Publications

Books, Book Chapters, and Edited Volumes

  • Proceedings of the 5th ACM Asia Public-Key Cryptography Workshop (APKC 2018)
    Author(s)
    K. Emura, J.H. Seo, and Y. Watanabe
    Publisher
    ACM
    Publication Year
    2018
    Abstract

    It is our great pleasure to welcome you to The 5th ACM Asia Public-Key Cryptography Workshop - APKC'18, held on June 4, 2018, in conjunction with The 13th ACM Asia Conference on Computer and Communications Security (AsiaCCS'18). Public-key cryptography plays an essential role in ensuring many security properties required in data processing of various kinds. The theme of this workshop is novel public-key cryptosystems for solving a wide range of reallife application problems. This workshop solicits original contributions on both applied and theoretical aspects of public-key cryptography. We also solicit systematization of knowledge (SoK) papers, which should aim to evaluate, systematize, and contextualize existing knowledge. The call for papers attracted 20 submissions from Asia, America, and Europe. The program committee accepted 7 papers based on their overall quality and novelty (acceptance ratio: 35%). We hope these proceedings will serve as a valuable reference for researchers and practitioners in the field of public-key cryptography and its applications.

Journal Articles

  • Adaptively Secure Revocable Hierarchical IBE from k-linear Assumption
    Author(s)
    K. Emura, A. Takayasu, and Y. Watanabe
    Journal
    Designs, Codes and Cryptography
    Publisher
    Springer
    Publication Year
    2021
    To appear.
    Abstract

    Revocable identity-based encryption (RIBE) is an extension of IBE with an efficient key revocation mechanism. Revocable hierarchical IBE (RHIBE) is its further extension with key delegation functionality. Although there are various adaptively secure pairing-based RIBE schemes, all known hierarchical analogs only satisfy selective security. In addition, the currently known most efficient adaptively secure RIBE and selectively secure RHIBE schemes rely on non-standard assumptions, which are referred to as the augmented DDH assumption and q-type assumptions, respectively. In this paper, we propose a simple but effective design methodology for RHIBE schemes. We provide a generic design framework for RHIBE based on an HIBE scheme with a few properties. Fortunately, several state-of-the-art pairing-based HIBE schemes have the properties. In addition, our construction preserves the sizes of master public keys, ciphertexts, and decryption keys, as well as the complexity assumptions of the underlying HIBE scheme. Thus, we obtain the first RHIBE schemes with adaptive security under the standard k-linear assumption. We prove adaptive security by developing a new proof technique for RHIBE. Due to the compactness-preserving construction, the proposed R(H)IBE schemes have similar efficiencies to the most efficient existing schemes.

  • Efficient Revocable Identity-Based Encryption with Short Public Parameters
    Author(s)
    K. Emura, J.H. Seo, and Y. Watanabe
    Journal
    Theoretical Computer Science
    Vol.
    863
    Pages
    127–155
    Publisher
    Elsevier
    Publication Year
    2021
    Abstract

    Revocation functionality is vital to real-world cryptographic systems for managing their reliability. In the context of identity-based encryption (IBE), Boldyreva, Goyal, and Kumar (ACM CCS 2008) first showed an efficient revocation method for IBE, and such an IBE scheme with the scalable revocation method is called revocable IBE (RIBE). Seo and Emura (PKC 2013) introduced a new security notion, called decryption key exposure resistance (DKER), which is a desirable security notion for RIBE. However, all existing RIBE schemes that achieve adaptive security with DKER require long public parameters or composite-order bilinear groups. In this paper, we first show an RIBE scheme that (1) satisfies adaptive security; (2) achieves DKER; (3) realizes constant-size public parameters; and (4) is constructed over prime-order bilinear groups. Our core technique relies on Seo and Emura's one (PKC 2013), which transform the Waters IBE (EUROCRYPT 2005) to the corresponding RIBE scheme. Specifically, we construct an IBE scheme that satisfies constant-size public parameters over prime-order groups and some requirements for the Seo-Emura technique, and then transform the IBE scheme to an RIBE scheme. We also discuss how to extend the proposed RIBE scheme to a chosen-ciphertext secure one and server-aided one (ESORICS 2015).

  • A single shuffle is enough for secure card-based computation of any Boolean circuit
    Author(s)
    K. Shinagawa and K. Nuida
    Journal
    Discrete Applied Mathematics
    Vol.
    289
    Pages
    248–261
    Publisher
    Elsevier
    Publication Year
    2020
    Abstract

    Secure computation enables a number of players each holding a secret input value to compute a function of the inputs without revealing the inputs. It is known that secure computation is possible physically when the inputs are given as a sequence of physical cards. This research area is called card-based cryptography. One of the important problems in card-based cryptography is to minimize the number of cards and shuffles, where a shuffle is the most important (and somewhat heavy) operation in card-based protocols. In this paper, we determine the minimum number of shuffles for achieving general secure computation. Somewhat surprisingly, the answer is just one, i.e., we design a protocol which securely computes any Boolean circuit with only a single shuffle. The number of cards required for our protocol is proportional to the size of the circuit to be computed.

  • Revocable Identity-based Encryption with Bounded Decryption Key Exposure Resistance: Lattice-based Construction and More
    Author(s)
    A. Takayasu and Y. Watanabe
    Journal
    Theoretical Computer Science
    Vol.
    849
    Pages
    64–98
    Publisher
    Elsevier
    Publication Year
    2021
    Abstract

    In general, identity-based encryption (IBE) does not support an efficient revocation procedure. In ACM CCS’08, Boldyreva et al. proposed revocable identity-based encryption (RIBE), which enables us to efficiently revoke (malicious) users in IBE. In PKC 2013, Seo and Emura introduced an additional security notion for RIBE, called decryption key exposure resistance (DKER). Roughly speaking, RIBE with DKER guarantees that the security is not compromised even if an adversary gets (a number of) short-term decryption keys. Therefore, DKER captures realistic scenarios and is an important notion. In this paper, we introduce bounded decryption key exposure resistance (B-DKER), where an adversary is allowed to get a-priori bounded number of short-term decryption keys in the security game. B-DKER is a weak version of DKER, but it seems to be sufficient for practical use. We obtain the following results:
    - We propose a lattice-based (anonymous) RIBE scheme with B-DKER, which is the first lattice-based construction resilient to decryption key exposure. Our lattice-based construction is secure under the learning with errors assumption. A previous lattice-based construction satisfies anonymity but is vulnerable even with a single decryption key exposure.
    - We propose the first pairing-based RIBE scheme that simultaneously realizes anonymity and B-DKER. Our pairing-based construction is adaptively secure under the symmetric external Diffie-Hellman assumption.
    Our two constructions rely on cover free families to satisfy B-DKER, whereas all the existing works rely on the key re-randomization property to achieve DKER.

  • Card-based Cryptography with Dihedral Symmetry
    Author(s)
    K. Shinagawa
    Journal
    New Generation Computing
    Publisher
    Springer
    Publication Year
    2021
    to appear
    Abstract

    It is known that secure computation can be done by using a deck of physical cards. This area is called card-based cryptography. Shinagawa et al. (ProvSec 2015) proposed regular n-sided polygon cards that enable to compute functions over Z/nZ. In particular, they designed efficient protocols for linear functions (e.g. addition and constant multiplication) over Z/nZ. Here, efficiency is measured by the number of cards used in the protocol. In this paper, we propose a new type of cards, dihedral cards, as a natural generalization of regular polygon cards. Based on them, we construct efficient protocols for various interesting functions such as carry of addition, equality, and greater-than, whose efficient construction has not been known before. Beside this, we introduce a new protocol framework that captures a wide class of card types including binary cards, regular polygon cards, dihedral cards, and so on.

  • How to Solve Millionaires’ Problem with Two Kinds of Cards
    Author(s)
    T. Nakai, Y. Misawa, Y. Tokushige, M. Iwamoto, and K. Ohta
    Journal
    New Generation Computing
    Vol.
    39
    Pages
    73–96
    Publisher
    Springer
    Publication Year
    2021
    Open Access
    Abstract

    Card-based cryptography, introduced by den Boer aims to realize multiparty computation (MPC) by using physical cards. We propose several efficient card-based protocols for the millionaires’ problem by introducing a new operation called Private Permutation (PP) instead of the shuffle used in most of existing card-based cryptography. Shuffle is a useful randomization technique by exploiting the property of card shuffling, but it requires a strong assumption from the viewpoint of arithmetic MPC because shuffle assumes that public randomization is possible. On the other hand, private randomness can be used in PPs, which enables us to design card-based protocols taking ideas of arithmetic MPCs into account. Actually, we show that Yao’s millionaires’ protocol can be easily transformed into a card-based protocol by using PPs, which is not straightforward by using shuffles because Yao’s protocol uses private randomness. Furthermore, we propose entirely novel and efficient card-based millionaire protocols based on PPs by securely updating bitwise comparisons between two numbers, which unveil a power of PPs. As another interest of these protocols, we point out they have a deep connection to the well-known logical puzzle known as “The fork in the road.”

