Takuro Hosoi

• Technical Associate

Interest

• Network security

Publications

• Kanta Matsuura, Takurou Hosoi. Mechanism Design of Data Sharing for Cybersecurity Research, IPSI Transactions on Advanced Research, Vol. 11, No. 1, pp. 35-40, 2015 [detail]

abstract

If we want to realize a scientific approach to cybersecurity, we need objective and reproducible evaluation of security. Although some of cryptographic technologies have rigorous security proofs, a lot of cybersecurity technologies rely on experimental evaluation which needs good datasets. One may expect that sharing such datasets would help at least the reproducibility of the evaluation. At the same time, one may be afraid that effective mechanism design is difficult because there have been a lot of studies on disincentive problems (e. g. free-riding) associated with information sharing in cybersecurity. However, the requirements and typical solutions for data sharing would be different from those for information sharing. In this paper, we comprehensively discuss the features of "data sharing for cybersecurity research" based on a systematic comparison with "information sharing for cybersecurity practice". We also report a Japanese case in the field of malware analysis. One important finding is that considering human resource development is an important factor in the activities associated with data sharing.
• Kanta Matsuura, Takurou Hosoi. Data Sharing for Cybersecurity Research and Information Sharing for Cybersecurity Practice, The 8th International Workshop on Security (IWSEC2013), 2013 [detail]

abstract

When we want to realize a scientific approach to cybersecurity, we need objective and reproducible evaluation of security properties. Although some of cryptographic technologies have rigorous security proofs, a lot of cybersecurity technologies rely on experimental security evaluation which needs good datasets. One may expect that sharing such datasets would help at least the reproducibility of the evaluation. At the same time, one may be afraid that effective mechanism design is not trivial because there have been a lot of studies on disincentive problems (e. g. free-riding) associated with information sharing for cybersecurity practice. However, the requirements and typical solutions for data sharing would be different from those for information sharing. In this poster, we comprehensively discuss the features of data sharing for cybersecurity research based on a systematic comparison with information sharing for cybersecurity practice. We also identify some intrinsic limitations of the data sharing approach.
• Takurou Hosoi, Kanta Matsuura. Effectiveness of a Change in TCP Retransmission Timer Management for Low-rate DoS Attack Mitigation and Attack Variants, The 8th International Workshop on Security (IWSEC2013), 2013 [detail]

abstract

The mechanism of TCP retransmission timeout is essential to the Internet congestion control. But existing research pointed out that this mechanism allows DoS attack with low-rate mean traffic. We proposed a change in TCP retransmission timeout management, in which length of TCP retransmission timer is increased not to precisely twice of the prior timer length in successive timeout waiting. We investigate its effectiveness in DoS attack mitigation analytically, and some attack variants under this countermeasure.
• Kanta Matsuura, Takurou Hosoi. Data Sharing for Cybersecurity Research: A Comparison with Information Sharing for Cybersecurity Practice, Ninth Annual Forum on Financial Information Systems and Cybersecurity: A Public Policy Perspective, 2013
• Takurou HOSOI, Kanta Matsuura. Evaluation of the Common Dataset Used in Anti-Malware Engineering Workshop 2009, Lecture Notes in Computer Science (Recent Advances in Intrusion Detection, 13th International Symposium on Recent Advances in Intrusion Detection: RAID 2010), Vol. 6307, pp. 496-497, 2010
• Takuro Hosoi, Kanta Matsuura, Hideki Imai. IP Trace Back by Packet Marking Method with Bloom Filters, Proceedings of the 2007 IEEE International Carnahan Conference on Security Technology (2007 ICCST) 41st Annual Conference, pp. 255-263, 2007

