Takeshi Miyamae

• Doctor Course

Interest

• Blockchain

Publications

• Takeshi Miyamae, Kanta Matsuura. Coin Transfer Unlinkability Under the Counterparty Adversary Model, Ledger, Vol.7, pp.17-34, 2022 [detail]

abstract

Unlinkability is a crucial property of cryptocurrencies that protects users from deanonymization attacks. However, currently, even anonymous cryptocurrencies do not necessarily attain unlinkability under specific conditions. For example, Mimblewimble, which is considered to attain coin unlinkability using its transaction kernel offset technique, is vulnerable under the assumption that privacy adversaries can send their coins to or receive coins from the challengers. This paper first illustrates the privacy issue in Mimblewimble that could allow two colluded adversaries to merge a person's two independent chunks of personally identifiable information (PII) into a single PII. To analyze the privacy issue, we formulate unlinkability between two sets of objects and a privacy adversary model in cryptocurrencies called the counterparty adversary model. On these theoretical bases, we define an abstract model of blockchain-based cryptocurrency transaction protocols called the coin transfer system, and unlinkability over it called coin transfer unlinkability (CT-unlinkability). Furthermore, we introduce zero-knowledgeness for the coin transfer systems to propose a method to easily prove the CT-unlinkability of cryptocurrency transaction protocols. Finally, we prove that Zerocash is CT-unlinkable by using our proving method to demonstrate its effectiveness.

Kittiphop Phalakarn

• Doctor Course

Interest

• Cryptography

Publications

• Kittiphop Phalakarn, Nuttapong Attrapadung, Kanta Matsuura. Efficient Oblivious Evaluation Protocol and Conditional Disclosure of Secrets for DFA, Lecture Notes in Computer Science (Applied Cryptography and Network Security - ACNS 2022 - 20th International Conference on Applied Cryptography and Network Security), Vol.13269, No., pp.605-625, 2022 [detail]

abstract

In oblivious finite automata evaluation, one party holds a private automaton, and the other party holds a private string of characters. The objective is to let the parties know whether the string is accepted by the automaton or not, while keeping their inputs secret. The applications include DNA searching, pattern matching, and more. Most of the previous works are based on asymmetric cryptographic primitives, such as homomorphic encryption and oblivious transfer. These primitives are significantly slower than symmetric ones. Moreover, some protocols also require several rounds of interaction. As our main contribution, we propose an oblivious finite automata evaluation protocol via conditional disclosure of secrets (CDS), using one (potentially malicious) outsourcing server. This results in a constant-round protocol, and no heavy asymmetric-key primitives are needed. Our protocol is based on a building block called "an oblivious CDS scheme for deterministic finite automata" which we also propose in this paper. In addition, we propose a standard CDS scheme for deterministic finite automata as an independent interest.
• Kittiphop Phalakarn, Vorapong Suppakitpaisarn, Nuttapong Attrapadung, Kanta Matsuura. Evolving Homomorphic Secret Sharing for Hierarchical Access Structures, Lecture Notes in Computer Science (Advances in Information and Computer Security - IWSEC 2021 - 16th International Workshop on Security), Vol.12835, pp.77-96, 2021 [detail]

abstract

Secret sharing is a cryptographic primitive that divides a secret into several shares, and allows only some combinations of shares to recover the secret. As it can also be used in secure multi-party computation protocol with outsourcing servers, several variations of secret sharing are devised for this purpose. Most of the existing protocols require the number of computing servers to be determined in advance. However, in some situations we may want the system to be "evolving". We may want to increase the number of servers and strengthen the security guarantee later in order to improve availability and security of the system. Although evolving secret sharing schemes are available, they do not support computing on shares. On the other hand, "homomorphic" secret sharing allows computing on shares with small communication, but they are not evolving. As the contribution of our work, we give the definition of "evolving homomorphic" secret sharing supporting both properties. We propose two schemes, one with hierarchical access structure supporting multiplication, and the other with partially hierarchical access structure supporting computation of low degree polynomials. Comparing to the work with similar functionality of Choudhuri et al. (IACR ePrint 2020), our schemes have smaller communication costs.
• Kittiphop Phalakarn, Vorapong Suppakitpaisarn, Nuttapong Attrapadung, Kanta Matsuura. Constructive t-secure Homomorphic Secret Sharing for Low Degree Polynomials, Lecture Notes in Computer Science (Progress in Cryptology - INDOCRYPT 2020 - 21st International Conference on Cryptology in India), Vol.12578, pp.763-785, 2020 [detail]

abstract

This paper proposes t-secure homomorphic secret sharing schemes for low degree polynomials. Homomorphic secret sharing is a cryptographic technique to outsource the computation to a set of servers while restricting some subsets of servers from learning the secret inputs. Prior to our work, at Asiacrypt 2018, Lai, Malavolta, and Schroder proposed a 1-secure scheme for computing polynomial functions. They also alluded to t-secure schemes without giving explicit constructions; constructing such schemes would require solving set cover problems, which are generally NP-hard. Moreover, the resulting implicit schemes would require a large number of servers. In this paper, we provide a constructive solution for threshold-t structures by combining homomorphic encryption with the classic secret sharing scheme for general access structure by Ito, Saito, and Nishizeki. Our scheme also quantitatively improves the number of required servers from O(t^2) to O(t), compared to the implicit scheme of Lai et al. We also suggest several ideas for future research directions.

Junichiro Hayata

• Doctor Course

Interest

• Cryptography

Publications

• Junichiro Hayata, Fuyuki Kitagawa, Yusuke Sakai, Goichiro Hanaoka, Kanta Matsuura. Equivalence between Non-Malleability against Replayable CCA and Other RCCA-Security Notions, IEICE Transactions on Fundamentals of Electronics, Communications and Computer Sciences, Vol.E104-A, No.1, pp.89-103, 2021 [detail]

abstract

Replayable chosen ciphertext (RCCA) security was introduced by Canetti, Krawczyk, and Nielsen (CRYPTO'03) in order to handle an encryption scheme that is "non-malleable except tampering which pre- serves the plaintext." RCCA security is a relaxation of CCA security and a useful security notion for many practical applications such as authentication and key exchange. Canetti et al. defined non-malleability against RCCA (NM-RCCA), indistinguishability against RCCA (IND-RCCA), and universal composability against RCCA (UC-RCCA). Moreover, they proved that these three security notions are equivalent when considering a PKE scheme whose plaintext space is super-polynomially large. Among these three security notions, NM-RCCA seems to play the central role since RCCA security was introduced in order to capture "non-malleability except tampering which preserves the plaintext." However, their definition of NM-RCCA is not a natural extension of that of original non-malleability, and it is not clear whether their NM-RCCA captures the requirement of original non- malleability. In this paper, we propose definitions of indistinguishability- based and simulation-based non-malleability against RCCA by extending definitions of original non-malleability. We then prove that these two notions of non-malleability and IND-RCCA are equivalent regardless of the size of plaintext space of PKE schemes.
• Junichiro Hayata, Jacob C. N. Schuldt, Goichiro Hanaoka, Kanta Matsuura. On Private Information Retrieval Supporting Range Queries, Lecture Notes in Computer Science (Computer Security - ESORICS 2020 - 25th European Symposium on Research in Computer Security, Proceedings, Part II), Vol.12309, pp.674-694, 2020 [detail]

abstract

Private information retrieval (PIR) allows a client to retrieve data from a database without the database server learning what data is being retrieved. Although many PIR schemes have been proposed in the literature, almost all of these focus on retrieval of a single database element, and do not consider more flexible retrieval queries such as basic range queries. Furthermore, while practically-oriented database schemes aiming at providing flexible and privacy-preserving queries have been proposed, to the best of our knowledge, no formal treatment of range queries has been considered for these. In this paper, we firstly highlight that a simple extension of the standard PIR security notion to range queries, is insufficient in many usage scenarios, and propose a stronger security notion aimed at addressing this. We then show a simple generic construction of a PIR scheme meeting our stronger security notion, and propose a more efficient direct construction based on function secret sharing - while the former has a round complexity logarithmic in the size of the database, the round complexity of the latter is constant. Finally, we report on the practical performance of our direct construction.
• Junichiro Hayata, Masahito Ishizaka, Yusuke Sakai, Goichiro Hanaoka, Kanta Matsuura. Generic Construction of Adaptively Secure Anonymous Key-Policy Attribute-Based Encryption from Public-Key Searchable Encryption, IEICE TRANSACTIONS on Fundamentals of Electronics, Communications and Computer Sciences, Vol.E103-A, No.1, pp.107-113, 2020 [detail]

abstract

Public-key encryption with keyword search (PEKS) is a cryptographic primitive that allows us to search for particular keywords over ciphertexts without recovering plaintexts. By using PEKS in cloud services, users can outsource their data in encrypted form without sacrificing search functionality. Concerning PEKS that can specify logical disjunctions and logical conjunctions as a search condition, it is known that such PEKS can be (generically) constructed from anonymous attribute-based encryption (ABE). However, it is not clear whether it is possible to construct this types of PEKS without using ABE which may require large computational/communication costs and strong mathematical assumptions. In this paper, we show that ABE is crucial for constructing PEKS with the above functionality. More specifically, we give a generic construction of anonymous key-policy ABE from PEKS whose search condition is specified by logical disjunctions and logical conjunctions. Our result implies such PEKS always requires large computational/communication costs and strong mathematical assumptions corresponding to those of ABE.
• Junichiro Hayata, Fuyuki Kitagawa, Yusuke Sakai, Goichiro Hanaoka, Kanta Matsuura. Equivalence Between Non-malleability Against Replayable CCA and Other RCCA-Security Notions, Lecture Notes in Computer Science (Advances in Information and Computer Security, The 14th International Workshop on Security: IWSEC2019), Vol.11689, pp.253-272, 2019 [detail]