  • An IC-level Countermeasure against Laser Fault Injection Attack by Information Leakage Sensing Based on Laser-Induced Opto-Electric Bulk Current Density
    Author(s)
    K. Matsuda, S. Tada, M. Nagata, Y. Komano, Y. Li, T. Sugawara, M. Iwamoto, K. Ohta, K. Sakiyama, and N. Miura
    Journal
    Japanese Journal of Applied Physics
    Vol.
    59
    No.
    SG
    Pages
    SGGL02
    Publisher
    IOPscience
    Publication Year
    2020
    Abstract

    Laser fault injection (LFI) attacks on cryptographic processor ICs are a critical threat to information systems. This paper proposes an IC-level integrated countermeasure employing an information leakage sensor against an LFI attack. Distributed bulk current sensors monitor abnormal bulk current density caused by laser irradiation for LFI. Time-interleaved sensor operation and sensitivity tuning can obtain partial secret key leakage bit information with small layout area penalty. Based on the leakage information, the secret key can be securely updated to realize high-availability resilient systems. The test chip was designed and fabricated in a 0.18 μm standard CMOS, integrating a 128-bit advanced encryption standard cryptographic processor with the proposed information leakage sensor. This evaluation successfully demonstrated bulk current density and leakage bit monitoring.

  • Key-Updatable Public-Key Encryption with Keyword Search (Or: How to Realize PEKS with Efficient Key Updates for IoT Environments)
    Author(s)
    H. Anada, A. Kanaoka, N. Matsuzaki, and Y. Watanabe
    Journal
    International Journal of Information Security
    Vol.
    19
    Pages
    15–38
    Publisher
    Springer
    Publication Year
    2020
    Abstract

    Security and privacy are the key issues for the Internet of Things (IoT) systems. Especially, secure search is an important functionality for cooperation among users’ devices and non-trusted servers. Public-key encryption with keyword search (PEKS) enables us to search encrypted data and is expected to be used between a cloud server and users’ mobile devices or IoT devices. However, those mobile devices might be lost or stolen. For IoT devices, it might be difficult to store keys in a tamper-proof manner due to prohibitive costs. In this paper, we deal with such a key-exposure problem on PEKS and introduce the concept of PEKS with key-updating functionality, which we call key-updatable PEKS (KU-PEKS). Specifically, we propose two models of KU-PEKS: the key-evolution model and the key-insulation model. In the key-evolution model, a pair of public and secret keys can be updated if needed (e.g., the secret key is exposed). In the key-insulation model, the public key remains fixed while the secret key can be updated if needed. The former model makes a construction simple and more efficient than the latter. On the other hand, the latter model is preferable for practical use since a user never updates their public key. We show constructions in each model in a black-box manner. We also give implementation results on Raspberry Pi 3, which can be regarded as a reasonable platform of IoT devices.

  • Multi-Party Computation for Modular Exponentiation Based on Replicated Secret Sharing
    Author(s)
    K. Ohara, Y. Watanabe, M. Iwamoto, and K. Ohta
    Journal
    IEICE Transactions on Fundamentals
    Vol.
    102-A
    No.
    9
    Pages
    1079–1090
    Publisher
    IEICE
    Publication Year
    2019
    Abstract

    In recent years, multi-party computation (MPC) frameworks based on replicated secret sharing schemes (RSSS) have attracted the attention as a method to achieve high efficiency among known MPCs. However, the RSSS-based MPCs are still inefficient for several heavy computations like algebraic operations, as they require a large amount and number of communication proportional to the number of multiplications in the operations (which is not the case with other secret sharing-based MPCs). In this paper, we propose RSSS-based three-party computation protocols for modular exponentiation, which is one of the most popular algebraic operations, on the case where the base is public and the exponent is private. Our proposed schemes are simple and efficient in both of the asymptotic and practical sense. On the asymptotic efficiency, the proposed schemes require O(n)-bit communication and O(1) rounds,where n is the secret-value size, in the best setting, whereas the previous scheme requires O(n2)-bit communication and O(n) rounds. On the practical efficiency, we show the performance of our protocol by experiments on the scenario for distributed signatures, which is useful for secure key management on the distributed environment (e.g., distributed ledgers). As one of the cases, our implementation performs a modular exponentiation on a 3,072-bit discrete-log group and 256-bit exponent with roughly 300ms, which is an acceptable parameter for 128-bit security, even in the WAN setting.

  • Identity-based Encryption with Hierarchical Key-Insulation in the Standard Model
    Author(s)
    J. Shikata and Y. Watanabe
    Journal
    Designs, Codes and Cryptography
    Vol.
    87
    No.
    5
    Pages
    1005–1033
    Publisher
    Springer
    Publication Year
    2019
    Abstract

    A key exposure problem is unavoidable since it seems human error can never be eliminated completely, and key-insulated encryption is one of the cryptographic solutions to the problem. At Asiacrypt’05, Hanaoka et al. introduced hierarchical key-insulation functionality, which is attractive functionality that enhances key exposure resistance, and proposed an identity-based hierarchical key-insulated encryption (hierarchical IKE) scheme in the random oracle model. In this paper, we first propose the hierarchical IKE scheme in the standard model (i.e., without random oracles). Our hierarchical IKE scheme is secure under the symmetric external Diffie–Hellman (SXDH) assumption, which is a static assumption. Particularly, in the non-hierarchical case, our construction is the first IKE scheme that achieves constant-size parameters including public parameters, secret keys, and ciphertexts. Furthermore, we also propose the first public-key-based key-insulated encryption (PK-KIE) in the hierarchical setting by using our technique.

  • Single-Round Pattern Matching Key Generation Using Physically Unclonable Function
    Author(s)
    Y. Komano, K. Ohta, K. Sakiyama, M. Iwamoto, and I. Verbauwhede
    Journal
    Security and Communication Networks
    Vol.
    2019
    Pages
    13 pages
    Publisher
    Hindawi
    Publication Year
    2019
    Article ID   1719585
    Abstract

    Paral and Devadas introduced a simple key generation scheme with a physically unclonable function (PUF) that requires no error correction, e.g., by using a fuzzy extractor. Their scheme, called a pattern matching key generation (PMKG) scheme, is based on pattern matching between auxiliary data, assigned at the enrollment in advance, and a substring of PUF output, to reconstruct a key. The PMKG scheme repeats a round operation, including the pattern matching, to derive a key with high entropy. Later, to enhance the efficiency and security, a circular PMKG (C-PMKG) scheme was proposed. However, multiple round operations in these schemes make them impractical. In this paper, we propose a single-round circular PMKG (SC-PMKG) scheme. Unlike the previous schemes, our scheme invokes the PUF only once. Hence, there is no fear of information leakage by invoking the PUF with the (partially) same input multiple times in different rounds, and, therefore, the security consideration can be simplified. Moreover, we introduce another hash function to generate a check string which ensures the correctness of the key reconstruction. The string enables us not only to defeat manipulation attacks but also to prove the security theoretically. In addition to its simple construction, the SC-PMKG scheme can use a weak PUF like the SRAM-PUF as a building block if our system is properly implemented so that the PUF is directly inaccessible from the outside, and, therefore, it is suitable for tiny devices in the IoT systems. We discuss its security and show its feasibility by simulations and experiments.

  • A Proactive Secret Image Sharing Scheme with Resistance to Machine Learning Based Steganalysis
    Author(s)
    A. Espejel-Trujillo, M. Iwamoto, and M. Nakano-Miyatake
    Journal
    Multimedia Tools And Applications
    Vol.
    77
    No.
    12
    Pages
    15161–15179
    Publisher
    Springer
    Publication Year
    2018
    Abstract

    In secret image sharing (SIS) schemes, a secret image is shared among a set of n images called stego-images. Each stego-image is preserved by a participant. In the recovery stage, at least k out of n stego-images are required to obtain the secret image, while k−1 cannot reveal the secret in the sense of perfect secrecy. Hence, SIS guarantees long-term security. However, as the longer the stego-images remain stored, the higher is the probability of being vulnerable against steganalysis. To resolve this issue, this paper proposes the use of proactive secret sharing in an SIS scheme (P-SIS). P-SIS allows the stego-images to be renewed frequently while these are stored, without changing both cover and secret images. However, direct implementation of a proactive SIS requires more embedding rate (ER), causing high steganalysis accuracy detection and loss of quality in the stego-images. Our proposal addresses this issue and presents the combination of a (k, L, n)-threshold ramp secret sharing scheme and least significant bit matching (LSBM) steganography to reduce the steganalysis accuracy detection. The results of the evaluation show effectiveness of the proposal in terms of good quality of the stego-images, accurate recovery of the secret, and reduce the ER. Note that, despite the extensive research of SIS presented until now, only a few previous work is found on steganalysis in SIS. Not only constructing P-SIS scheme, but we also experimented the tolerance of the proposed P-SIS scheme against stganalysis in this paper. As a result, it is shown that the proposed scheme can withstand steganalysis based on machine learning (i.e., based on subtractive pixel adjacency matrix, SPAM).