Kensuke Tamura

• Lecturer

Interest

• Digital forensics, Control system security

Publications

• Kensuke Tamura, Kanta Matsuura. Improvement of Anomaly Detection Performance using Packet Flow Regularity in Industrial Control Networks, IEICE Transactions on Fundamentals of Electronics, Communications and Computer Sciences, Vol. E102-A, No. 1, pp. 65-73, 2019 [detail]

abstract

Since cyber attacks such as cyberterrorism against Industrial Control Systems (ICSs) and cyber espionage against companies managing them have increased, the techniques to detect anomalies in early stages are required. To achieve the purpose, several studies have developed anomaly detection methods for ICSs. In particular, some techniques using packet flow regularity in industrial control networks have achieved high-accuracy detection of attacks disrupting the regularity, i. e. normal behavior, of ICSs. However, these methods cannot identify scanning attacks employed in cyber espionage because the probing packets assimilate into a number of normal ones. For example, the malware called Havex is customized to clandestinely acquire information from targeting ICSs using general request packets. The techniques to detect such scanning attacks using widespread packets await further investigation. Therefore, the goal of this study was to examine high performance methods to identify anomalies even if elaborate packets to avoid alert systems were employed for attacks against industrial control networks. In this paper, a novel detection model for anomalous packets concealing behind normal traffic in industrial control networks was proposed. For the proposal of the sophisticated detection method, we took particular note of packet flow regularity and employed the Markov-chain model to detect anomalies. Moreover, we regarded not only original packets but similar ones to them as normal packets to reduce false alerts because it was indicated that an anomaly detection model using the Markov-chain suffers from the ample false positives affected by a number of normal, irregular packets, namely noise. To calculate the similarity between packets based on the packet flow regularity, a vector representation tool called word2vec was employed. Whilst word2vec is utilized for the calculation of word similarity in natural language processing tasks, we applied the technique to packets in ICSs to calculate packet similarity. As a result, the Markov-chain with word2vec model identified scanning packets assimilating into normal packets in higher performance than the conventional Markov-chain model. In conclusion, employing both packet flow regularity and packet similarity in industrial control networks contributes to improving the performance of anomaly detection in ICSs.

Kaname Shimada

• Associate Research Fellow

Interest

• Anonymous communication system

Iifan Tyou

• Doctor Course 3*

Interest

• Blockchain

Publications

• Haocheng Jiang, Iifan Tyou, Kanta Matsuura. TLS2VC: A DecentralizedWebProof Framework Enabling Verifiable Credentials for TLS Sessions, The 41st ACM/SIGAPP Symposium on Applied Computing (SAC 2026), pp. 405-407, 2026 [detail]

abstract

This work proposes TLS2VC, a decentralized WebProof framework that distributes trust across multiple Notaries. Notaries attest to TLS session authenticity-server identity and encrypted transcript integrity - without accessing plaintext, then issue Verifiable Credentials (VCs) that Verifiers can validate. To prevent concentration of malicious Notaries, we employ verifiable random selection via Verifiable Random Function (VRF) combined with threshold signatures. We provide probabilistic security analysis showing that honest Notaries are included with high probability, and derive formulas to compute the minimum number of Notaries k required for a target security level. A lightweight prototype demonstrates practical feasibility, enabling reuse of existing web information as trusted credentials in Web3 and self-sovereign identity environments.
• Iifan Tyou, Ryuya Hayashi, Kanta Matsuura. Interoperability between Permissioned Distributed Ledgers without External Trust Anchor, IEEE International Conference on Blockchain and Cryptocurrency, Vol. 2025, 2025 [detail]

abstract

Permissioned ledgers offer efficient, cost-effective alternatives to public blockchains, ideal for enterprises. However, secure interoperability remains challenging without external trust anchors. We propose a scheme that eliminates this reliance, ensuring data correctness and verifiability via digital signatures and vector commitments, resisting forgery and ledger committee collusion.
• Iifan Tyou, Shigenori Ohashi, Justin Yu, Takayuki Miura, Takuro Hosoi, Kanta Matsuura. Leveraging Timestamps to Create Secure and Feeless Evidence Management, IEEE International Conference on Blockchain, Vol. 7th, 2024