abstract

Replayable chosen ciphertext (RCCA) security was introduced by Canetti, Krawczyk, and Nielsen (CRYPTO 03) in order to handle an encryption scheme that is "non-malleable except tampering which preserves the plaintext". RCCA security is a relaxation of CCA security and a useful security notion for many practical applications such as authentication and key exchange. Canetti et al. defined non-malleability against RCCA (NM-RCCA), indistinguishability against RCCA (IND-RCCA), and universal composability against RCCA (UC-RCCA). Moreover, they proved that these three security notions are equivalent when considering a PKE scheme whose plaintext space is super-polynomially large. Among these three security notions, NM-RCCA seems to play the central role since RCCA security was introduced in order to capture "non-malleability except tampering which preserves the plaintext." However, their definition of NM-RCCA is not a natural extension of that of classical non-malleability, and it is not clear whether their NM-RCCA captures the requirement of classical non-malleability. In this paper, we propose definitions of indistinguishability-based and simulation-based non-malleability against RCCA by extending definitions of classical non-malleability. We then prove that these two notions of non-malleability and IND-RCCA are equivalent regardless of the size of plaintext space of PKE schemes.
• Junichiro Hayata, Masahito Ishizaka, Yusuke Sakai, Goichiro Hanaoka, Kanta Matsuura. Generic Construction of Adaptively Secure Anonymous Key-Policy Attribute-Based Encryption from Public-Key Searchable Encryption, Proceeding of the 2018 International Symposium on Information Theory and its Applications (ISITA2018), pp.739-743, 2018 [detail]

abstract

Public-key encryption with keyword search (PEKS) is a cryptographic primitive that allows us to search encrypted data for those of including particular keywords without decrypting them. PEKS is expected to be used for enhancing security of cloud storages. It is known that PEKS can be constructed from anonymous identity-based encryption (IBE), anonymous attribute-based encryption (ABE) and so on. It is believed that it is difficult to construct PEKS schemes that can specify a flexible search condition such as logical disjunctions and logical conjunctions from weaker cryptographic tools than ABE. However, this intuition has not been rigorously justified. In this paper, we formally prove it by constructing key-policy ABE from PEKS for monotone boolean formulas.

Taiki Asano

• Master Course

Interest

• Cryptography

Hajime Kuno

• Master Course

Interest

• Vulnerability assessment

Publications

• Hajime Kuno, Kanta Matsuura. Towards Automation of Penetration Testing for Web Applications by Deep Reinforcement Learning, Proceedings of the 37th Annual Computer Security Applications Conference (ACSAC '21), 2021 [detail]

abstract

Penetration testing (PT) that assesses vulnerabilities by considering and executing all possible attacks is important in security engineering but very expensive due to the need of experienced professionals. As a countermeasure, there are attempts to partially automate and improve the efficiency of PT. Their common feature is the use of existing PT tools (e.g. Metasploit) and machine learning (ML). Such approaches do not embed ML in PT tools, and would not improve the tools themselves. In this work, we use deep reinforcement learning to automate search and exploit executions for various vulnerabilities existing in Web applications so that a wide variety of PT tools can be integrated in an effective manner with such embedded ML. This poster will show two preliminary experiments in this direction.

Toshinori Usui

• Doctor Course

Interest

• Anti-malware

Publications

• Toshinori Usui, Yuto Otsuki, Yuhei Kawakoya, Makoto Iwamura, Kanta Matsuura. Script Tainting Was Doomed From The Start (By Type Conversion): Converting Script Engines into Dynamic Taint Analysis Frameworks, Proceedings of the 25th International Symposium on Research in Attacks, Intrusions and Defenses (RAID 2022), pp.380-394, 2022 [detail]

abstract

Data flow analysis is an essential technique for understanding the complicated behavior of malicious scripts. For tracking the data flow in scripts, dynamic taint analysis has been widely adopted by existing studies. However, the existing taint analysis techniques have a problem that each script engine needs to be separately designed and implemented. Given the diversity of script languages that attackers can choose for their malicious scripts, it is unrealistic to prepare taint analysis tools for the various script languages and engines. In this paper, we propose an approach that automatically builds a taint analysis framework for scripts on top of the framework designed for native binaries. We first conducted experiments to reveal that the semantic gaps in data types between binaries and scripts disturb our approach by causing under-tainting. To address this problem, our approach detects such gaps and bridges them by generating force propagation rules, which can eliminate the under-tainting. We implemented a prototype system with our approach called STAGER T. We built taint analysis frameworks for Python and VBScript with STAGER T and found that they could effectively analyze the data flow of real-world malicious scripts.
• Toshinori Usui, Yuto Otsuki, Tomonori Ikuse, Yuhei Kawakoya, Makoto Iwamura, Jun Miyoshi, Kanta Matsuura. Automatic Reverse Engineering of Script Engine Binaries for Building Script API Tracers, Digital Threats: Research and Practice, Vol.2, No.1, pp.1-31, 2021 [detail]

abstract

Script languages are designed to be easy-to-use and require low learning costs. These features provide attackers options to choose a script language for developing their malicious scripts. This diversity of choice in the attacker side unexpectedly imposes a significant cost on the preparation for analysis tools in the defense side. That is, we have to prepare for multiple script languages to analyze malicious scripts written in them. We call this unbalanced cost for script languages asymmetry problem. To solve this problem, we propose a method for automatically detecting the hook and tap points in a script engine binary that is essential for building a script Application Programming Interface (API) tracer. Our method allows us to reduce the cost of reverse engineering of a script engine binary, which is the largest portion of the development of a script API tracer, and build a script API tracer for a script language with minimum manual intervention. This advantage results in solving the asymmetry problem. The experimental results showed that our method generated the script API tracers for the three script languages popular among attackers (Visual Basic for Applications (VBA), Microsoft Visual Basic Scripting Edition (VBScript), and PowerShell). The results also demonstrated that these script API tracers successfully analyzed real-world malicious scripts.
• Toshinori Usui, Yuto Otsuki, Yuhei Kawakoya, Makoto Iwamura, Jun Miyoshi, Kanta Matsuura. My Script Engines Know What You Did In The Dark: Converting Engines into Script API Tracers, Proceedings of the 35th Annual Computer Security Applications Conference (ACSAC '19), pp.466-477, 2019 [detail]

abstract

Malicious scripts have been crucial attack vectors in recent attacks such as malware spam (malspam) and fileless malware. Since malicious scripts are generally obfuscated, statically analyzing them is difficult due to reflections. Therefore, dynamic analysis, which is not affected by obfuscation, is used for malicious script analysis. However, despite its wide adoption, some problems remain unsolved. Current designs of script analysis tools do not fulfill the following three requirements important for malicious script analysis. (1) Universally applicable to various script languages, (2) capable of outputting analysis logs that can precisely recover the behavior of malicious scripts, and (3) applicable to proprietary script engines. In this paper, we propose a method for automatically generating script API tracer by analyzing the target script engine binaries. The method mine the knowledge of script engine internals that are required to append behavior analysis capability. This enables the addition of analysis functionalities to arbitrary script engines and generation of script API tracers that can fulfill the above requirements. Experimental results showed that we can apply this method for building malicious script analysis tools.
• Toshinori Usui, Tomonori Ikuse, Yuto Otsuki, Yuhei Kawakoya, Makoto Iwamura, Jun Miyoshi, Kanta Matsuura. ROPminer: Learning-based Static Detection of ROP Chain Considering Linkability of ROP Gadgets, IEICE Transactions on Information and Systems, Vol.E103-D, No.7, pp.1-17, 2020 [detail]

abstract

Return-oriented programming (ROP) has been crucial for attackers to evade the security mechanisms of recent operating systems. Although existing ROP detection approaches mainly focus on host-based intrusion detection systems (HIDSes), network-based intrusion detection systems (NIDSes) are also desired to protect various hosts including IoT devices on the network. However, existing approaches are not enough for network-level protection due to two problems: (1) Dynamic approaches take the time with second- or minute-order on average for inspection. For applying to NIDSes, millisecond-order is required to achieve near real time detection. (2) Static approaches generates false positives because they use heuristic patterns. For applying to NIDSes, false positives should be minimized to suppress false alarms. In this paper, we propose a method for statically detecting ROP chains in malicious data by learning the target libraries (i.e., the libraries that are used for ROP gadgets). Our method accelerates its inspection by exhaustively collecting feasible ROP gadgets in the target libraries and learning them separated from the inspection step. In addition, we reduce false positives inevitable for existing static inspection by statically verifying whether a suspicious byte sequence can link properly when they are executed as a ROP chain. Experimental results showed that our method has achieved millisecond-order ROP chain detection with high precision.

Kensuke Tamura

• Associate Research Fellow

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.

Daisuke Sumida

• Associate Research Fellow

Interest

• Digital forensics

Shuntarou Miyazato

• Master Course

Interest

• Blockchain, Attack detection

Shi Weizhi

• Research Student*

Interest

• Blockchain

Takahiro Nagamine

• Master Course

Interest

• Blockchain

Publications

• Takahiro Nagamine, Kanta Matsuura. A New Protocol for Fair Addition of a Transaction Fee When Closing a Payment Channel Uncooperatively, 24th International Conference on Financial Cryptography and Data Security (FC'20), Poster presentation, 2020 [detail]

abstract

A technique called transaction replacement using timelocks is used in payment channels for Bitcoin. When closing a payment channel uncooperatively, the latest time-locked transaction is broadcasted to the Bitcoin network. However, if the Bitcoin network is crowded, the latest one with a lower fee might not be added to a block preferentially, and hence the transaction replacement might fail. This problem can be solved by adding a fee to the latest one (e.g. using SIGHASH_ANYONECANPAY). However, it is difficult to divide the additional fee cooperatively because this scenario happens in the uncooperative case. We propose a protocol that allows the transaction fee added by a single party to be divided equally between two parties. In this protocol, each party deposits funds for the additional fee to the payment channel in advance. A party can add the transaction fee alone by creating a child transaction referring to the funds (Child Pays for Parent). Then, the remains of the funds are returned to each party on two outputs of the child transaction. Regarding these two outputs, one party decides the values of outputs, and the other has a right to choose either output. As a result, a party who decides the values is motivated to specify the same value.