  • Implementation and Analysis of Fully Homomorphic Encryption in Resource-Constrained Devices
    Author(s)
    A. Prasitsupparote, Y. Watanabe, J. Sakamoto, J. Shikata, and T. Matsumoto
    Journal
    International Journal of Digital Information and Wireless Communications (IJDIWC)
    Vol.
    8
    No.
    4
    Pages
    288–303
    Publisher
    SDIWC Library
    Publication Year
    2018
    Abstract

    Currently, resource-constrained devices, which are known as one of the Internet of things (IoT) devices, have been widely used for healthcare systems. Most healthcare systems store users’ health data, which is encrypted by ordinary symmetric-key encryption and/or public-key encryption schemes, in a (cloud) server. However, the encrypted data needs to be decrypted for data analysis, and it means that sensitive information would be leaked to the server. One promising solution is to use fully homomorphic encryption (FHE), which enables ones to perform any computation among encrypted data while keeping it encrypted, though FHE generally requires high computational and communication costs in the theoretical sense.
    In this paper, we investigate practical feasibility of FHE in resource-constrained devices for healthcare systems. First, we define a privacy-preserving protocol for healthcare systems, and implement it on PC and Raspberry Pi by using a network simulator to measure its communication overhead, computational cost, and energy consumption over wireless body area network (WBAN). For this implementation, we suppose PC and Raspberry Pi as a cloud server and a resource-constrained device such as a smartphone or a wearable device, respectively. We use two FHE libraries, HElib and SEAL, for the implementation. Our result shows that the protocol with SEAL is better than that with HElib in terms of the communication overhead and energy consumption in transmission. On the other hand, HElib is better than SEAL regarding the running time, while SEAL can perform more homomorphic operations than HElib for the almost same plaintext size. Furthermore, the energy to execute each algorithm in the libraries is very small compared to the energy required in transmission. SEAL produces smaller sizes of ciphertexts than HElib, and therefore consumes few energy consumptions. As a result, we observe that both HElib and SEAL would be used on restricted resource devices, and in particular, SEAL would be more suitable for practical use in resource-constrained devices from our analysis.

  • Timed-Release Computational Secret Sharing and Threshold Encryption
    Author(s)
    Y. Watanabe and J. Shikata
    Journal
    Designs, Codes and Cryptography
    Vol.
    86
    No.
    1
    Pages
    17–54
    Publisher
    Springer
    Publication Year
    2018
    Abstract

    In modern cryptography, a secret sharing scheme is an important cryptographic primitive. In particular, Krawczyk proposed a computational secret sharing (CSS) scheme, which is a practical, simple secret sharing scheme. In this paper, we focus on a CSS scheme with timed-release functionality, which we call a timed-release computational secret sharing (TR-CSS) scheme. In TR-CSS, participants more than or equal to a threshold number can reconstruct a secret by using their shares only when the time specified by a dealer has come. Our main purpose is to realize a TR-CSS scheme in a generic and efficient way in terms of the share size. Specifically, we first introduce a model and formalization of security of TR-CSS. In addition, we propose two kinds of constructions of TR-CSS: the first one is a simple and generic construction starting from an identity-based key encapsulation mechanism (IB-KEM); the second one, which is a more efficient construction than the first one, is built using a specific IB-KEM as the underlying IB-KEM. As a result, we can regard TR-CSS as a natural extension of Krawczyk’s CSS in terms of both a model and constructions, and we finally succeed to add timed-release functionality to Krawczyk’s CSS with small overhead, which is almost optimal. Moreover, our proposal of TR-CSS is important for constructing threshold encryption and multiple encryption with timed-release functionality in a generic and efficient way. Dodis and Katz showed (i) a simple and generic construction of threshold encryption from multiple encryption; and (ii) a simple, elegant and generic construction of multiple encryption. By using TR-CSS, we can effectively apply the Dodis–Katz paradigm even in the context of timed-release security.

  • Security Formalizations and Their Relationships for Encryption and Key Agreement in Information-Theoretic Cryptography
    Author(s)
    M. Iwamoto, K. Ohta, and J. Shikata
    Journal
    IEEE Transactions on Information Theory
    Vol.
    64
    No.
    1
    Pages
    654–685
    Publisher
    IEEE
    Publication Year
    2018
    Abstract

    This paper analyzes the formalizations of information-theoretic security for the fundamental primitives in cryptography: symmetric-key encryption and key agreement. Revisiting the previous results, we can formalize information-theoretic security using different methods, by extending Shannon's perfect secrecy, by information-theoretic analogues of indistinguishability and semantic security, and by the frameworks for composability of protocols. We show the relationships among the security formalizations and obtain the following results. First, in the case of encryption, there are significant gaps among the formalizations, and a certain type of relaxed perfect secrecy or a variant of information-theoretic indistinguishability is the strongest notion. Second, in the case of key agreement, there are significant gaps among the formalizations, and a certain type of relaxed perfect secrecy is the strongest notion. In particular, in both encryption and key agreement, the formalization of composable security is not stronger than any other formalizations. Furthermore, as an application of the relationships in encryption and key agreement, we simultaneously derive a family of lower bounds on the size of secret keys and security quantities required under the above formalizations, which also implies the importance and usefulness of the relationships.

  • Q-class Authentication System for Double Arbiter PUF
    Author(s)
    R. Yashiro, T. Sugawara, M. Iwamoto, and K. Sakiyama
    Journal
    IEICE Transactions on Fundamentals
    Vol.
    E101–A
    No.
    1
    Pages
    129–137
    Publisher
    IEICE
    Publication Year
    2018
    Abstract

    Physically Unclonable Function (PUF) is a cryptographic primitive that is based on physical property of each entity or Integrated Circuit (IC) chip. It is expected that PUF be used in security applications such as ID generation and authentication. Some responses from PUF are unreliable, and they are usually discarded. In this paper, we propose a new PUF-based authentication system that exploits information of unreliable responses. In the proposed method, each response is categorized into multiple classes by its unreliability evaluated by feeding the same challenges several times. This authentication system is named Q-class authentication, where Q is the number of classes. We perform experiments assuming a challenge-response authentication system with a certain threshold of errors. Considering 4-class separation for 4-1 Double Arbiter PUF, it is figured out that the advantage of a legitimate prover against a clone is improved form 24% to 36% in terms of success rate. In other words, it is possible to improve the tolerance of machine-learning attack by using unreliable information that was previously regarded disadvantageous to authentication systems.

  • CCA-secure Revocable Identity-Based Encryption Schemes with Decryption Key Exposure Resistance
    Author(s)
    Y. Ishida, J. Shikata, and Y. Watanabe
    Journal
    International Journal of Applied Cryptography (IJACT)
    Vol.
    3
    No.
    3
    Pages
    288–311
    Publisher
    Inderscience Publishers
    Publication Year
    2017
    Abstract

    Key revocation functionality is important for identity-based encryption (IBE) to manage users dynamically. Revocable IBE (RIBE) realises such revocation functionality with scalability. In PKC 2013, Seo and Emura first considered decryption key exposure resistance (DKER) as a new realistic threat, and proposed the first RIBE scheme with DKER. Their RIBE scheme is adaptively secure against chosen plaintext attacks (CPA), and there is no concrete RIBE scheme adaptively secure against chosen ciphertext attacks (CCA) even without DKER so far. In this paper, we first propose three constructions of adaptively CCA-secure RIBE schemes with DKER. The first and second schemes are based on an existing transformation, which is called a BCHK transformation, that a CPA-secure hierarchical IBE scheme can be transformed into a CCA-secure scheme. The third scheme is constructed via the KEM/DEM framework. Specifically, we newly propose a revocable identity-based key encapsulation mechanism (RIB-KEM), and we show a generic construction of a CCA-secure RIBE scheme from the RIB-KEM and a data encapsulation mechanism (DEM). The third scheme is more efficient than the first and second ones in terms of the ciphertext size.

  • Unconditionally Secure Broadcast Encryption Schemes with Tradeoffs between Communication and Storage
    Author(s)
    Y. Watanabe and J. Shikata
    Journal
    IEICE Transactions on Fundamentals
    Vol.
    99-A
    No.
    6
    Pages
    1097–1106
    Publication Year
    2016
    Abstract

    An (≤n,≤ω)-one-time secure broadcast encryption scheme (BES) allows a sender to choose any subset of receivers so that only the designated users can decrypt a ciphertext. In this paper, we first show an efficient construction of an (≤n,≤ω)-one-time secure BES with general ciphertext sizes. Specifically, we propose a generic construction of an (≤n,≤ω)-one-time secure BES from key predistribution systems (KPSs) when its ciphertext size is equal to integer multiple of the plaintext size, and our construction includes all known constructions. However, there are many possible combinations of the KPSs to realize the BES in our construction methodology, and therefore, we show that which combination is the best one in the sense that secret-key size can be minimized. Our (optimized) construction provides a flexible parameter setup (i.e. we can adjust the secret-key sizes) by setting arbitrary ciphertext sizes based on restrictions on channels such as channel capacity and channel bandwidth.

  • Information-Theoretically Secure Timed-Release Secret Sharing Schemes
    Author(s)
    Y. Watanabe and J. Shikata
    Journal
    Journal of Information Processing
    Vol.
    24
    No.
    4
    Pages
    680–689
    Publisher
    IPSJ
    Publication Year
    2016
    Abstract

    In modern cryptography, the secret sharing scheme is an important cryptographic primitive, and it is used in various situations. In this paper, timed-release secret sharing (TR-SS) schemes with information-theoretic security is first studied. TR-SS is a secret sharing scheme with the property that more than a threshold number of participants can reconstruct a secret by using their shares only when the time specified by a dealer has come. Specifically, in this paper we first introduce models and formalization of security for two kinds of TR-SS based on the traditional secret sharing scheme and information-theoretic timed-release security. We also derive tight lower bounds on the sizes of shares, time-signals, and entities' secret-keys required for each TR-SS scheme. In addition, we propose direct constructions for the TR-SS schemes. Each direct construction is optimal in the sense that the construction meets equality in each of our bounds, respectively. As a result, it is shown that timed-release security can be realized without any additional redundancy on the share size.