Yuichi Tanishita

• Doctor Course 3

Interest

• Cryptography

Publications

• Yuichi Tanishita, Takahiro Matsuda, Kanta Matsuura. Anamorphic Signatures with Stealthiness under Chosen Anamorphic Signature Attacks, APKC '26: Proceedings of the ACM ASIA Public-Key Cryptography Workshop, pp. 11-20, 2026 [detail]

abstract

Anamorphic signature is a cryptographic primitive that enables a covert channel even in the presence of a dictator who prohibits end-to-end encryption in order to censor all communications. A standard security notion for anamorphic signature is stealthiness, which guarantees that the dictator who possesses the signing key cannot distinguish whether signatures are anamorphic or not, as long as it does not know a secret key for covert communication (called a double key in the context of anamorphic signature). This notion is intended to provide the confidentiality of anamorphic signature, regarded as a kind of symmetric-key encryption. For a symmetric-key encryption scheme, it is natural to consider chosen ciphertext attacks (CCA), which are called chosen anamorphic signature attacks (CASA) in the context of anamorphic signature. Jaeger and Stracovsky recently formalized indistinguishability under CASA, denoted IND-CASA, and showed an implication that if an anamorphic signature scheme satisfies stealthiness and dictator unforgeability (a kind of unforgeability notion for anamorphic signature), then it also satisfies IND-CASA security. However, IND-CASA security is essentially analogous to IND-CCA security in that an adversary is required to be unable to distinguish which covert message was used to generate a given anamorphic signature. It is not about confidentiality of the existence of covert messages in signatures, and does not capture the threats of stealthiness under CASA. In this paper, we provide a formal definition of stealthiness under CASA for anamorphic signature. Furthermore, we investigate relationships among stealthiness under CASA and other security notions for anamorphic signature. Specifically, we show that (1) CASA stealthiness is equivalent to the combination of both stealthiness and dictator unforgeability, (2) CASA stealthiness implies IND-CASA but the converse does not hold, and (3) CASA stealthiness and recipient unforgeability (another unforgeability notion for anamorphic signature) are incomparable. Our first and second results give a strictly stronger implication than the one shown by Jaeger and Stracovsky.
• Yuichi Tanishita, Takahiro Matsuda, Kanta Matsuura. Revisiting Security Definitions of Sender-Anamorphic Encryption, Lecture Notes in Computer Science (Public-Key-Cryptography - PKC 2026, 29th IACR International Conference on Practice and Theory in Public-Key-Cryptography: PKC 2026), Vol. 16554, No. 4, pp. 130-161, 2026 [detail]

abstract

Sender-anamorphic encryption is a cryptographic primitive that allows a sender to covertly embed an alternative message into the ciphertext. This enables the sender to transmit the message they truly wish to send without an authority's knowledge, even if they are coerced into sending a message against their will. The concrete scenario considered here is one where the authority demands that the sender provide the public key, the plaintext, and the internal randomness used to generate the ciphertext, and then requires a proof that the coerced message was indeed encrypted correctly. Persiano et al. (Eurocrypt 2022) formulated the security of sender-anamorphic encryption to capture this situation. Building on that, Wang et al. (Asiacrypt 2023) proposed sender-anamorphic encryption along with its security definition. However, in the formal security definitions for sender-anamorphic encryption in these existing works, the randomness used to generate the challenge ciphertext is not given to an adversary, and thus, the potential threats are not fully accounted for. Therefore, in this study, we redefine security for sender-anamorphic encryption so that the randomness used to generate the challenge ciphertext is provided to the adversary. We then investigate whether the existing sender-anamorphic encryption schemes by Persiano et al. and Wang et al. satisfy our refined notions of security.
• Yuichi Tanishita, Ryuya Hayashi, Ryu Ishii, Takahiro Matsuda, Kanta Matsuura. Updatable Encryption Secure against Randomness Compromise, IEICE Transactions on Fundamentals of Electronics, Communications and Computer Sciences, Vol. E109-A, No. 3, pp. 232-245, 2026 [detail]