Huang Ke

• Master Course

Interest

• Cryptography

Masahito Ishizaka

• Doctor Course

Interest

• Cryptography

Publications

• Masahito Ishizaka, Kanta Matsuura. Identity-Based Encryption Resilient to Auxiliary Leakage under the Decisional Linear Assumption, Lecture Notes in Computer Science (The 17th International Conference on Cryptology and Network Security: CANS2018), Vol.11124, pp.417-439, 2018 [detail]

abstract

Leakage-resilience guarantees that even if some information about the secret key is partially leaked, the security is maintained. Several security models considering leakage-resilience have been proposed. Among them, auxiliary leakage model proposed by Dodis et al. in STOC'09 is especially important, since it can deal with a leakage caused by a function which information-theoretically reveals the secret key, e.g., one-way permutation. Contribution of this work is two-fold. Firstly, we propose an identity based encryption (IBE) scheme and prove that it is fully secure and resilient to the auxiliary leakage under the decisional linear assumption in the standard model. Secondly, although the IBE scheme proposed by Yuen et al. in Eurocrypt'12 has been considered to be the only IBE scheme resilient to auxiliary leakage, we prove that the security proof for the IBE scheme is defective. We insist that our IBE scheme is the only IBE scheme resilient to auxiliary leakage.
• Masahito Ishizaka, Kanta Matsuura. Strongly Unforgeable Signature Resilient to Polynomially Hard-to-Invert Leakage under Standard Assumptions, Lecture Notes in Computer Science (The 21st Information Security Conference), Vol.11060, pp.422-441, 2018 [detail]

abstract

A signature scheme is said to be weakly unforgeable, if it is hard to forge a signature on a message not signed before. A signature scheme is said to be strongly unforgeable, if it is hard to forge a signature on any message. In some applications, the weak unforgeability is not enough and the strong unforgeability is required, e.g., the Canetti, Halevi and Katz transformation. Leakage-resilience is a property which guarantees that even if secret information such as the secret-key is partially leaked, the security is maintained. Some security models with leakage-resilience have been proposed. The auxiliary (input) leakage model, or hard-to-invert leakage model, proposed by Dodis et al. in STOC'09 is especially meaningful one, since the leakage caused by a function which information-theoretically reveals the secret-key, e.g., one-way permutation, is considered. In this work, we propose a generic construction of a signature scheme strongly unforgeable and resilient to polynomially hard-to-invert leakage which can be instantiated under standard assumptions such as the decisional linear assumption. We emphasize that our signature scheme is not only the first one resilient to polynomially hard-to-invert leakage under standard assumptions, but also the first one which is strongly unforgeable and has hard-to-invert leakage-resilience.
• Masahito Ishizaka, Satsuya Ohata, Kanta Matsuura. Generic Construction of Ciphertext-Policy Attribute-Based Signcryption Secure in the Adaptive Predicate Model, IPSI Transactions on Advanced Research, Special issue - "Advances in Cryptology and Information Security", Vol.12, No.2, pp.16-26, 2016 [detail]

abstract

Ciphertext-policy attribute-based signcryption (CP-ABSC) is a cryptographic primitive which performs simultaneously both the functionalities of ciphertext-policy attribute-based encryption and signature-policy attribute-based signature. CP-ABSC guarantees both message confidentiality and authenticity and is considered to be a useful tool for fine-grained data access control in attribute-based environments such as a cloud service. In this paper, we provide a generic construction of CP-ABSC which achieves ciphertext indistinguishability under adaptively chosen ciphertext attacks in the adaptive predicate model (AP-IND-CCA), strongly existentially unforgeability of signcryptext under adaptively chosen message attacks in the adaptive predicate model (AP-sEUF-CMA) and perfect privacy. Our generic construction uses as building blocks, ciphertext-policy attribute-based key encapsulation mechanism, signature-policy attribute-based signature and data encapsulation mechanism.

Arnaud PIRIOU

• Exchange Student

Interest

• Cryptography, Steganography

Yuya Senzaki

• Master Course

Interest

• Machine Learning, Malware

Publications

• Yuya Senzaki, Satsuya Ohata, Kanta Matsuura. Simple Black-box Adversarial Examples Generation with Very Few Queries, IEICE Transactions on Information and Systems, Vol.E103-D, No.2, pp.212-221, 2020 [detail]

abstract

Research on adversarial examples for machine learning has received much attention in recent years. Most of previous approaches are white-box attacks; this means the attacker needs to obtain before-hand internal parameters of a target classifier to generate adversarial examples for it. This condition is hard to satisfy in practice. There is also research on black-box attacks, in which the attacker can only obtain partial information about target classifiers; however, it seems we can prevent these attacks, since they need to issue many suspicious queries to the target classifier. In this paper, we show that a naive defense strategy based on surveillance of number query will not suffice. More concretely, we propose to generate not pixel-wise but block-wise adversarial perturbations to reduce the num ber of queries. Our experiments show that such rough perturbations can confuse the target classifier. We succeed in reducing the number of queries to generate adversarial examples in most cases. Our simple method is an untargeted attack and may have low success rates compared to previous results of other black-box attacks, but needs in average fewer queries. Surprisingly, the minimum number of queries (one and three in MNIST and CIFAR-10 dataset, respectively) is enough to generate adversarial examples in some cases. Moreover, based on these results, we propose a detailed classification for black-box attackers and discuss countermeasures against the above attacks.

Takemasa Imada

• Master Course

Interest

• Public Blockchain

Akira Takenouchi

• Master Course 2

Interest

• Anonymous Communication

Shogo Hayashi

• Master Course 2

Interest

• WEB Application Security

Sun Da

• Master Course 2

Interest

• Computer Security, Network Security

Satsuya Ohata

• Doctor Course

Interest

• Cryptography

Publications

• Satsuya Ohata, Takahiro Matsuda, Kanta Matsuura. On Rigorous Security of Password Recovery Protocols, The Tenth International Workshop on Security (IWSEC2015), Poster Session, 2015 [detail]

abstract

Many online services adopt a password-based user authentication system because of its usability. However, several problems have been pointed out on it, and one of the well-known problems is that a user forgets his/her password and cannot login the services. To solve this problem, most online services support a mechanism with which a user can recover a password. In this poster, we discuss rigorous security for a password recovery protocol.
• Satsuya Ohata, Yutaka Kawai, Takahiro Matsuda, Goichiro Hanaoka, Kanta Matsuura. Re-encryption Verifiability: How to Detect Malicious Activities of a Proxy in Proxy Re-encryption, Lecture Notes in Computer Science (Topics in Cryptology: CT-RSA2015), Vol.9048, pp.410-428, 2015 [detail]

abstract

In this paper, we introduce a new functionality for proxy re-encryption (PRE) that we call re-encryption verifiability. In a PRE scheme with re-encryption verifiability (which we simply call verifiable PRE, or VPRE), a receiver of a re-encrypted ciphertext can verify whether the received ciphertext is correctly transformed from an original ciphertext by a proxy, and thus can detect illegal activities of the proxy. We formalize the security model for a VPRE scheme, and show that the single-hop uni-directional PRE scheme by Hanaoka et al. (CT-RSA 2012) can be extended to a secure VPRE scheme.
• Satsuya Ohata. On the Key Re-splittability of Threshold Public Key Encryption, 2014
• Satsuya Ohata, Takahiro Matsuda, Goichiro Hanaoka, Kanta Matsuura. More Constructions of Re-splittable Threshold Public Key Encryption, Lecture Notes in Computer Science (Advances in Information and Computer Security, The 9th International Workshop on Security: IWSEC2014), Vol.8639, pp.109-118, 2014 [detail]

abstract

The concept of threshold public key encryption (TPKE) with the special property called key re-splittability (re-splittable TPKE, for short) was introduced by Hanaoka et al.(CT-RSA 2012), and used as one of the building blocks for constructing their proxy re-encryption scheme. In a re-splittable TPKE scheme, a secret key can be split into a set of secret key shares not only once, but also multiple times, and the security of the TPKE scheme is guaranteed as long as the number of corrupted secret key shares under the same splitting is smaller than the threshold. In this paper, we show several new constructions of re-splittable TPKE scheme by extending the previous (ordinary) TPKE schemes. Our results suggest that key re-splittability is a very natural property for TPKE.
• Satsuya Ohata, Takahiro Matsuda, Goichiro Hanaoka, Kanta Matsuura. More Constructions of Re-Splittable Threshold Public Key Encryption, IEICE Transactions on Fundamentals of Electronics, Communications and Computer Sciences, Vol.E101-A, No.9, pp.1473-1483, 2018 [detail]

abstract

The concept of threshold public key encryption (TPKE) with the special property called key re-splittability (re-splittable TPKE, for short) was introduced by Hanaoka et al. (CT-RSA 2012), and used as one of the building blocks for constructing their proxy re-encryption scheme. In a re-splittable TPKE scheme, a secret key can be split into a set of secret key shares not only once, but also multiple times, and the security of the TPKE scheme is guaranteed as long as the number of corrupted secret key shares under the same splitting is smaller than the threshold. In this paper, we show several new constructions of re-splittable TPKE scheme by extending the previous (ordinary) TPKE schemes.