  • A New Arbiter PUF for Enhancing Unpredictability on FPGA
    Author(s)
    T. Machida, D. Yamamoto, M. Iwamoto, and K. Sakiyama
    Journal
    The Scientific World Journal
    Vol.
    2015
    Pages
    13 pages
    Publisher
    Hindawi
    Publication Year
    2015
    Article ID   864812
    Abstract

    In general, conventional Arbiter-based Physically Unclonable Functions (PUFs) generate responses with low unpredictability. The N-XOR Arbiter PUF, proposed in 2007, is a well-known technique for improving this unpredictability. In this paper, we propose a novel design for Arbiter PUF, called Double Arbiter PUF, to enhance the unpredictability on field programmable gate arrays (FPGAs), and we compare our design to conventional N-XOR Arbiter PUFs. One metric for judging the unpredictability of responses is to measure their tolerance to machine-learning attacks. Although our previous work showed the superiority of Double Arbiter PUFs regarding unpredictability, its details were not clarified. We evaluate the dependency on the number of training samples for machine learning, and we discuss the reason why Double Arbiter PUFs are more tolerant than the N-XOR Arbiter PUFs by evaluating intrachip variation. Further, the conventional Arbiter PUFs and proposed Double Arbiter PUFs are evaluated according to other metrics, namely, their uniqueness, randomness, and steadiness. We demonstrate that 3-1 Double Arbiter PUF archives the best performance overall.

  • A New Method for Enhancing Variety and Maintaining Reliability of PUF Responses and Its Evaluation on ASICs
    Author(s)
    D. Yamamoto, K. Sakiyama, M. Iwamoto, K. Ohta, M. Takenaka, K. Itoh, and N. Torii
    Journal
    Journal of Cryptographic Engineering
    Vol.
    5
    No.
    3
    Pages
    187–199
    Publisher
    Springer
    Publication Year
    2015
    Abstract

    Physically unclonable functions (PUFs) are expected to provide a breakthrough in anti-counterfeiting devices for secure ID generation and authentication, etc. Factory-manufactured PUFs are generally more secure if the number of outputs (the variety of responses) is larger (e.g., a 256-bit full-entropy response is more secure than a 128-bit response). In Yamamoto et al. (J. Cryptogr. Eng. 3(4):197–211, 2013), we presented a latch-based PUF structure, which enhances the variety of responses by utilizing the location information of the RS (Reset-Set) latches outputting random numbers. We confirmed the effectiveness of this method using two kinds of different Xilinx FPGA chips: Spartan-3E and Spartan-6. In this paper, we propose a novel method of further enhancing the variety of responses while maintaining the reliability of responses, i.e., consistency over repeated measurements. The core idea in this method is to effectively utilize the information on the proportion of `1’s in the random number sequence output by the RS latches. This proportion information is determined during the manufacturing process, making it relatively stable and reliable once PUFs are manufactured. We estimated the variety of responses generated by the PUFs to which the proposed method was applied. According to our experiment with 73 ASIC chips fabricated by a 0.18-㎛ CMOS process, latch-based PUFs with 256 RS latches can improve the variety of responses to as much as 2379. This is much larger than 2220 for conventional methods, and 2314 for our previous method presented in Yamamoto et al., (J. Cryptogr. Eng. 3(4):197–211, 2013). The average error rate (reliability) of responses is only 0.064 when both temperature and voltage are changed to −20∼60℃ and 1.80 ± 0.15V, respectively. Our proposed PUF enhances the variety of responses dramatically while maintaining reliability.

  • Information-Theoretically Secure Blind Authentication Codes without Verifier’s Secret Keys
    Author(s)
    N. Takei, Y. Watanabe, and J. Shikata
    Journal
    Josai Mathematical Monograph
    Vol.
    8
    Pages
    115–133
    Publisher
    Graduate School of Sciences, Josai University
    Publication Year
    2015
    Abstract

    In modern cryptography, information-theoretic security is formalized by means of some probability (e.g., success probability of adversary’s guessing) or some information-theoretic measure (e.g., Shannon entropy), and the study on cryptographic protocols with information-theoretic security is one of effective applications of the probability theory, statistics, and information theory. In this paper, we study the blind authentication code (BA-code), a kind of information-theoretically secure authentication protocols, in which verifier’s secret keys are not required. For realizing it, we utilize a unidirectional low-bandwidth auxiliary channel which is called a manual channel. Specifically, in this paper we propose a model, a security definition, and a construction of BA-codes in the manual channel model. Furthermore, we consider BA-codes in other models, i.e., the noisy channel model and the bounded storage model, in which no verifier's secret key is required.

  • Information-Theoretically Secure Anonymous Group Authentication with Arbitration: Formal Definition and Construction
    Author(s)
    T. Seito, Y. Watanabe, K. Kinose, and J. Shikata
    Journal
    Josai Mathematical Monograph
    Journal/Conference
    Josai Mathematical Monograph
    Vol.
    7
    Pages
    85–110
    Publisher
    Graduate School of Sciences, Josai University
    Publication Year
    2014
    Abstract

    In cryptographic applications, there is often a need for protecting privacy of users besides integrity of message transmitted in a public channel. In information-theoretic (or unconditional) security setting, a model of GA-codes (Group Authentication codes) which can ensure both the integrity of the message and the anonymity for senders was proposed. In this model, there are multiple senders and a single receiver. And, one of the senders can generate an authenticated message anonymously. That is, the receiver can verify the validity of the authenticated message, but he cannot specify the sender of it. In GA-codes, it is assumed that both the sender and receiver are honest. However, it may be unnatural and an ideal assumption in several situations. In this paper, we remove the assumption and newly propose a formal definition (i.e., the model and security definitions) of GA2-codes (Group Authentication codes with Arbitration). In GA2-codes, it is assumed that the sender or the receiver can be dishonest and thus a dispute between them may occur. To resolve such a dispute, we introduce an honest arbiter in GA2-codes. This model can be considered as natural extension of that of both the GAcodes and the traditional A2-codes (Authentication codes with Arbitration). In addition, we propose a construction which meets our security definition of GA2-codes by using polynomials over finite fields. We also consider the case that the arbiter is not always honest and call this model GA3-codes (GA2-codes with protection against arbiter’s attack), which is further extension of GA2-codes and be naturally considered from a similar setting of the traditional A3-codes (A2-code with protection against arbiter’s attack).

  • Practical DFA Strategy for AES Under Limited-Access Conditions
    Author(s)
    K. Sakiyama, Y. Li, S. Gomisawa, Y. Hayashi, M. Iwamoto, N. Homma, T. Aoki, and K. Ohta
    Journal
    Journal of Information Processing
    Vol.
    55
    No.
    2
    Pages
    142–151
    Publisher
    IPSJ
    Publication Year
    2014
    Abstract

    Secret data in embedded devices can be revealed by injecting computational faults using the fault analysis attacks.The fault analysis researches on a cryptographic implementation by far first assumed a certain fault model, and then discussed the key recovery method under some assumptions. We note that a new remote-fault injection method has emerged, which is threatening in practice. Due to its limited accessibility to cryptographic devices, the remote-fault injection, however, can only inject uncertain faults. In this surroundings, this paper gives a general strategy of the remote-fault attack on the AES block cipher with a data set of faulty ciphertexts generated by uncertain faults. Our method effectively utilizes all the information from various kinds of faults, which is more realistic than previous researches. As a result, we show that it can provide a decent success probability of key identification even when only a few intended faults are available among 32 millions fault injections.

  • Variety Enhancement of PUF Responses Using the Locations of Random Outputting RS Latches
    Author(s)
    D. Yamamoto, K. Sakiyama, M. Iwamoto, K. Ohta, M. Takenaka, and K. Itoh
    Journal
    Journal of Cryptographic Engineering
    Vol.
    3
    No.
    4
    Pages
    197–211
    Publisher
    Springer
    Publication Year
    2013
    Abstract

    Physical Unclonable Functions (PUFs) are expected to represent an important solution for secure ID generation and authentication etc. In general, manufactured PUFs are considered to be more secure when the pattern of outputs (the variety of responses) is larger, i.e., the response bit length is longer (e.g., 192-bit response is more secure than 128-bit one). However, the actual bit length is reduced because some response bits are inconsistent (random) for repeated measurements, which are regarded as unnecessary for ID generation and discarded. Latch-based PUFs with N RS latches, for example, generate ideally 2N responses depending on binary values output from RS latches (0/1). However, some RS latches output random responses which are inconsistent and cannot be used for reliable ID generation, so the variety of responses becomes smaller than 2N. In this paper, we propose a novel Latch-based PUF structure, which outputs larger variety of responses by utilizing location information of the RS latches outputting the random responses. Differently from random responses themselves, this location information is determined during a manufacturing process, so almost fixed once PUFs are manufactured. The proposed PUF generates 3N≈ 21.58N responses by considering random responses as the third stable value: using ternary values (0/1/random). We estimate the variety of responses generated by the proposed PUFs. According to our experiment with 40 FPGAs, a Latch-based PUF with 128 RS latches can improve it from 2116 to 2192.7, this being maximized when the 128 latches outputs 0s, 1s, or random outputs with equal probability. We also show the appropriate RS latch structure for satisfying this condition, and validate it using two kinds of different Xilinx FPGAs: Spartan-3E and Spartan-6. The average error rate of responses is only 5.3% when the core voltage is changed within the rated voltage range of the FPGAs. Our proposed PUF using ternary values enhances dramatically the variety of responses while keeping the reliability.