abstract

Updatable encryption (UE) allows a third-party server to update outsourced encrypted data without exposing keys and plaintexts. The server can update ciphertexts to ones under a new key using an update token provided by the client. UE can realize efficient key rotation and is effective against key compromise. The standard security notions of UE capture the property that even if keys or update tokens are compromised, the confidentiality of messages is maintained by the key update and ciphertext update. In general, the randomnesses used in the encryption and ciphertext update algorithms must be kept secret in the same way as the keys. On the other hand, while key compromise is considered in existing security notions, randomness compromise is not. In this paper, we define a new security notion for UE, IND-UE-R security, that is resilient to the compromise of randomnesses used to generate or update ciphertexts. Furthermore, we prove that the UE construction RISE (EUROCRYPT'18) satisfies our proposed security notion.
• Yuichi Tanishita, Ryuya Hayashi, Ryu Ishii, Takahiro Matsuda, Kanta Matsuura. On the Implications from Updatable Encryption to Public-Key Cryptographic Primitives, IEICE Transactions on Fundamentals of Electronics, Communications and Computer Sciences, Vol. E109-A, No. 3, pp. 216-231, 2026 [detail]

abstract

Updatable encryption (UE) is a special type of symmetric-key encryption (SKE) that allows a third party to update ciphertexts while protecting plaintexts. Alamati et al. (CRYPTO 2019) showed a curious connection between UE and public-key encryption (PKE) that PKE can be constructed from UE. This implication result is somewhat surprising since it is well-known that PKE cannot be constructed from (ordinary) SKE in a black-box manner. In this paper, we continue to study the relationships between UE and other cryptographic primitives to obtain further insights into the existence and power of UE, and assumptions required for it. More specifically, we introduce some security properties that are natural to consider for UE (and are indeed satisfied by some existing UE schemes), and then investigate what types of public-key cryptographic primitives can be constructed from UE with the additional properties. Specifically, we show the following results: - 2-round oblivious transfer (OT) can be constructed from UE that satisfies the oblivious samplability of original ciphertexts (i. e. those generated by the ordinary encryption algorithm, as opposed to those generated by the ciphertext-update algorithm) and the oblivious samplability of update tokens (that are used for updating ciphertexts). - 3-round OT can be constructed from UE with the oblivious samplability of updated ciphertexts (i. e. those generated by the ciphertext-update algorithm). - Lossy encryption and PKE secure against selective-opening attacks can be constructed from UE if it satisfies what we call statistical confidentiality of original ciphertexts. IND-CPA secure PKE can be constructed from another variant of UE, ciphertext-dependent UE, if its algorithm to generate an update token is deterministic.
• Yuichi Tanishita, Ryuya Hayashi, Ryu Ishii, Takahiro Matsuda, Kanta Matsuura. Updatable Encryption Secure Against Randomness Compromise, Lecture Notes in Computer Science (Cryptology and Network Security, 23rd International Conference, CANS 2024), Vol. 14906, No. 1, pp. 47-69, 2024 [detail]

abstract

Updatable encryption (UE) allows a third-party server to update outsourced encrypted data without exposing keys and plaintexts. The server can update ciphertexts to ones under a new key using an update token provided by the client. UE can realize efficient key rotation and is effective against key compromise. The standard security notions of UE capture the property that even if keys or update tokens are compromised, the confidentiality of messages is maintained by the key update and ciphertext update. In general, the randomnesses used in the encryption and ciphertext update algorithms must be kept secret in the same way as the keys. On the other hand, while key compromise is considered in existing security notions, randomness compromise is not. In this paper, we define a new security notion for UE, IND-UE-R security, that is resilient to the compromise of randomnesses used to generate or update ciphertexts. Furthermore, we prove that the UE construction RISE (EUROCRYPT'18) satisfies our proposed security notion.
• Yuichi Tanishita, Ryuya Hayashi, Ryu Ishii, Takahiro Matsuda, Kanta Matsuura. On the Implications from Updatable Encryption to Public-Key Cryptographic Primitives, Lecture Notes in Computer Science (Information Security and Privacy, The 29th Australasian Conference on Information Security and Privacy: ACISP2024), Vol. 14895, No. 1, pp. 303-323, 2024 [detail]