Kenjiro Nakata

• Master Course

Interest

• Anonymous Communication

Shiori Shinoda

• Master Course

Interest

• Security Management

Tobias Fuchs

• Visiting student

Interest

• Physical Unclonable Function, Anonymous Communication

Bongkot Jenjarrussakul

• Doctor Course

Interest

• Security Management

Publications

• Bongkot Jenjarrussakul, Kanta Matsuura. Analysis of Japanese Loyalty Programs Considering Liquidity, Security Efforts, and Actual Security Levels, 13th Workshop on the Economics of Information Security (WEIS2014), on web, 2014 [detail]

abstract

Virtual currency is an important medium of exchange in cyber space, and loyalty program (LP) can be considered as a type of virtual currency. In the U.S., according to a report in COLLOQUY talk[5], the total number of LP memberships is more than 2.6 billion in 2012 after 26.7% growth from 2010. In addition, the number of LPs is also reported to show a clear increasing trend. LPs are very popular in Japan, too; there are more than 200 LPs in Japan and the use of them is widespread among Japanese people. People collect their LP points and redeem them to obtain goods and enjoy services. In addition, many LP points can be converted into points of different LPs. LPs in Japan are thus increasing redemption options, and getting more and more popular and liquid virtual currencies. This situation can motivate malicious people to abuse such increasingly useful LPs for crimes, and in fact, there are some reports of such crimes. However, the security issues of LPs have not been well studied. In this paper, we investigate Japanese LPs with focuses on their liquidity, their operating firms' security efforts, and the LP systems' actual security levels.
• Bongkot Jenjarrussakul, Hideyuki Tanaka, Kanta Matsuura. Sectoral and Regional Interdependency of Japanese Firms under the Influence of Information Security Risks, 11th Workshop on the Economics of Information Security (WEIS2012), on web, 2012 [detail]

abstract

Although there are some studies on inter-sectoral information security interdependency, the lack of regional interdependency analysis is one of their limitations. In this empirical study, we used an inter-regional input-output table in order to analyze both sectoral and regional interdependencies under the influence of information technology and information security of Japanese firms. Our analysis showed that the economic scale of a region has a great influence on the characteristics of the interdependency. Furthermore, we found that the demand-side sectors can be classified into five classes based on the characteristics. Among them, the groups with high self-dependency get more benefits from simultaneous understanding of regional characteristics; for the sectors in these classes, investment advice obtained from sectoral characteristics only is very limited, whereas they can obtain much more from regional characteristics. Since these classes include a majority of the sectors, we can recognize the importance of regional interdependency analysis. In the above basic study, what we see is the situation before The Great East Japan Earthquake on March 11, 2011. As an extended study, we estimated reductions in interdependency resulting from investment-dependent economic damages in order to study how security investment can change the impact of the earthquake. Our main finding in regional perspective is that the interdependency characteristics of the most damaged region (Tohoku) and of the economically largest region (Kanto) are impacted most significantly. Both in the basic study and in the extended study, we can see that considering not only sectoral but also regional characteristics is an effective approach to the task of empirically deriving implications related to the interdependency. There are many possibilities of more extended studies based on our methodology.

keywords

Interdependency, Backward dependency, Empirical analysis, Supply chain, Input-output table, The Great East Japan Earthquake
• Bongkot Jenjarrussakul, Hideyuki Tanaka, Kanta Matsuura. Impact on Information Security from the Great East Japan Earthquake on March 11, 2011, Eighth Annual Forum on Financial Information Systems and Cybersecurity: A Public Policy Perspective, 2012 [detail]

abstract

The Great East Japan Earthquake on March 11, 2011 introduced vast impact on supply-chain in Tohoku region. Although there are some reports regarding impact in economic viewpoint as well as information, communication, and telecommunication (ICT) viewpoint, non of them shows possibility about impact on information security. Here we simulate the possible effect from the impact of the earthquake. The methodology is applied to predict impact from the earthquake. With this concept, Japanese offcial statistical economic data as well as offcial data regarding information technology and information security are used. The results show that limited effect from the loss in IT-related capital stock due to the Great East Japan Earthquake likely affects some regions and industrial sectors such as other manufacturing(6) and services(12).
• Bongkot Jenjarrussakul, Hideyuki Tanaka, Kanta Matsuura. Empirical Study on Interdependency of Information Security Between Industrial Sectors and Regions, Seventh Annual Forum on Financial Information Systems and Cybersecurity: A Public Policy Perspective, 2011 [detail]

abstract

This paper broadens the concept of measurement methodology of information security interdependency in industrial sectoral perspective into industrial regional perspective to analyze inter-regional and inter-sectoral interdependency between specific industrial sectors and regions on demand-side perspective. Previous study of cross-sectoral information security interdependency demonstrated that different industrial sectors is one of the factors that affect interdependency in information security. Nevertheless, regional interdependency analysis was one of their limitations. In our implementation, we apply methodology to the statistical economic data of Japanese industrial sectors separated into regions in order to show their information security interdependency influenced by information technology and the level of information security measure.

Feng Fei

• Master Course

Interest

• Anonymous Communication

Toshinori Usui

• Master Course

Interest

• Anti-malware, Anomaly Detection

Keisuke Katayama

• Research Student

Interest

• Biometrics

Takao Murakami

• Doctor Course

Interest

• Biometrics

Publications

• Kenta Takahashi, Takao Murakami. A Measure of Information Gained through Biometric Systems, Elsevier Image and Vision Computing, Vol.32, No.12, pp.1194-1203, 2014 [detail]

abstract

We propose a measure of information gained through biometric matching systems. Firstly, we discuss how the information about the identity of a person is derived from biometric samples through a biometric system, and define the "biometric system entropy" or BSE based on mutual information. We present several theoretical properties and interpretations of the BSE, and show how to design a biometric system which maximizes the BSE. Then we prove that the BSE can be approximated asymptotically by the relative entropy D(fG(x) || fI(x)) where fG(x) and fI(x) are probability mass functions of matching scores between samples from individuals and among population. We also discuss how to evaluate the BSE of a biometric systemand show experimental evaluation of the BSE of face, fingerprint and multimodal biometric systems.
• Takao Murakami, Kenta Takahashi, Kanta Matsuura. A General Framework and Algorithms for Score Level Indexing and Fusion in Biometric Identification, IEICE Transactions on Information and Systems, Vol.E97-D, No.3, pp.510-523, 2014 [detail]

abstract

Biometric identification has recently attracted attention because of its convenience: it does not require a user ID nor a smart card. However, both the identification error rate and response time increase as the number of enrollees increases. In this paper, we combine a score level fusion scheme and a metric space indexing scheme to improve the accuracy and response time in biometric identification, using only scores as information sources. We firstly propose a score level indexing and fusion framework which can be constructed from the following three schemes: (I) a pseudo-score based indexing scheme, (II) a multi-biometric search scheme, and (III) a score level fusion scheme which handles missing scores. A multi-biometric search scheme can be newly obtained by applying a pseudo-score based indexing scheme to multi-biometric identification. We secondly propose the NBS (Naive Bayes search) scheme as a multi-biometric search scheme and discuss its optimality with respect to the retrieval error rate. We evaluated our proposal using the datasets of multiple fingerprints and face scores from multiple matchers. The results showed that our proposal significantly improved the accuracy of the unimodal biometrics while reducing the average number of score computations in both the datasets.
• Takao Murakami, Kenta Takahashi, Kanta Matsuura. Toward Optimal Fusion Algorithms with Security against Wolves and Lambs in Biometrics, IEEE Transactions on Information Forensics and Security (IEEE TIFS), Vol.9, No.2, pp.259-271, 2014 [detail]

abstract

It is known that different users have different degrees of accuracy in biometric authentication, and claimants and enrollees who cause false accepts against many others are referred to as wolves and lambs, respectively. The aim of this paper is to develop a fusion algorithm, which has security against both of the animals while minimizing the number of query samples a genuine claimant has to input. To achieve our aim, we first introduce a taxonomy of wolves and lambs, and propose a minimum log-likelihood ratio-based sequential fusion scheme (MLR scheme). We prove that this scheme keeps wolf attack probability and lamb accept probability, the maximum of the claimant-specific false accept probability (FAP), and the enrolleespecific FAP, less than a desired value if log-likelihood ratios are perfectly estimated, except in the case of adaptive spoofing wolves. We also prove that this scheme is optimal with regard to false reject probability (FRP), and asymptotically optimal with respect to the average number of inputs (ANIs) under some conditions. We further propose an input order decision scheme based on the Kullback.Leibler (KL) divergence, which maximizes the expectation of a genuine log-likelihood ratio, to further reduce ANI of the MLR scheme in the case where the KL divergence differs from one modality to another. The results of the experimental evaluation using a virtual multimodal (one face and eight fingerprints) data set showed the effectiveness of our schemes.
• Takao Murakami, Kenta Takahashi, Susumu Serita, Yasuhiro Fujii. Probabilistic Enhancement of Approximate Indexing in Metric Spaces, Elsevier Information Systems, Vol.38, No.7, pp.1007-1018, 2013 [detail]

abstract

Some approximate indexing schemes have been recently proposed in metric spaces which sort the objects in the database according to pseudo-scores. It is known that (1) some of them provide a very good trade-off between response time and accuracy, and (2) probability-based pseudo-scores can provide an optimal trade-off in range queries if the probabilities are correctly estimated. Based on these facts, we propose a probabilistic enhancement scheme which can be applied to any pseudo-score based scheme. Our scheme computes probability-based pseudo-scores using pseudo-scores obtained from a pseudo-score based scheme. In order to estimate the probability-based pseudoscores, we use the object-specific parameters in logistic regression and learn the parameters using MAP (Maximum a Posteriori) estimation and the empirical Bayes method. We also propose a technique which speeds up learning the parameters using pseudo-scores. We applied our scheme to the two state-of-the-art schemes: the standard pivot-based scheme and the permutation-based scheme, and evaluated them using various kinds of datasets from the Metric Space Library. The results showed that our scheme outperformed the conventional schemes, with regard to both the number of distance computations and the CPU time, in all the datasets.
• Takao Murakami, Kenta Takahashi, Kanta Matsuura. Towards Optimal Countermeasures against Wolves and Lambs in Biometrics, Proceedings of the IEEE Fifth International Conference on Biometrics: Theory, Applications and Systems (BTAS 2012), 2012 [detail]