  • Coding Theorems for a (2, 2)–Threshold Scheme with Detectability of Impersonation Attacks
    Author(s)
    M. Iwamoto, H. Koga, and H. Yamamoto
    Journal
    IEEE Transactions on Information Theory
    Vol.
    58
    No.
    9
    Pages
    6194–6206
    Publisher
    IEEE
    Publication Year
    2012
    Abstract

    In this paper, we discuss coding theorems on a (2,2)–threshold scheme in the presence of an opponent who impersonates one of the two shareholders in an asymptotic setup. We consider a situation where n secrets Sn from a memoryless source is blockwisely encoded to two shares and the two shares are decoded to Sn with permitting negligible decoding error. We introduce correlation level of the two shares and characterize the minimum attainable rates of the shares and a uniform random number for realizing a (2,2)–threshold scheme that is secure against the impersonation attack by an opponent. It is shown that, if the correlation level between the two shares equals to an ℓ≥0, the minimum attainable rates coincide with H(S)+ℓ, where H(S) denotes the entropy of the source, and the maximum attainable exponent of the success probability of the impersonation attack equals to ℓ. We also give a simple construction of an encoder and a decoder using an ordinary (2,2)–threshold scheme where the two shares are correlated and attains all the bounds.

  • A Cheating Prevention EVC Scheme Using Watermarking Techniques
    Author(s)
    A. E. Torujillo, M. N. Miyatake, M. Iwamoto, and H. P. Maena
    Journal
    Revista Facultad de Ingeniería
    No.
    63
    Pages
    30–42
    Publisher
    The University of Antioquia
    Publication Year
    2012
    Abstract

    Visual Cryptography (VC), proposed by Naor and Shamir in 1994, is a variation of the conventional secret sharing scheme. In VC, instead of a numerical secret key, a secret image is shared among participants in the form of images called shares. Each participant possesses his own share which cannot reveal the secret image being alone, making it necessary to stack more than one share of a qualified participant in order to reveal the secret image. Thus in VC the stacking of shares is equivalent to the decryption process, where neither extra computations nor previous knowledge are required to reveal the secret image. Until now some important VC schemes, such as the (k,n)-VC scheme, the general access structure for VC and the extended VC (EVC), have been proposed. Unfortunately all schemes can be cheated, if one or more participants try to generate their fake shares to force the revealed secret image to be a faked one. In this paper, we propose a cheating prevention VC scheme, in which the shares can be identified and authenticated using the EVC scheme and watermarking techniques. In the proposed VC scheme, the share of each participant can be identified by its meaningful appearance instead of noise-like image used in the conventional VC scheme. For the purpose of authentication of each share two binary watermark images are encrypted using shift operation. Before the secret image is revealed, the validation of the shares must be carried out, extracting two watermark images. If they can be extracted correctly, the revealed secret image is considered as authentic; otherwise it is determined as a faked one. The simulation results show the desirable performance of the proposed EVC scheme.

  • A Weak Security Notion for Visual Secret Sharing Schemes
    Author(s)
    M. Iwamoto
    Journal
    IEEE Transactions on Information Forensics and Security
    Vol.
    7
    No.
    2
    Pages
    372–382
    Publisher
    IEEE
    Publication Year
    2012
    Abstract

    We propose a weak security notion for visual secret sharing (VSS) schemes. Under such a weak security notion, VSS schemes are designed to be secure against attackers' eyesight, but are not unconditionally secure, in general. In this paper, we theoretically discuss the relation between unconditionally secure (US) and weakly secure (WS) VSS schemes and present two constructions of WS-VSS schemes for color images. We show that WS-VSS schemes can achieve clearer color reproduced images with a smaller pixel expansion compared to those using US-VSS schemes, while we clarify that the basis matrices in both types of VSS schemes for black-white binary images are the same. These results suggest that the proposed VSS schemes can be regarded as ramp (or nonperfect) VSS schemes for color secret images.

  • Information-Theoretic Approach to Optimal Differential Fault Analysis
    Author(s)
    K. Sakiyama, Y. Li, M. Iwamoto, and K. Ohta
    Journal
    IEEE Transactions on Information Forensics and Security
    Vol.
    7
    No.
    1
    Pages
    109–120
    Publisher
    IEEE
    Publication Year
    2012
    Abstract

    This paper presents a comprehensive analysis of differential fault analysis (DFA) attacks on the Advanced Encryption Standard (AES) from an information-theoretic perspective. Injecting faults into cryptosystems is categorized as an active at tack where attackers induce an error in operations to retrieve the secret internal information, e.g., the secret key of ciphers. Here, we consider DFA attacks as equivalent to a special kind of passive attack where attackers can obtain leaked information without measurement noise. The DFA attacks are regarded as a conversion process from the leaked information to the secret key. Each fault model defines an upper bound for the amount of leaked information. The optimal DFA attacks should be able to exploit fully the leaked information in order to retrieve the secret key with a practical level of complexity. This paper discusses a new DFA methodology to achieve the optimal DFA attack by deriving the amount of the leaked information for various fault models from an information-theoretic perspective. We review several previous DFA at tacks on AES variants to check the optimality of their attacks. We also propose improved DFA attacks on AES-192 and AES-256 that reach the theoretical limits.

  • Optimal Multiple Assignments Based on Integer Programming in Secret Sharing Schemes with General Access Structures
    Author(s)
    M. Iwamoto, H. Yamamoto, and H. Ogawa
    Journal
    IEICE Transactions on Fundamentals
    Vol.
    E90–A
    No.
    1
    Pages
    101–112
    Publisher
    IEICE
    Publication Year
    2007
    Abstract

    It is known that for any general access structure, a secret sharing scheme (SSS) can be constructed from an (m,m)-threshold scheme by using the so-called cumulative map or from a (t,m)-threshold SSS by a modified cumulative map. However, such constructed SSSs are not efficient generally. In this paper, a new method is proposed to construct a SSS from a (t,m)-threshold scheme for any given general access structure. In the proposed method, integer programming is used to derive the optimal (t,m)-threshold scheme and the optimal distribution of the shares to minimize the average or maximum size of the distributed shares to participants. From the optimality, it can always attain lower coding rate than the cumulative maps because the cumulative maps cannot attain the optimal distribution in many cases. The same method is also applied to construct SSSs for incomplete access structures and/or ramp access structures.

  • Visual Secret Sharing Schemes for Multiple Secret Images Allowing the Rotation of Shares
    Author(s)
    M. Iwamoto, L. Wang, K. Yoneyama, N. Kunihiro, and K. Ohta
    Journal
    IEICE Transactions on Fundamentals
    Vol.
    E89–A
    No.
    5
    Pages
    1382–1395
    Publisher
    IEICE
    Publication Year
    2006
    Abstract

    In this paper, a method is proposed to construct a visual secret sharing (VSS) scheme for multiple secret images in which each share can be rotated with 180 degrees in decryption. The proposed VSS scheme can encrypt more number of secret images compared with the normal VSS schemes. Furthermore, the proposed technique can be applied to the VSS scheme that allows to turn over some shares in decryption. From the theoretical point of view, it is interesting to note that such VSS schemes cannot be obtained from so-called basis matrices straightforwardly.

  • Strongly Secure Ramp Secret Sharing Schemes for General Access Structures
    Author(s)
    M. Iwamoto and H. Yamamoto
    Journal
    Information Processing Letters
    Vol.
    97
    No.
    2
    Pages
    52–57
    Publisher
    Elsevier
    Publication Year
    2006
    Abstract

    Ramp secret sharing (SS) schemes can be classified into strong ramp SS schemes and weak ramp SS schemes. The strong ramp SS schemes do not leak out any part of a secret explicitly even in the case that some information about the secret leaks out from some set of shares, and hence, they are more desirable than the weak ramp SS schemes. In this paper, it is shown that for any feasible general access structure, a strong ramp SS scheme can be constructed from a partially decryptable ramp SS scheme, which can be considered as a kind of SS scheme with plural secrets. As a byproduct, it is pointed out that threshold ramp SS schemes based on Shamir's polynomial interpolation method are not always strong.

  • Quantum Secret Sharing Schemes and Reversibility of Quantum Operations
    Author(s)
    T. Ogawa, A. Sasaki, M. Iwamoto, and H. Yamamoto
    Journal
    Physical Review A
    Vol.
    72
    No.
    3
    Publisher
    APS Physics
    Publication Year
    2005
    Abstract

    Quantum secret sharing schemes encrypting a quantum state into a multipartite entangled state are treated. The lower bound on the dimension of each share given by Gottesman [Phys. Rev. A 61, 042311 (2000)] is revisited based on a relation between the reversibility of quantum operations and the Holevo information. We also propose a threshold ramp quantum secret sharing scheme and evaluate its coding efficiency.

  • A Construction Method of Visual Secret Sharing Schemes for Plural Secret Images
    Author(s)
    M. Iwamoto and H. Yamamoto
    Journal
    IEICE Transactions on Fundamentals
    Vol.
    E86–A
    No.
    10
    Pages
    2577–2588
    Publisher
    IEICE
    Publication Year
    2003
    Abstract

    In this paper, a new method is proposed to construct a visual secret sharing scheme with a general access structure for plural secret images. Although the proposed scheme can be considered as an extension of Droste's method that can encode only black-white images, it can encode plural gray-scale and/or color secret images.