abstract

Updatable encryption (UE) is a special type of symmetric-key encryption (SKE) that allows a third party to update ciphertexts while protecting plaintexts. Alamati et al. (CRYPTO 2019) showed a curious connection between UE and public-key encryption (PKE) that PKE can be constructed from UE. This implication result is somewhat surprising since it is well-known that PKE cannot be constructed from (ordinary) SKE in a black-box manner. In this paper, we continue to study the relationships between UE and other cryptographic primitives to obtain further insights into the existence and power of UE, and assumptions required for it. More specifically, we introduce some security properties that are natural to consider for UE (and are indeed satisfied by some existing UE schemes), and then investigate what types of public-key cryptographic primitives can be constructed from UE with the additional properties. Specifically, we show the following results: - 2-round oblivious transfer (OT) can be constructed from UE that satisfies the \emph{oblivious samplability (OS)} of original ciphertexts (i. e. those generated by the ordinary encryption algorithm, as opposed to those generated by the ciphertext-update algorithm) and the OS of update tokens (that are used for updating ciphertexts). - 3-round OT can be constructed from UE with OS of updated ciphertexts (i. e. those generated by the ciphertext-update algorithm). - Lossy encryption and PKE secure against selective-opening attacks can be constructed from UE if it satisfies what we call \emph{statistical confidentiality of original ciphertexts}.

Taiki Asano

• Doctor Course 2

Interest

• Cryptography

Ryoma Kanai

• Master Course 2

Interest

• Security Economics

Publications

• Ryoma Kanai, Kanta Matsuura. Analyzing Investment Incentives under METI's New Rating System via a Stepwise Extension of the Gordon-Loeb Model, The 25th Workchop on the Economics of Information Security (WEIS2026), 2026 [detail]

abstract

Against the backdrop of increasingly severe supply chain attacks, Japan's Ministry of Economy, Trade and Industry (METI) is formulating a new institutional system to evaluate and visualize companies' security measures in a stepwise manner, ranging from a three-star rank to a five-star rank. This system features a discrete rank structure, comprised of basic measures based on self-declaration (for three-star) to advanced measures requiring third-party certification (for four-star and five-star). However, conventional models premised on the "continuity" of investment effectiveness, such as the Gordon-Loeb model (GL model) and the Matsuura model, fail to sufficiently explain corporate investment decision-making under such a "discrete" scheme or the network externalities associated with its diffusion. In this study, building upon the Matsuura model, we propose a stepwise extension model that incorporates the specific star structure of the system and network externalities where transaction opportunities fluctuate. Through analysis using the proposed model, we quantitatively derive the threshold of diffusion (critical mass) required for companies to shift their investment behavior from the status quo (zero-star) to participation, and further to higher stars. Our analysis reveals that participation (zero-star to three-star) is governed by "survival" incentives to avoid market exclusion, whereas transitioning to higher stars (three-star to four-star) is driven by "growth" incentives aimed at capturing further market opportunities, representing decisively different investment incentives. Furthermore, numerical simulation results demonstrate the risk that excessive support measures (such as subsidies) aimed at lowering the entry barrier to three-star (Basic) may relatively diminish the incentive to transition to four-star (Standard). This can cause economic stagnation - a phenomenon practically known as the "checkbox compliance trap" - where corporate investment behavior becomes fixed at lower stars, satisfying minimum requirements without pursuing actual security growth. These findings provide policy implications based on mathematical evidence for designing institutional systems and optimizing subsidy policies to improve the security level of the entire supply chain.

Sogo Hosomi

• Master Course 1

Interest

• TBD