abstract

Claimants and enrollees who have high similarity scores against many others are referred to as wolves and lambs, respectively. These animals can cause false accepts against many others and compromise the security of the system. The aim of this paper is to develop a fusion algorithm which has security against both of them while minimizing the number of query samples the genuine claimant has to input. To achieve this aim, we first propose a minimum log-likelihood ratio based sequential fusion scheme (MLR scheme). We prove that the MLR scheme keeps WAP (Wolf Attack Probability) and LAP (Lamb Accept Probability), the maximum of the claimant-specific FAR and the enrollee-specific FAR, less than the desired value if the score distributions are perfectly estimated, except in the case of adaptive spoofing wolves. We also prove that the MLR scheme is nearly optimal with respect to the average number of inputs (ANI) of the genuine claimant under some conditions. We secondly propose an input order decision scheme based on the KL (Kullback-Leibler) divergence which maximizes the expectation of the genuine loglikelihood ratio, to further reduce ANI of the MLR scheme in the case where the KL divergence differs from one modality to another. The results of the experimental evaluation using the CASIA-FingerprintV5 showed the effectiveness of our schemes.
• Takao Murakami, Kenta Takahashi. Fast and Accurate Biometric Identification Using Score Level Indexing and Fusion, Proceedings of IEEE/IAPR International Joint Conference on Biometrics (IJCB 2011), CD-ROM, 2011 [detail]

abstract

Biometric identification provides a very convenient way to authenticate a user because it does not require the user to claim an identity. However, both the identification error rates and the response time increase almost in proportion to the number of enrollees. A technique which decreases both of them using only scores has the advantage that it can be applied to any kind of biometric system that outputs scores. In this paper, we propose such a technique by combining score level fusion and distance-based indexing. In order to reduce the retrieval error rate in multibiometric identification, our technique takes a strategy to select the template of the enrollee whose posterior probability of being identical to the claimant is the highest as a next to be matched. The experimental evaluation using the Biosecure DS2 dataset and the CASIA-FingerprintV5 showed that our technique significantly reduced the identification error rates while keeping down or even reducing the number of score calculations, compared to the unimodal biometrics.
• Takao Murakami, Kenta Takahashi, Susumu Serita, Yasuhiro Fujii. Versatile Probability-based Indexing for Approximate Similarity Search, Proceedings of 4th International Conference on SImilarity Search and APplications (SISAP 2011), pp.51-58, 2011 [detail]

abstract

We aim at reducing the number of distance computations as much as possible in the inexact indexing schemes which sort the objects according to some promise values. To achieve this aim, we propose a new probability-based indexing scheme which can be applied to any inexact indexing scheme that uses the promise values. Our scheme (1) uses the promise values obtained from any inexact scheme to compute the new probability-based promise values. In order to estimate the new promise values, we (2) use the object-specific parameters in logistic regression and learn the parameters using MAP (Maximum a Posteriori) estimation. We also propose a technique which (3) speeds up learning the parameters using the promise values. We applied our scheme to the standard pivot-based scheme and the permutation-based scheme, and evaluated them using various kinds of datasets from the Metric Space Library. The results showed that our scheme improved the conventional schemes, in all cases.

Kenichi Yokote

• Master Course

Interest

• Network Security, Software Security

Teppei Takagi

• Master Course

Interest

• Network Security, Machine Learning

Andreas Gutmann

• Visiting student

Interest

• Asymmetric Cryptography, Provable Security

Ken Ichikawa

• Master Course

Interest

• Network Security, Software Security

Publications

• Ken Ichikawa, Kanta Matsuura. Preventing execution of JIT shellcode by isolating running process, Annual Computer Security Applications Conference 2011, 2011

Takahiro Matsuda

• Doctor Course

Interest

• Cryptography, Provable Security, Computer Security

Publications

• Takao Murakami, Kenta Takahashi, Susumu Serita, Yasuhiro Fujii. Probabilistic Enhancement of Approximate Indexing in Metric Spaces, Elsevier Information Systems, 2012 [detail]

abstract

Some approximateindexing schemes have been recently proposed in metricspaces which sort the objects according to pseudo scores. It is known that (1) some of them provide a very good trade-off between response time and accuracy, and (2) the probability-based pseudo scores can provide an optimal trade-off in range queries if the probability is correctly estimated. Based on these facts, we propose a probabilistic enhancement scheme which can be applied to any approximateindexing scheme that computes a pseudo score for each object. Our scheme uses the pseudo scores obtained from the approximate scheme to compute the new probability-based pseudo scores. In order to estimate the new pseudo scores, we use the object-specific parameters in logistic regression and learn the parameters using MAP (Maximum a Posteriori) estimation and the empirical Bayes method. We also propose a technique which speeds up learning the parameters using the pseudo scores. We applied our scheme to the two state-of-the-art schemes: the standard pivot-based scheme and the permutation-based scheme, and evaluated them using various kinds of datasets from the MetricSpace Library. The results showed that our scheme outperformed the conventional schemes in all the datasets.
• Takahiro Matsuda, Goichiro Hanaoka, Kanta Matsuura. Relations between Constrained and Bounded Chosen Ciphertext Security for Key Encapsulation Mechanisms, Lecture Notes in Computer Science (Public Key Cryptography - PKC 2012, 15th International Conference on Practice and Theory in Public Key Cryptography: PKC 2012), Vol.7293, pp.576-594, 2012 [detail]

abstract

In CRYPTO 2007, Hofheinz and Kiltz formalized a security notion for key encapsulation mechanisms (KEMs), called constrained chosen ciphertext (CCCA) security, which is strictly weaker than ordinary chosen ciphertext (CCA) security, and showed a new composition paradigm for CCA secure hybrid encryption. Thus, CCCA security of a KEM turned out to be quite useful. However, since the notion is relatively new and its definition is slightly complicated, relations among CCCA security and other security notions have not been clarified well. In this paper, in order to better understand CCCA security and the construction of CCCA secure KEMs, we study relations between CCCA and bounded CCA security, where the latter notion considers security against adversaries that make a-priori bounded number of decapsulation queries, and is also strictly weaker than CCA security. Specifically, we show that in most cases there are separations between these notions, while there is some unexpected implication from (a slightly stronger version of) CCCA security to a weak form of 1-bounded CCA security. We also revisit the construction of a KEM from a hash proof system (HPS) with computational security properties, and show that the HPS-based KEM, which was previously shown CCCA secure, is actually 1-bounded CCA secure as well. This result, together with the above general implication, suggests that 1-bounded CCA security can be essentially seen as a "necessary" condition for a CCCA secure KEM.
• Takahiro Matsuda, Kanta Matsuura. On Black-Box Separations among Injective One-Way Functions, Lecture Notes in Computer Science (Theory of Cryptography, 8th Theory of Cryptography Conference: TCC 2011), Vol.6597, pp.597-614, 2011
• Takahiro Matsuda, Kanta Matsuura. Parallel Decryption Queries in Bounded Chosen Ciphertext Attacks, Lecture Notes in Computer Science (Public Key Cryptography - PKC 2011, 14th International Conference on Practice and Theory in Public Key Cryptography: PKC 2011), Vol.6571, pp.246-264, 2011 [detail]

abstract

Whether it is possible to construct a chosen ciphertext secure (CCA secure) public key encryption (PKE) scheme only from a chosen plaintext secure (CPA secure) one is a well-known fundamental open problem, and the best known positive results regarding this problem are the construction of so-called bounded CCA secure schemes. Since we can achieve the best possible security in the bounded CCA security notions, in order to further tackle this problem, we would need other new security notions that capture intermediate security notions that lie between CPA and CCA security. Motivated by this situation, in order to provide a theoretical foundation for further tackling the above problem, we focus on ''parallel'' decryption queries (originally introduced by Bellare and Sahai[BS99]) for the extension of bounded CCA security, and introduce a new security notion which we call mixed CCA security. It captures the security against adversaries that make single and parallel decryption queries in a predetermined order, where each parallel query can contain unboundedly many ciphertexts. Moreover, how the decryption oracle is available before and after the challenge is also taken into account in this new security definition, which enables us to capture existing major security notions that lie between CPA and CCA security, including complex notion like non-malleability against bounded CCA, in a unified security notion. We investigate the relations among mixed CCA security notions, and show a necessary and sufficient condition regarding implications/separations between any given two notions in mixed CCA security. We then show two black-box constructions of PKE schemes from CPA secure ones, one of which satisfies strictly stronger security notions than the security notions achieved by the existing constructions of PKE schemes constructed only from a CPA secure one. We also discuss the consequences of our results regarding security with parallel decryption queries, give several observations, and leave some open problems.
• Takahiro Matsuda, Yasumasa Nakai, Kanta Matsuura. Efficient Generic Constructions of Timed-Release Encryption with Pre-open Capability, Lecture Notes in Computer Science (Pairing-Based Cryptography, 4th International Conference on Pairing-Based Cryptography: Pairing 2010), Vol.6487, pp.225-245, 2010 [detail]

abstract

Timed-release encryption with pre-open capability (TRE-PC), introduced by Hwang et al.[HYL05], is a cryptosystem with which a sender can make a ciphertext so that a receiver can decrypt it by using a timed-release key provided from a trusted agent called time-server at a predetermined release-time, or by using a special information called pre-open key provided from the sender before the release-time, and thus adds flexibility to ordinary TRE schemes in many practical situations. Recently, Nakai et al.[NMKM09] proposed a generic construction of a TRE-PC scheme from a public-key encryption scheme, an identity-based encryption scheme (with some special property), and a signature scheme. Although their construction enables us to construct many TRE-PC schemes based on existing primitives, Concrete TRE-PC schemes derived via their generic construction are, however, not so practical because of the used building block primitives. Motivated by this situation, in this paper we propose two new generic consructions of TRE-PC schemes. Both of our generic constructions employ the KEM/DEM structure, are quite generic in the sense that they enable us to construct TRE-PC schemes from a combination of ordinary KEMs, identity-based KEMs, and symmetric-key primitives, and do not use a one-time signature scheme. Our first construction utilizes a [tag-KEM] as one of main building blocks, which has been widely studied since it was introduced by Abe et al.[AGK08] and can also be built from any ordinary KEM, and is suitable for constructing standard model TRE-PC schemes. Our second construction uses a random oracle, and is suitable for constructing TRE-PC schemes based on existing building block primitives that already use random oracles. Concrete TRE-PC schemes derived from our generic constructions are comparable to or more efficient than the currently known TRE-PC schemes in terms of ciphertext overhead size and computation costs.
• Takahiro Matsuda, Kanta Matsuura, Jacob C. N. Schuldt. Efficient Constructions of Signcryption Schemes and Signcryption Composability, Lecture Notes in Computer Science (Progress in Cryptology, 10th International Conference on Cryptology in India: INDOCRYPT 2009), Vol.5922, pp.321-342, 2009 [detail]