  • The Optimal n-out-of-n Visual Secret Sharing Scheme for Gray-Scale Images
    Author(s)
    M. Iwamoto and H. Yamamoto
    Journal
    IEICE Transactions on Fundamentals
    Vol.
    E85–A
    No.
    10
    Pages
    2238–2247
    Publisher
    IEICE
    Publication Year
    2002
    Abstract

    In this paper, a method is proposed to construct an n-out-of-n visual secret sharing scheme for gray-scale images, for short an (n, n)-VSS-GS scheme, which is optimal in the sense of contrast and pixel expansion, i.e., resolution. It is shown that any (n, n)-VSS-GS scheme can be constructed based on the so- called polynomial representation of basis matrices treated in (15), (16). Furthermore, it is proved that such construction can attain the optimal (n, n)-VSS-GS scheme.

  • An Analytic Construction of the Visual Secret Sharing Scheme for Color Images
    Author(s)
    H. Koga, M. Iwamoto, and H. Yamamoto
    Journal
    IEICE Transactions on Fundamentals
    Vol.
    E84–A
    No.
    1
    Pages
    262– 272
    Publisher
    IEICE
    Publication Year
    2001
    Abstract

    This paper proposed a new construction of the visual secret sharing scheme for the (n, n)-threshold access structure applicable to color images. The construction uses matrices with n rows that can be identifield with homogeneous polynomials of degree n. It is shown that, if we find a set of homogeneous polynomials of degree n satisfying a certain system of simultaneous partial differential equations, we can construct a visual secret sharing scheme for the (n, n)-threshold access structure by using the matrices corresponding to the homogeneous polynomials. The construction is easily extended to the cases of the (t, n)-threshold access structure and more general access structures.

Conference Papers

  • Efficient Threshold Public Key Encryption from the Computational Bilinear Diffie-Hellman Assumption
    Author(s)
    M. Ebina, J. Mita, J. Shikata, and Y. Watanabe
    Conference
    APKC 2021
    Publisher
    ACM Press
    Publication Year
    2021
    To appear.
    Abstract

    In this paper, we show the first efficient threshold public-key encryption (TPKE) scheme under the difficulty of search problems. More specifically, our TPKE scheme simultaneously achieves: (1) Chosen ciphertext security (CCA security) under the computational bilinear Diffie-Hellman (CBDH) assumption in the standard model; (2) re-splittability, which is a useful property that makes partial secret keys refreshable; and (3) O(kappa)-bit ciphertexts and public keys. Most previous CCA-secure TPKE schemes rely on decisional complexity assumptions or random oracles. Although there exist CCA-secure TPKE schemes under the difficulty of search problems, all such schemes are inefficient or work over small plaintext spaces. Technically, we begin with a direct construction of a threshold identity-based key encapsulation mechanism (TIB-KEM) with a weak security notion. Then, we transform the weakly-secure TIB-KEM into a CCA-secure TPKE scheme via the tag-KEM/DEM approach.

  • Anonymous Broadcast Authentication for Securely Remote-Controlling IoT Devices
    Author(s)
    Y. Watanabe, N. Yanai, and J. Shikata
    Conference
    AINA 2021
    Vol.
    LNNS 226
    Pages
    679–690
    Publisher
    Springer
    Publication Year
    2021
    Date Presented
    2021/05/14
    Abstract

    In this paper, we present a basic system for controlling IoT devices in remote environments with the following requirements: (1) in a situation where an operation center broadcasts information to IoT devices, e.g., wireless environment, only the designated devices can identify operations sent from the center; (2) the devices can detect manipulation of the broadcast information and hence prevents maliciously generated operations from being executed. We formalize a model of the basic system and its essential requirements and propose anonymous broadcast authentication (ABA) as its core cryptographic primitive. We formally define the syntax and security notions for ABA and show provably-secure ABA constructions.

  • A Key Recovery Algorithm Using Random Key Leakage from AES Key Schedule
    Author(s)
    T. Uemura, Y. Watanabe, Y. Li, N. Miura, M. Iwamoto, K. Sakiyama, and K. Ohta
    Conference
    ISITA 2020
    Pages
    382–386
    Publisher
    IEEE
    Publication Year
    2020
    Date Presented
    2020/10/24
    Abstract

    A key recovery algorithm using parts of the key schedules is proposed for evaluating the threat of probing attack. Suppose that we have an information leakage sensor, and we can detect a leak (attacked) point where an attacker makes electrical/physical contact with a laser, a probe, etc. We assume that the attacked bits (leaked bits) are completely known to the attacker, whereas the other non-attacked bits are not leaked at all. We also assume that each bit leaks with a constant probability. Our key recovery algorithm is constructed by modifying the pruning phase that for cold boot attacks proposed by Tsow. Experimental result shows that, using our algorithm, more than 15% leakage recovers the key with almost probability 1, whereas less than 10% is recovered with small probability close to 0.

  • How to Detect Malicious Behaviors in a Card-Based Majority Voting Protocol with Three Inputs
    Author(s)
    Y. Abe, M. Iwamoto, and K. Ohta
    Conference
    ISITA 2020
    Pages
    377–381
    Publisher
    IEEE
    Publication Year
    2020
    Abstract

    Card-based protocol is a multi-party computation using cards. The card-based protocol using operations called private operation has an advantage that the number of cards and the number of times of communication are smaller than the card-based protocol using operations called shuffle. However, there is a disadvantage that private operation allows dishonest players to perform malicious behaviors. Although the method to detect malicious behaviors in private operations was proposed, the method was available only in committed-format protocols, where inputs and outputs are represented by a pair of cards called commitment. In this paper, we show how to detect malicious behaviors in non-committed-format protocol with an example of a three-input majority voting protocol using private operations. Our majority voting protocol requires a smaller number of cards than the minimum number of cards required for committed-format protocols.

  • On the Power of Interaction in Signcryption
    Author(s)
    J. Ida, J. Shikata, and Y. Watanabe
    Conference
    ISITA 2020
    Pages
    348–352
    Publisher
    IEEE
    Publication Year
    2020
    Abstract

    Signcryption (SC) achieves the goal with lower computational costs than simply combining public-key encryption (PKE) and digital signatures (DS). Meanwhile, at SCN 2014, Dodis and Fiore formalized interactive PKE and DS. In particular, in the interactive setting, they showed a CCA-secure PKE scheme can be constructed assuming only CPA-secure PKE schemes in a black-box manner. In this paper, we focus on SC schemes in the interactive setting (ISC for short). Specifically, we newly define a model and security notions for ISC schemes. We then propose generic constructions of ISC schemes by using CPA-secure PKE schemes rather than CCA-secure ones, whereas such a realization is unknown in the context of non-interactive SC schemes. We show that two rounds are sufficient to construct an ISC scheme from only CPA-secure PKE schemes. Furthermore, we also show the first SC scheme that can be efficiently instantiated from simple assumptions in the standard model without pairings or lattices by allowing interaction.

  • Card-Based Secure Ranking Computations
    Author(s)
    K. Takashima, Y. Abe, T. Sasaki, D. Miyahara, K. Shinagawa, T. Mizuki, and H. Sone
    Conference
    COCOA 2019
    Vol.
    LNCS 11949
    Pages
    461–472
    Publisher
    Springer
    Publication Year
    2019
    Date Presented
    2019/12/14
    Abstract

    Consider a group of people who want to know the “rich list” among them, namely the ranking in terms of their total assets, without revealing any information about the actual value of their assets. This can be achieved by a “secure ranking computation,” which was first considered by Jiang and Gong (CT-RSA 2006); they constructed a secure ranking computation protocol based on a public-key cryptosystem. In this paper, instead of using a public-key cryptosystem, we use a deck of physical cards to provide secure ranking computation protocols. Therefore, our card-based protocols do not rely on computers, and they are simple and easy for humans to implement. Specifically, we design four protocols considering tradeoffs between the number of cards and the number of shuffles required to execute protocols. We also present a guide to choose an appropriate protocol according to the number of people participating in the protocol and the size of the input range.

  • Efficient Private PEZ Protocols for Symmetric Functions
    Author(s)
    Y. Abe, M. Iwamoto, and K. Ohta
    Conference
    TCC 2019
    Vol.
    LNCS 11891
    Pages
    372–392
    Publisher
    Springer
    Publication Year
    2019
    Date Presented
    Dec. 3, 2019
    Abstract

    A private PEZ protocol is a variant of secure multi-party computation performed using a (long) PEZ dispenser. The original paper by Balogh et al. presented a private PEZ protocol for computing an arbitrary function with n inputs. This result is interesting, but no follow-up work has been presented since then, to the best of our knowledge. We show herein that it is possible to shorten the initial string (the sequence of candies filled in a PEZ dispenser) and the number of moves (a player pops out a specified number of candies in each move) drastically if the function is symmetric. Concretely, it turns out that the length of the initial string is reduced from O(2n!) for general functions in Balogh et al.’s results to O(n·n!)$ for symmetric functions, and 2n moves for general functions are reduced to n2 moves for symmetric functions. Our main idea is to utilize the recursive structure of symmetric functions to construct the protocol recursively. This idea originates from a new initial string we found for a private PEZ protocol for the three-input majority function, which is different from the one with the same length given by Balogh et al. without describing how they derived it.

