor Signatures and Trapdoor Commitments
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Verifiable Credential (VC) have emerged as a self-sovereign framework for proving user attributes. While effective for static claims, existing schemes cannot natively prove dynamic facts such as condition fulfillment or state changes. For example, a VC can represent a warranty period but cannot securely incorporate the actual activation date. We propose a VC issuance and verification method that proves dynamic condition fulfillment by combining adaptor signatures, trapdoor commitments, and a blockchain-based Verifiable Data Registry (VDR). The design supports use cases such as warranties activated upon product delivery or conference passes validated upon attendance. This approach extends VC beyond static attributes to securely attest both the occurrence of a condition and related contextual details.
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Proposal for Verifiable Credentials with Designated-Verifier
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Since Boneh et al. proposed the BBS signature, various security proofs and efficient algorithms have been proposed as applications for Verifiable Credentials (VCs). As of 2026, the IETF is proceeding with the standardization of BBS signatures, primarily based on the framework proposed by Tessaro and Zhu. In the context of VCs, a use case should be considered where the Holder discloses their Credential only to a specific Verifier, allowing the Verifier to verify its validity (with a zero-knowledge). On the other hand, the validity verification in BBS signatures described in Tessaro et al.'s paper imposes no constraints on the Verifier and cannot be said to reproduce an ideal VC scenario. To address this, this paper proposes an extended model called DV-VC, a verifier-specified VC, constructs it using designated verifier signatures, and discusses its security proof.
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Web Security
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A Web Information Proof Method Based on TLS Communication Authenticity Verification
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Recently, ensuring the authenticity of online information has become increasingly important. However, existing methods often require server-side modifications or rely heavily on zero-knowledge proofs, making verification complex. In particular, users have faced challenges in proving information obtained from arbitrary websites to third parties without server cooperation. This study proposes a method to directly issue Verifiable Credentials (VCs) from web information acquired through TLS communication. It also introduces a new Notary node selection mechanism to maintain security even in the presence of malicious actors.
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Network Security
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An Overlay Communication System for Traffic Confirmation Attack Against Tor Hidden Services
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Tor hidden services are services hosted on the Tor network. The IP addresses of these services are hidden by onion routing. Previous research has shown that traffic confirmation attack methods can reveal IP addresses of hidden services. However, the methods are subject to false positives when multiple entities use the same method. In this research, we propose an overlay communication system on the Tor network to confirm the sender of signals in a traffic confirmation attack.
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Blockchain
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Load Evaluation of Proof-of-Verification
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Bitcoin, a cryptocurrency on blockchain, maintains its soundness by successive block generation. Block generation offers a reward for success on a first-come-first-served basis. It consumes a huge computational cost, so block generation triers (miners) have an incentive to skip other computations. A block contains many transaction data of coins, whose validity check, especially signature verification needs cryptographic calculations and can be the first skip target. To address this issue, a method called "Proof-of-Verification" (PoV) was proposed, which can indicate signatures in the block have been verified. We introduce the load evaluation of PoV.
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Security Economics
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Analyzing Investment Incentives under METI's New Rating System via a Stepwise Extension of the Gordon-Loeb Model
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Against the backdrop of severe supply chain attacks, Japan's Ministry of Economy, Trade and Industry (METI) is formulating a new stepwise security rating system (rank 3 to 5) for companies. However, conventional models assuming continuous investment effectiveness, such as the Gordon-Loeb model, cannot adequately explain corporate investment decisions under this discrete framework. This study proposes a stepwise extension model that incorporates the discrete rank structure and network externalities associated with fluctuating transaction opportunities. Our quantitative analysis reveals that initial participation (rank 3) is driven by "survival" incentives to avoid market exclusion, while transitioning to higher ranks (rank 4) is driven by "growth" incentives. Furthermore, we demonstrate the risk that excessive subsidies lowering the barrier for basic ranks may diminish the incentive to upgrade, causing corporate security investments to stagnate at lower levels.
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IIS Open House 2026, Matsuura Lab.
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