abstract

In this paper, we investigate simple, but efficient constructions of signcryption schemes. Firstly, we show how symmetric primitives can be used to efficiently achieve outsider security, leading to a signcryption scheme with the currently lowest ciphertext and computational overhead. This scheme provides outsider multi-user security. For the mixed security notions outsider confidentiality/insider unforgeability and insider confidentiality/outsider unforgeability, this approach yields lower ciphertext overhead and a higher level of security, respectively, compared to the current schemes. Secondly, we show a simple optimization to the well known ``sign-then-encrypt'' and ``encrypt-then-sign'' approaches to the construction of signcryption schemes, by using tag-based encryption. Instantiations with our proposed tag-based schemes yield multi-user insider secure signcryption schemes in the random oracle model which is at least as efficient as any other existing scheme both in terms of ciphertext overhead and computational cost. Furthermore, very efficient standard model signcryption schemes are achieved as well. Lastly, we show how signatures and encryption can be combined in a non-black-box manner to achieve higher efficiency than schemes based on the above approach. We refer to signature and encryption schemes which can be combined in this way as signcryption composable, and we show that a number of the most efficient standard model encryption and signature schemes satisfy this, leading to the most efficient standard model signcryption schemes. Since all of our constructions are fairly simple and efficient, they provide a benchmark which can be used to evaluate future signcryption schemes.
• Takahiro Matsuda, Goichiro Hanaoka, Kanta Matsuura, Hideki Imai. An Efficient Encapsulation Scheme from Near Collision Resistant Pseudorandom Generators and Its Application to IBE-to-PKE Transformations, Lecture Notes in Computer Science (Topics in Cryptology: CT-RSA2009), Vol.5473, pp.16-31, 2009 [detail]

abstract

In [BK05], Boneh and Katz introduced a primitive called encapsulation scheme, which is a special kind of commitment scheme. Using the encapsulation scheme, they improved the generic transformation by Canetti, Halevi, and Katz[CHK04] which transforms any semantically secure identity-based encryption (IBE) scheme into a chosen-ciphertext secure public key encryption (PKE) scheme (we call the BK transformation). The ciphertext size of the transformed PKE scheme directly depends on the parameter sizes of the underlying encapsulation scheme. In this paper, by designing a size-efficient encapsulation scheme, we further improve the BK transformation. With our proposed encapsulation scheme, the ciphertext overhead of a transformed PKE scheme via the BK transformation can be that of the underlying IBE scheme plus 384-bit, while the original BK scheme yields that of the underlying IBE scheme plus at least 704-bit, for 128-bit security. Our encapsulation scheme is constructed from a pseudorandom generator (PRG) that has a special property called near collision resistance, which is a fairly weak primitive. As evidence of it, we also address how to generically construct a PRG with such a property from any one-way permutation.
• Takahiro Matsuda, Goichiro Hanaoka, Kanta Matsuura, Hideki Imai. Simple CCA-Secure Public Key Encryption from Any Non-Malleable Identity-Based Encryption, Lecture Notes in Computer Science (The 11th International Conference on Information Security and Cryptology: ICISC2008), Vol.5461, pp.1-19, 2008 [detail]

abstract

In this paper, we present a simple and generic method for constructing public key encryption (PKE) secure against chosen ciphertext attacks (CCA) from identity-based encryption (IBE). Specifically, we show that a CCA-secure PKE scheme can be generically obtained by encrypting (m||r) under identity ``f(r)'' with the encryption algorithm of the given IBE scheme, assuming that the IBE scheme is non-malleable and f is one-way. In contrast to the previous generic methods (such as Canetti-Halevi-Katz), our method requires stronger security for the underlying IBE schemes, non-malleability, and thus cannot be seen as a direct improvement of the previous methods. However, once we have an IBE scheme which is proved (or can be assumed) to be non-malleable, we will have a PKE scheme via our simple method, and we believe that the simpleness of our proposed transformation itself is theoretically interesting. Our proof technique for security of the proposed scheme is also novel. In the security proof, we show how to deal with certain types of decryption queries which cannot be handled by straightforwardly using conventional techniques.
• Takahiro Matsuda, Nuttapong Attrapadung, Goichiro Hanaoka, Kanta Matsuura, Hideki Imai. A Strongly Unforgeable Signature under the CDH Assumption without Collision Resistant Hash Functions, IEICE Transactions on Information and Systems, Vol.E91-D, No.5, pp.1466-1476, 2008 [detail]

abstract

Unforgeability of digital signatures is closely related to the security of hash functions since hashing messages, such as hash-and-sign paradigm, is necessary in order to sign (arbitrarily) long messages. Recent successful collision finding attacks against practical hash functions would indicate that constructing practical collision resistant hash functions is difficult to achieve. Thus, it is worth considering to relax the requirement of collision resistance for hash functions that is used to hash messages in signature schemes. Currently, the most efficient strongly unforgeable signature scheme in the standard model which is based on the CDH assumption (in bilinear groups) is the Boneh-Shen-Waters (BSW) signature proposed in 2006. In their scheme, however, a collision resistant hash function is necessary to prove its security. In this paper, we construct a signature scheme which has the same properties as the BSW scheme but does not rely on collision resistant hash functions. Instead, we use a target collision resistant hash function, which is a strictly weaker primitive than a collision resistant hash function. Our scheme is, in terms of the signature size and the computational cost, as efficient as the BSW scheme.
• Takahiro Matsuda, Nuttapong Attrapadung, Goichiro Hanaoka, Kanta Matsuura, Hideki Imai. A CDH-Based Strongly Unforgeable Signature Without Collision Resistant Hash Function, Lecture Notes in Computer Science (Provable Security-First International Conference: ProvSec 2007), Vol.4784, pp.68-84, 2007 [detail]

abstract

Unforgeability of digital signatures is closely related to the security of hash functions since hashing messages, such as hash-and-sign paradigm, is necessary in order to sign (arbitrarily) long messages. Recent successful collision finding attacks against practical hash functions would indicate that constructing practical collision resistant hash functions is difficult to achieve. Thus, it is worth considering to relax the requirement of collision resistance for hash functions that is used to hash messages in signature schemes. Currently, the most efficient strongly unforgeable signature scheme in the standard model which is based on the CDH assumption (in bilinear groups) is the Boneh-Shen-Waters (BSW) signature proposed in 2006. In their scheme, however, a collision resistant hash function is necessary to prove its security. In this paper, we construct a signature scheme which has the same properties as the BSW scheme but does not rely on collision resistant hash functions. Instead, we use a target collision resistant hash function, which is a strictly weaker primitive than a collision resistant hash function. Our scheme is, in terms of the signature size and the computational cost, as efficient as the BSW scheme.

links

• Link (SpringerLink)
• Takahiro Matsuda, Goichiro Hanaoka, Kanta Matsuura, Hideki Imai. A Practical Provider Authentication System for Bidirectional Broadcast Service, Lecture Notes in Computer Science (11th International Conference on Knowledge-Based and Intelligent Information & Engineering Systems:KES 2007), Vol.4694, pp.967-974, 2007 [detail]

abstract

Several content distribution services via the Internet have been developed, and a number of bidirectional broadcasting services will be provided in the near future. Since such bidirectional broadcasting treats personal information of the users, provider authentication is necessary. Taking the currently existing broadcasting system using CAS cards into account, Ohtake et al. recently proposed the provider authentication system which utilizes key-insulated signature (KIS) schemes. However, the authors did not refer to details of what kind of KIS should be used. In this paper we supplement their works in terms of KIS specification. We carefully identify what kind of KIS should be used and propose concrete KIS schemes which realize both the reliability and the robustness required for the bidirectional broadcasting service.

links

• Link (SpringerLink)

Daiki Chiba

• Master Course

Interest

• Network Security, Cryptography

Publications

• Daiki Chiba, Takahiro Matsuda, Jacob C.N. Schuldt, Kanta Matsuura. Efficient Generic Constructions of Signcryption with Insider Security in the Multi-user Setting, Lecture Notes in Computer Science (9th International Conference on Applied Cryptography and Network Security: ACNS 2011), Vol.6715, pp.220-237, 2011

Takashi Kitagawa

• Project Researcher

Interest

• Information Security

Jacob Schuldt

• Doctor Course

Interest

• Cryptography, Provable Security

Publications

• Jacob C.N. Schuldt, Kanta Matsuura. On-line Non-transferable Signatures Revisited, Lecture Notes in Computer Science (Public Key Cryptography - PKC 2011, 14th International Conference on Practice and Theory in Public Key Cryptography: PKC 2011), Vol.6571, pp.369-386, 2011 [detail]

abstract

Undeniable signatures, introduced by Chaum and van Antwerpen, and designated confirmer signatures, introduced by Chaum, allow a signer to control the verifiability of his signatures by requiring a verifier to interact with the signer to verify a signature. An important security requirement for these types of signature schemes is nontransferability which informally guarantees that even though a verifier has confirmed the validity of a signature by interacting with the signer, he cannot prove this knowledge to a third party. Recently Liskov and Micali pointed out that the commonly used notion of non-transferability only guarantees security against an off-line attacker which cannot influence the verifier while he interacts with the signer, and that almost all previous schemes relying on interactive protocols are vulnerable to online attacks. To address this, Liskov and Micali formalized on-line nontransferable signatures which are resistant to on-line attacks, and proposed a generic construction based on a standard signature scheme and an encryption scheme. In this paper, we revisit on-line non-transferable signatures. Firstly, we extend the security model of Liskov and Micali to cover not only the sign protocol, but also the confirm and disavow protocols executed by the confirmer. Our security model furthermore considers the use of multiple (potentially corrupted or malicious) confirmers, and guarantees security against attacks related to the use of signer specific confirmer keys. We then present a new approach to the construction of on-line non-transferable signatures, and propose a new concrete construction which is provably secure in the standard model. Unlike the construction by Liskov and Micali, our construction does not require the signer to issue ``fake'' signatures to maintain security, and allows the confirmer to both confirm and disavow signatures. Lastly, our construction provides noticeably shorter signatures than the construction by Liskov and Micali.