  • Identity-Based Encryption with Security against the KGC: A Formal Model and Its Instantiation from Lattices
    Author(s)
    K. Emura, S. Katsumata, and Y. Watanabe
    Conference
    ESORICS 2019
    Vol.
    LNCS 11736
    Pages
    113–133
    Publisher
    Springer
    Publication Year
    2019
    Date Presented
    Sep. 25, 2019
    Abstract

    The key escrow problem is one of the main barriers to the widespread real-world use of identity-based encryption (IBE). Specifically, a key generation center (KGC), which generates secret keys for a given identity, has the power to decrypt all ciphertexts. At PKC 2009, Chow defined a notion of security against the KGC, that relies on assuming that it cannot discover the underlying identities behind ciphertexts. However, this is not a realistic assumption since, in practice, the KGC manages an identity list and hence it can easily guess the identities corresponding to given ciphertexts. Chow later closed the gap between theory and practice by introducing a new entity called an identity-certifying authority (ICA) and proposed an anonymous key-issuing protocol. Essentially, this allows the users, KGC, and ICA to interactively generate secret keys without users ever having to reveal their identities to the KGC. Unfortunately, the proposed protocol did not include a concrete security definition, meaning that all of the subsequent works following Chow lack the formal proofs needed to determine whether or not it delivers a secure solution to the key escrow problem.
    In this paper, based on Chow’s work, we formally define an IBE scheme that resolves the key escrow problem and provide formal definitions of security against corrupted users, KGC, and ICA. Along the way, we observe that if we are allowed to assume a fully trusted ICA, as in Chow’s work, then we can construct a trivial (and meaningless) IBE scheme that is secure against the KGC. Finally, we present a lattice-based construction in our new security model based on the Gentry–Peikert–Vaikuntanathan (GPV) IBE scheme (STOC 2008) and Rückert’s lattice-based blind signature scheme (ASIACRYPT 2010).

  • Optimal Multiple Assignment Schemes Using Ideal Multipartite Secret Sharing Schemes
    Author(s)
    R. Eriguchi, N. Kunihiro, and M. Iwamoto
    Conference
    IEEE ISIT 2019
    Pages
    3047–3051
    Publisher
    IEEE
    Publication Year
    2019
    Date Presented
    Jul. 12, 2019
    Abstract

    A multiple assignment scheme (MAS) is a method to construct secret sharing schemes (SSSs) for general access structures. There are MASs using threshold and ramp SSSs. The paper proposes new MASs using ideal SSSs realizing compartmented access structures and those using SSSs realizing multi-level access structures. Since the ideal SSSs realizing compartmented access structures and SSSs realizing multi-level access structures are natural generalizations of threshold and ramp SSSs, respectively, the new MASs cannot be less efficient than those using threshold or ramp SSSs.

  • Light Cryptography
    Author(s)
    P. Lafourcade, T. Mizuki, A. Nagao, and K. Shinagawa
    Conference
    WISE 2019
    Vol.
    IFIPAICT 552
    Pages
    89–101
    Publisher
    Springer
    Publication Year
    2019
    Date Presented
    Jun. 26, 2019
    Abstract

    Physical cryptography provides cryptographic protocols using physical objects like cards and envelopes instead of using computers. In this paper, we introduce a new model for physical cryptography, called light cryptography. It uses transparent sheets and some properties of light and shadows. We design several secure light cryptographic protocols: one for set-intersection (which can solve the scheduling problem), one for maximum (which can solve the Yao's Millionaires' problem), one for computing the sum of integers. We believe that our protocols using light cryptography are a powerful tool for information security education because they are fairly simple and fun to use.

  • Secure Computation of Any Boolean Function Based on Any Deck of Cards
    Author(s)
    K. Shinagawa and T. Mizuki
    Conference
    FAW 2019
    Vol.
    LNCS 11458
    Pages
    63–75
    Publisher
    Springer
    Publication Year
    2019
    Date Presented
    May 1, 2019
    Abstract

    It is established that secure computation can be achieved by using a deck of physical cards. Almost all existing card-based protocols are based on a specific deck of cards. In this study, we design card-based protocols that are executable using any deck of cards (e.g., playing cards, UNO, and trading cards). Specifically, we construct a card-based protocol for any Boolean function based on any deck of cards. As corollaries of our result, a standard deck of playing cards (having 52 cards) enables secure computation of any 22-variable Boolean function, and UNO (having 112 cards) enables secure computation of any 53-variable Boolean function.

  • Card-Based Cryptography with Invisible Ink
    Author(s)
    K. Shinagawa
    Conference
    TAMC 2019
    Vol.
    LNCS 11436
    Pages
    566–577
    Publisher
    Springer
    Publication Year
    2019
    Date Presented
    Apr. 16, 2019
    Abstract

    It is known that secure computation can be done by using a deck of physical cards; card-based cryptography makes people understand the correctness and security of secure computation, even for people who are not familiar with mathematics. In this paper, we propose a new type of cards, layered polygon cards, based on the use of invisible ink. A deck of cards with invisible ink naturally hides the contents of cards and allows to open some pieces of contents, which we referred to it as partial opening. Based on them, we construct novel protocols for various interesting functions such as carry of addition, equality, and greater-than.

  • An Abstraction Model for 1-bit Probing Attack on Block Ciphers
    Author(s)
    N. Shoji, T. Sugawara, M. Iwamoto, and K. Sakiyama
    Conference
    ICCCS 2019
    Pages
    502–506
    Publisher
    IEEE
    Publication Year
    2019
    Date Presented
    Feb. 23–25, 2019
    Abstract

    The threat of physical attacks on crypto devices has been reported. Attack efficiency is determined by the attacker's ability to measure the physical information leaked from the device, for example, side-channel information, or to control physical disturbance against a device, for example, laser fault injection. Intuitively, the higher the ability of the attacker, the more information can be retrieved. In order to assess attack efficiency, this paper focuses on the 1-bit probing attack, which is one of the strongest attacks in a real-world setting. In the 1-bit probing attack, the attacker can observe a specific 1-bit intermediate value at a certain timing in the cryptographic operation. Firstly, we explain previous studies on an abstraction model for physical attacks. Secondly, we introduce a new model with an ideal property with regard to the probing attacks. Finally, we compare the attack efficiency of seven reported block ciphers with the proposed model value.

  • Implementation and Analysis of Fully Homomorphic Encryption in Wearable Devices
    Author(s)
    A. Prasitsupparote, Y. Watanabe, and J. Shikata
    Conference
    ISDF 2018
    Pages
    1–14
    Publisher
    SDIWC Library
    Publication Year
    2018
    Date Presented
    2018
    Abstract

    Currently, wearable devices, which are known as one of the Internet of things (IoT) devices, have been widely used for healthcare systems. Most of the healthcare systems store users’ healthcare data, which is encrypted by ordinary symmetric-key en- cryption and/or public-key encryption schemes, in a (cloud) server. However, the encrypted data needs to be decrypted for data analysis, and it means that sensitive information is leaked to the server. One promising solution is to use fully homomorphic encryption (FHE), which enables ones to perform any computation among encrypted data while keep- ing it encrypted. Although FHE generally requires high computational and communication costs in the theoretical sense, several researchers have imple- mented FHE schemes to measure their practical efficiency. In this paper, we consider a privacy- preserving protocol for healthcare systems employ- ing wearable devices, and implement this proto- col over Raspberry Pi, which is a popular single- board computer, to measure the actual efficiency of FHE over wearable devices. Specifically, we implemented the protocol by using two FHE li- braries, HElib and SEAL, on Raspberry Pi and net- work simulator to measure both computational and communication costs in wireless body area network (WBAN). In terms of the communication overhead, our result shows that the protocol with SEAL is bet- ter than that with HElib. In particular, the proto- col with SEAL has almost the same communication costs as the trivial protocol, which is the same pro- tocol without encryption. On the other hand, HE- lib is better than SEAL regarding the running time, while SEAL can perform more homomorphic op- erations than HElib for the almost same plaintext- size. Therefore, HElib is suitable for applications which require small time complexity, and SEAL is suitable for applications which require many homo- morphic operations.

  • Card-Based Majority Voting Protocols with Three Inputs Using Three Cards
    Author(s)
    Y. Watanabe, Y. Kuroki, S. Suzuki, Y. Koga, M. Iwamoto, and K. Ohta
    Conference
    ISITA 2018
    Pages
    218–222
    Publisher
    IEEE
    Publication Year
    2018
    Abstract

    Private operations (private permutations) were independently introduced by Nakai et al. and Marcedone et al. for implementing card-based cryptographic protocols efficiently. Recently, Nakai et al. showed that, if the private operations are available, secure computations of AND and OR operations for two inputs can be realized simultaneously by using four cards, and the protocol is applied to four-card majority voting protocol with three inputs. In this paper, it is shown that only three cards are sufficient to construct the majority voting protocol with three inputs. Specifically, we propose two constructions of three-input majority voting protocols. First, assuming that players are allowed to announce their outputs, we show that one card can be reduced from Nakai et al.'s protocol without any additional private operations and communications. Our second construction requires two more private operations and communications, whereas it removes the assumption on announcement from the first construction.