links

• pdf
• Jacob Schuldt, Kanta Matsuura. Efficient Convertible Undeniable Signatures with Delegatable Verification, IEICE Transactions on Fundamentals of Electronics, Communications and Computer Sciences, Vol.94-A, No.1, pp.71-83, 2011
• Jacob C. N. Schuldt, Kanta Matsuura. An Efficient Convertible Undeniable Signature Scheme with Delegatable Verification, Lecture Notes in Computer Science (6th International Conference on Information Security Practice and Experience: ISPEC2010), Vol.6047, pp.276-293, 2010
• Jacob C. N. Schuldt, Kanta Matsuura, Kenneth G. Paterson. Proxy Signatures Secure Against Proxy Key Exposure, Lecture Notes in Computer Science (11th International Workshop on Practice and Theory in Public Key Cryptography : PKC'08), Vol.4939, pp.141-161, 2008 [detail]

abstract

We provide an enhanced security model for proxy signatures that captures a more realistic set of attacks than previous models of Boldyreva et al. and of Malkin et al. Our model is motivated by concrete attacks on existing schemes in scenarios in which proxy signatures are likely to be used. We provide a generic construction for proxy signatures secure in our enhanced model using sequential aggregate signatures; our construction provides a benchmark by which future specific constructions may be judged. Finally, we consider the extension of our model and constructions to the identity-based setting.
• James Birkett, Alexander W. Dent, Gregory Neven, Jacob C. N. Schuldt. Efficient Chosen-Ciphertext Secure Identity-Based Encryption with Wildcards, Lecture Notes in Computer Science (12th Australasian Conference on Information Security and Provacy: ACISP 2007), Vol.4586, pp.274-292, 2007 [detail]

abstract

We propose new instantiations of chosen-ciphertext secure identity-based encryption schemes with wildcards (WIBE). Our schemes outperform all existing alternatives in terms of efficiency as well as security. We achieve these results by extending the hybrid encryption (KEM?DEM) framework to the case of WIBE schemes. We propose and prove secure one generic construction in the random oracle model, and one direct construction in the standard model.

Yi SHI

• Master Course

Interest

• Anonymous Communication, Network Security

Publications

• Yi Shi, Kanta Matsuura. Fingerprinting Attack on the Tor Anonymity System, Lecture Notes in Computer Science (11th International Conference on Information and Communications Security: ICICS 2009), Vol.5927, pp.425-438, 2009 [detail]

abstract

We present a novel way to implement a fingerprinting attack against Onion Routing anonymity systems such as Tor. Our attack is a realistic threat in the sense that it can be mounted by nothing but controller of entrance routers; the required resource is very small. However, the conventional fingerprinting attack based on incoming traffic does not work straightforwardly against Tor due to its multiplex and quantized nature of traffic. By contrast, our novel attack can degrade this Tor's anonymity by a metric based on both incoming and outgoing packets. In addition, our method keeps the fingerprinting attack's advantage of being realistic in terms of the required small resource. Regarding more evaluation, the effectiveness of our method is discussed in a comprehensive manner: experimentally and theoretically. In order to enhance further studies and show the significance of our idea, we also discuss methods for defending against our attack and other applications of our idea.

Shaojing Fu

• Visiting Student

Interest

• Coding, Public Key Cryptography

Publications

• Shaojing Fu, Kanta Matsuura, Chao Li, Longjiang Qu. Construction of highly nonlinear resilient S-boxes with given degree, Designs, Codes and Cryptography, Vol.64, No.3, pp.241-253, 2012 [detail]

abstract

We provide two new construction methods for nonlinear resilient S-boxes with given degree. The first method is based on the use of linear error correcting codes together with highly nonlinear S-boxes. Given a [u,m, t+1] linear code where u = n-d-1, d > m, we showthat it is possible to construct (n,m, t, d) resilient S-boxes which have currently best known nonlinearity. Our second construction provides highly nonlinear (n,m, t, d) resilient S-boxes which do not have linear structure, then an improved version of this construction is given.

keywords

Cryptography, Linear code, Resiliency, Linear structure, Nonlinearity
• Shaojing Fu, Chao Li, Kanta Matsuura, Longjiang Qu. Construction of Even- variable Rotation Symmetric Boolean Functions with Maximum Algebraic Immunity, SCIENCE CHINA Information Sciences, Vol.56, No.3, pp.1-9, 2013 [detail]

abstract

Rotation symmetric Boolean functions (RSBFs) have been used as components of different cryptosystems. In this paper, we investigate n-variable (n even and n >= 12) RSBFs to achieve maximum algebraic immunity (AI), and provide a construction of RSBFs with maximum AI and nonlinearity. These functions have higher nonlinearity than the previously known nonlinearity of RSBFs with maximum AI. We also prove that our construction provides high algebraic degree in some case.

keywords

Boolean function, rotation symmetry, algebraic immunity, nonlinearity
• Shaojing Fu, Chao Li, Kanta Matsuura, Longjiang Qu. Blanced 2p-variable Rotation Symmetric Boolean Functions with Maximum Algebraic Immunity, Applied mathematical Letters, Vol.24, No.12, pp.2093-2096, 2011 [detail]

abstract

In this paper, we study the construction of Rotation Symmetric Boolean Functions (RSBFs) which achieve a maximum algebraic immunity (AI). For the first time, a construction of balanced 2p-variable (p is an odd prime) RSBFs with maximum AI was provided, and the nonlinearity of the constructed RSBFs is not less than 2^(2p-1)-(2p-1)C(p)+(p-2)(p-3)+2; this nonlinearity result is significantly higher than the previously best known nonlinearity of RSBFs with maximum AI.

keywords

Stream cipher, Rotation symmetry, Boolean function, Algebraic immunity
• Shaojing Fu, Chao Li, Kanta Matsuura, Longjiang Qu.. Construction of Odd- variable Resilient Boolean Functions with Optimal Degree, IEICE Transactions on Fundamentals of Electronics, Communications and Computer Sciences, Vol.VE94-A, pp.265-267, 2011 [detail]

abstract

Constructing degree-optimized resilient Boolean functions with high nonlinearity is a significant study area in Boolean function. In this letter, we provide a construction of degree-optimized n-variable (n odd and n>=35) resilient Boolean functions, and it is shown that the resultant functions achieve the currently best known nonlinearity.

keywords

stream cipher, boolean function, resiliency, nonlinearity
• Shaojing Fu, Chao Li, Kanta Matsuura, Longjiang Qu. Enumeration of Balanced Symmetric Functions over GF(p), Information Processing Letters, Vol.110, pp.544-548, 2010
• Shaojing Fu, Kanta Matsuura, Chao Li, Longjiang Qu. Construction of Rotation Symmetric Boolean Functions with Maximum Algebraic Immunity, Lecture Notes in Computer Science (The 8th International Conference on Cryptology and Network Security: CANS 2009), Vol.5888, pp.402-412, 2009

Peng YANG

• Doctor Course

Interest

• Information security, encryption schemes with high functionalities, security notinos.

Publications

• Peng Yang, Kanta Matsuura. An Introduction of A Users' Guideline to Japan Cryptographic Module Validation Program, ASIACCS 2010 (5th ACM Symposium on Information, Computer and Communications Security), 2010
• Peng Yang, Rui Zhang, Kanta Matsuura, Hideki Imai. Generic Construction of Stateful Identity Based Encryption, Lecture Notes in Computer Science (Information Security, 12th International Conference: ISC2009), Vol.5735, pp.338-346, 2009
• Peng Yang, Kanta Matsuura. A Forward Secure Identity Based Encryption Scheme with Master Key Update, Proceedings of 2008 International Symposium on Information Theory and its Applications (ISITA2008), CD-ROM, 2008 [detail]

abstract

We propose an identity based encryption scheme with forward security. Especially in our scheme, the top secret, called the master key, evolves through time. Our scheme is provably secure in the sense of FS-IND-ID-CPA based on DBDH assumption in standard model.

keywords

Foward Security, identity based encryption, master key update.
• Peng Yang, Kanta Matsuura. Stateful Public Key Encryption: How to Remove Gap Assumptions and Maintaining Tight Reductions, Proceedings of 2008 International Symposium on Information Theory and its Applications (ISITA2008), CD-ROM, 2008 [detail]

abstract

Stateful public key encryption schemes are introduced recently with much efficiency improvement over traditional stateless schemes. However, previous proposals are either based on strong assumptions, or admitting loose security reductions (a barrier for the proofs being practically-meaningful). In this paper, we present a stateful public key encryption scheme with tight security reduction to the computational Diffie Hellman assumption (cf. gap Diffie-Hellman), as well as a stateful identity based encryption scheme with tighter security reduction to the computational bilinear Diffie-Hellman problem.

keywords

Stateful public key encryption, security reduction.
• Takashi Kitagawa, Peng Yang, Goichro Hanaoka, Rui Zhang, Hajime Watanabe, Kanta Matsuura, Hideki Imai. Generic Transforms to Acquire CCA-Security for Identity Based Encryptions: The Cases of FOpkc and REACT, Lecture Notes in Computer Science (11th Australasian Conference on Information Security and Provacy: ACISP 2006), Vol.4058, pp.348-359, 2006 [detail]

abstract

Fujisaki-Okamoto (FOpkc) conversion [13] and REACT[17] are widely known to be able to generically convert a weak public key encryption scheme to a strong encryption scheme. In this paper, we discuss applications of FOpkc conversion and REACT to Identity Based Encryptions (IBE). It has not been formally verified yet that whether these conversions are generic in the IBE setting. Our results show that both conversions are effective in the IBE case: plain REACT already achieves a good security reduction while the plain FOpkc conversion results in bad running time of the simulator. We further propose a simple modification to the plain FOpkc that solves this problem. Finally, we choose some concrete parameters to explain (visually) the effect of how the modified FOpkc substantially improves reduction cost regarding the plain conversion.

keywords

Fujisaki-Okamoto, identity based encryption, security enhancement.
• Nuttapong Attrapadung, Yang Cui, David Galindo, Goichiro Hanaoka, Ichiro Hasuo, Hideki Imai, Kanta Matsuura, Peng Yang, Rui Zhang. Relations among Notions of Security for Identity Based Encryption Schemes, Lecture Notes in Computer Science (LATIN 2006: Theoretical Informatics: 7th Latin American Symposium), Vol.3887, pp.130-141, 2006 [detail]

abstract

This paper shows that the standard security notion for iden- tity based encryption schemes (IBE), that is IND-ID-CCA2, captures the essence of security for all IBE schemes. To achieve this intention, we first describe formal definitions of the notions of security for IBE, and then present the relations among OW, IND, SS and NM in IBE, along with rigorous proofs. With the aim of comprehensiveness, notions of security for IBE in the context of encryption of multiple messages and/or to mul- tiple receivers are finally presented. All of these results are proposed with the consideration of the particular attack in IBE, namely the adaptive chosen identity attack.

keywords

Identity based encryption, security notions.
• Peng Yang, Takashi Kitagawa, Goichiro Hanaoka, Rui Zhang, Kanta Matsuura, Hideki Imai. Applying Fujisaki-Okamoto to Identity-Based Encryption, Lecture Notes in Computer Science (Applied Algebra, Algebraic Algorithms and Error-Correcting Codes: 16th International Symposium: AAECC-16), Vol.3857, pp.183-192, 2006 [detail]

abstract

The Fujisaki-Okamoto (FO) conversion is widely known to be able to generically convert a weak public key encryption scheme, say one-way against chosen plaintext attacks (OW-CPA), to a strong one, namely, indistinguishable against adaptive chosen ciphertext attacks (IND-CCA). It is not known that if the same holds for identity-based encryption (IBE) schemes, though many IBE and variant schemes are in fact specifically using the FO conversion. In this paper, we investigate this issue and confirm that the FO conversion is generically effective also in the IBE case. However, straightforward application of the FO conversion only leads to an IBE scheme with a loose (but polynomial) reduction. We then propose a simple modification to the FO conversion, which results in considerably more efficient security reduction.

keywords

Fujisaki-Okamoto, identity based encryption, security enhancement.
• Peng Yang, Takashi Kitagawa, Goichiro Hanaoka, Rui Zhang, Hajime Watanabe, Kanta Matsuura, Hideki Imai. A Simple Approach to Evaluate Fujisaki-Okamoto Conversion in Identity Based Encryption, Proceedings of the 2006 International Symposium on Information Theory and Its Applications (ISITA'06), pp.507-512, 2006 [detail]

abstract

The Fujisaki-Okamoto (FO) conversion is a very powerful security enhancement method in public key encryption (PKE) schemes. The generality of the plain FO in identity based encryption (IBE) schemes was verified, and a slightly different version, the modified FO, was proposed. Both of the plain FO and the modified FO could achieve the goal of converting a weak IBE scheme, i.e., one-way against adaptive chosen identity and chosen plaintext attacks ({\sf OW-ID-CPA}), to the strongest one, namely, indistinguishability against adaptive chosen identity and adaptive chosen ciphertext attacks (\textsf{IND-ID-CCA}). This work aims to evaluate the plain FO and the modified FO by substituting proper concrete values. By mainly focusing on the time costs of security reductions, we show the modified FO is better than the plain one.

keywords

Fujisaki-Okamoto, identity based encryption, security enhancement.

Yasumasa Nakai

• Master Course 2

Interest

• Cryptography, Computer Security

Publications

• Yasumasa Nakai, Takahiro Matsuda, Wataru Kitada, Kanta Matsuura. A Generic Construction of Timed-Release Encryption with Pre-open Capability, Lecture Notes in Computer Science (Advances in Information and Computer Security, The 4th International Workshop on Security: IWSEC2009), Vol.5824, pp.53-70, 2009 [detail]

abstract

In 2005, Hwang et al. proposed a concept of timed-release encryption with pre-open capability (TRE-PC), where a receiver can decrypt a ciphertext not only by using a time-release key which is provided after its release-time, but also using a secret information called a preopen key provided from a sender even before the release-time. Though there are several concrete constructions of TRE-PC proposed so far, no generic construction has been known. In this paper, we show a generic construciton of TRE-PC. Specifically, we construct a TRE-PC scheme from a chosen-ciphertext secure public key encryption scheme (PKE), a chosen plaintext secure identity-based encryption (IBE) scheme with specific property that we call target collision resistance for randomness, and a one-time signature scheme. Interestingly, our proposed construction of TRE-PC is essentially the same as the generic construciton of (normal) TRE based on multiple encryption of IBE and PKE. As one of the consequences of our result, we can build a TRE-PC scheme secure in the standard model based on weaker assumptions than the ones used by the existing standard model TRE-PC scheme.

Naoko Hashizume

• Secretary

Interest

Yu Watanabe

• Master Course

Interest

• Computer Security, Parallel and Distributed Computing

Wataru Kitada

• Master Course

Interest

• Cryptology

Publications

• Wataru Kitada, Goichiro Hanaoka, Kanta Matsuura, Hideki Imai. Unconditionally Secure Chaffing-and-Winnowing for Multiple Use, Lecture Notes in Computer Science (International Conference on Information Theoretic Security: ICITS 2007), Vol.4883, pp.133-145, 2007 [detail]

abstract

Chaffing-and-winnowing is a cryptographic technique which does not require encryption but instead use a message authentication code (MAC) to provide the same function as encryption. Hanaoka et al. showed that an unconditionally secure chaffing-and-winnowing with one-time security can be constructed from any authentication code (A-code) (with one-time security). In this paper, we show a construction of unconditionally secure chaffing-and-winnowing for multiple use and prove the security of perfect secrecy and non-malleability. Additionally, we investigate a relation between encryption and authentication in more detail. Particularly, we show through chaffing-and-winnowing that a fully secure A-code with a specific property can be converted to a non-malleable one-time pad with a short ciphertext size. Interestingly, when applying this method to a known A-code, this becomes a known construction of a non-malleable one-time pad. This fact implies that the notions of authentication and encryption can be seamlessly connected by chaffing-and-winnowing mechanism.

Vadim Jefte Zendejas Samano

• Master Course

Interest

• Mail Security

Publications

• Vadim Jefte Zendejas Samano, Takuro Hosoi, Kanta Matsuura. Time Categorization in a Social-Network-Analysis Spam Filter, Workshop on Information Security Applications (WISA2008), CD-ROM, 2008 [detail]

abstract

In this article, the introduction of a new method of language- independent e-mail classification using Social Network Analysis (SNA) for spam filtering is proposed. Our approach uses a time categorization of different instances of the e-mail to improve the classification of the filter. The proposal reduced the complexity of the classification and increases the accuracy of the filter. Although the naive SNA suffers from a high unclassification rate, our proposal decreases the number of unclassified e-mails.

Phan Thi Lan Anh

• Master Course

Interest

• Cryptography

Publications

• Thi Lan Anh Phan, Goichiro Hanaoka, Kanta Matsuura, Hideki Imai. Key-Insulated Public Key Encryption with Auxiliary Helper Key: Model, Construcrtions and Formal Security Proofs, IEICE Transactions on Fundamentals of Electronics, Communications and Computer Sciences, Vol.E90-A, No.9, pp.1814-1829, 2007 [detail]

abstract

To deal with the problem of secret key exposure, Dodis, Katz, Xu and Yung [6] proposed a key-insulated public key encryption (KIE), which uses the secure helper key to update the secret key more often and helps to minimize the damage overall. We take this idea further in improving the helper key security by introducing an auxiliary helper key besides of the main helper key. The auxiliary helper key is used less than the main helper key and can help the system to reduce the damage even in case of helper key exposure. This paper give two different schemes of KIE with auxiliary helper key. There are trade-offs between public key length and ciphertext size, as well as between encryption algorithms and decryption algorithms in these two schemes. With these trade-offs, it is flexible to choose an efficient one to implement in reality. We also show concrete constructions with chosen-ciphertext security. The formal security proofs for all schemes are given in this paper, based on the CBDH assumption [2] in the random oracle model.
• Thi Lan Anh Phan, Yumiko Hanaoka, Goichiro Hanaoka, Kanta Matsuura, Hideki Imai. Reducing the Spread of Damage of Key Exposure in Key Insulated Encryption, Lecture Notes in Computer Science (First International Conference on Cryptology in Vietnam: VietCrypt 2006), Vol.4341, pp.366-384, 2006 [detail]

abstract

A proposal for key exposure resilient cryptography called, key-insulated public key encryption (KIPE), has been proposed by Dodis, Katz, Xu, and Yung [6] in which the secret key is changed over time so that the exposure of current key minimizes the damage overall. We take this idea further toward betterment by introducing new schemes with improved helper key security: in our schemes, we introduce an auxiliary helper key to update the secret key less frequently than the main helper key (and only one of these keys is used at each key updates,) as a result, this gives added protection to the system, by occasional auxiliary key updates, reducing the spread of further harm that may be caused by key exposure when compared to the original KIPE. Our proposed schemes are proven to be semantically secure in the random oracle model.