  • Key-Updatable Public-Key Encryption with Keyword Search: Models and Generic Constructions
    Author(s)
    H. Anada, A. Kanaoka, N. Matsuzaki, and Y. Watanabe
    Conference
    ACISP 2018
    Vol.
    LNCS 10946
    Pages
    341–359
    Publisher
    Springer
    Publication Year
    2018
    Abstract

    Public-key encryption with keyword search (PEKS) enables us to search over encrypted data, and is expected to be used between a cloud server and users’ devices such as laptops or smartphones. However, those devices might be lost accidentally or be stolen. In this paper, we deal with such a key-exposure problem on PEKS, and introduce a concept of PEKS with key-updating functionality, which we call key-updatable PEKS (KU-PEKS). Specifically, we propose two models of KU-PEKS: The key-evolution model and the key-insulation model. In the key-evolution model, a pair of public and secret keys can be updated if needed (e.g., the secret key is exposed). In the key-insulation model, a public key remains fixed while a secret key can be updated if needed. The former model makes a construction simple and more efficient than the latter model. On the other hand, the latter model is preferable for practical use since a user never updates his/her public key. We show constructions of a KU-PEKS scheme in each model in a black-box manner. We also give an experimental result for the most efficient instantiation, and show our proposal is practical.

  • Broadcast Encryption with Guessing Secrecy
    Author(s)
    Y. Watanabe
    Conference
    ICITS 2017
    Vol.
    LNCS 10681
    Pages
    39–57
    Publisher
    Springer
    Publication Year
    2017
    Abstract

    Perfect secrecy, which is a fundamental security notion introduced by Shannon, guarantees that no information on plaintexts is leaked from corresponding ciphertexts in the information-theoretic sense. Although it captures the strongest security, it is well-known that the secret-key size must be equal or larger than the plaintext-size to achieve perfect secrecy. Furthermore, probability distribution on secret keys must be uniform. Alimomeni and Safavi-Naini (ICITS 2012) proposed a new security notion, called guessing secrecy, to relax the above two restrictions, and showed that unlike perfect secrecy, even non-uniform keys can be used for providing guessing secrecy. Iwamoto and Shikata (ISIT 2015) showed secure concrete constructions of a symmetric-key encryption scheme with non-uniform keys in the guessing secrecy framework. In this work, we extend their results to the broadcast encryption setting. We first define guessing secrecy of broadcast encryption, and show relationships among several guessing-secrecy notions and perfect secrecy. We derive lower bounds on secret keys, and show the Fiat-Naor one-bit construction with non-uniform keys is also secure in the sense of guessing secrecy.

  • Four Cards Are Enough for Card-Based Three-Input Voting Protocol Utilizing Private Permutations
    Author(s)
    T. Nakai, S. Shirouchi, M. Iwamoto, and K. Ohta
    Conference
    ICITS 2017
    Vol.
    LNCS 10681
    Pages
    153–165
    Publisher
    Springer
    Publication Year
    2017
    Abstract

    The card-based cryptographic protocol is a variant of multi-party computation that enables us to compute a certain function securely by using playing cards. In existing card-based cryptographic protocols, a special operation of cards called a shuffle is used to achieve the information-theoretic security. Recently, card-based cryptographic protocols have been reconsidered from the viewpoint of multi-party computations. In this direction, a new model of card-based cryptographic protocol including a new assumption called Private Permutations (PP, for short) is introduced and succeeds in constructing efficient protocols for the millionaires’ protocol. In this paper, we construct efficient card-based cryptographic OR and XOR protocols based on the existing AND protocol. Furthermore, by unifying AND and OR protocols, it is shown that a majority voting protocol with three inputs is efficiently obtained. Our construction requires only four cards thanks to PPs, whereas the previous work requires eight cards.

  • Lattice-Based Revocable Identity-Based Encryption with Bounded Decryption Key Exposure Resistance
    Author(s)
    A. Takayasu and Y. Watanabe
    Conference
    ACISP 2017
    No.
    LNCS 10342
    Pages
    184–204
    Publisher
    Springer
    Publication Year
    2017
    Abstract

    A revocable identity-based encryption (RIBE) scheme, proposed by Boldyreva et al., provides a revocation functionality for managing a number of users dynamically and efficiently. To capture a realistic scenario, Seo and Emura introduced an additional important security notion, called decryption key exposure resistance (DKER), where an adversary is allowed to query short-term decryption keys. Although several RIBE schemes that satisfy DKER have been proposed, all the lattice-based RIBE schemes, e.g., Chen et al.’s scheme, do not achieve DKER, since they basically do not have the key re-randomization property, which is considered to be an essential requirement for achieving DKER. In particular, in every existing lattice-based RIBE scheme, an adversary can easily recover plaintexts if the adversary is allowed to issue even a single short-term decryption key query. In this paper, we propose a new lattice-based RIBE scheme secure against exposure of a-priori bounded number of decryption keys (for every identity). We believe that this bounded notion is still meaningful and useful from a practical perspective. Technically, to achieve the bounded security without the key re-randomization property, key updates in our scheme are short vectors whose corresponding syndrome vector changes in each time period. For this approach to work correctly and for the scheme to be secure, cover free families play a crucial role in our construction.

  • Unconditionally Secure Searchable Encryption
    Author(s)
    T. Yoshizawa, Y. Watanabe, and J. Shikata
    Conference
    CISS 2017
    Pages
    1–6
    Publisher
    IEEE
    Publication Year
    2017
    Abstract

    Searchable symmetric encryption (SSE) enables us to search encrypted data with an arbitrarily chosen keyword without leaking information on the data and keyword. SSE is expected to be used in, for example, cloud computing and genome analyses. In particular, privacy of genome data must be guaranteed for long periods, and therefore unconditionally secure cryptographic protocols, rather than computationally secure ones, should be used for protecting genome data. For this reason, we propose new constructions of unconditionally secure SSE schemes in this paper. Specifically, we define a model and security of unconditionally secure SSE, and we show a lower bound on secret-key sizes. We propose two kinds of constructions of unconditionally secure SSE schemes: One is asymptotically optimal in the sense of the secret-key size with some restriction on the security definition; and the other achieves full security at the sacrifice of the secret-key size.

  • New Revocable IBE in Prime-Order Groups: Adaptively Secure, Decryption Key Exposure Resistant, and with Short Public Parameters
    Author(s)
    Y. Watanabe, K. Emura, and J.H. Seo
    Conference
    CT-RSA 2017
    Vol.
    LNCS 10159
    Pages
    432–449
    Publisher
    Springer
    Publication Year
    2017
    Abstract

    Revoking corrupted users is a desirable functionality for cryptosystems. Since Boldyreva, Goyal, and Kumar (ACM CCS 2008) proposed a notable result for scalable revocation method in identity-based encryption (IBE), several works have improved either the security or the efficiency of revocable IBE (RIBE). Currently, all existing scalable RIBE schemes that achieve adaptively security against decryption key exposure resistance (DKER) can be categorized into two groups; either with long public parameters or over composite-order bilinear groups. From both practical and theoretical points of views, it would be interesting to construct adaptively secure RIBE scheme with DKER and short public parameters in prime-order bilinear groups.
    In this paper, we address this goal by using Seo and Emura’s technique (PKC 2013), which transforms the Waters IBE to the corresponding RIBE. First, we identify necessary requirements for the input IBE of their transforming technique. Next, we propose a new IBE scheme having several desirable properties; satisfying all the requirements for the Seo-Emura technique, constant-size public parameters, and using prime-order bilinear groups. Finally, by applying the Seo-Emura technique, we obtain the first adaptively secure RIBE scheme with DKER and constant-size public parameters in prime-order bilinear groups.

  • Unconditionally Secure Revocable Storage: Tight Bounds, Optimal Construction, and Robustness
    Author(s)
    Y. Watanabe, G. Hanaoka, and J. Shikata
    Conference
    ICITS 2016
    Vol.
    LNCS 10015
    Pages
    213–237
    Publisher
    Springer
    Publication Year
    2016
    Abstract

    Data stored in cloud storage sometimes requires long-term security due to its sensitivity (e.g., genome data), and therefore, it also requires flexible access control for handling entities who can use the data. Broadcast encryption can partially provide such flexibility by specifying privileged receivers so that only they can decrypt a ciphertext. However, once privileged receivers are specified, they can be no longer dynamically added and/or removed. In this paper, we propose a new type of broadcast encryption which provides long-term security and appropriate access control, which we call unconditionally secure revocable-storage broadcast encryption (RS-BE). In RS-BE, privileged receivers of a ciphertext can be dynamically updated without revealing any information on the underlying plaintext. Specifically, we define a model and security of RS-BE, and derive tight lower bounds on sizes of secret keys required for a one-time secure RS-BE scheme when the ciphertext size is equal to the plaintext size. Our lower bounds can be applied to traditional broadcast encryption. We then construct a one-time secure RS-BE scheme with a trade-off between sizes of ciphertexts and secret keys, and our construction for the smallest ciphertext size meets all bounds with equalities. Furthermore, to detect an improper update, we consider security against modification attacks to a ciphertext, and present a concrete construction secure against this type of attacks.

  • Efficient Card-Based Cryptographic Protocols for Millionaires’ Problem Utilizing Private Permutations
    Author(s)
    T. Nakai, Y. Misawa, Y. Tokushige, M. Iwamoto, and K. Ohta
    Conference
    CANS 2016
    Vol.
    LNCS 10052
    Pages
    350–364
    Publisher
    Springer
    Publication Year
    2016
    Date Presented
    Nov. 15, 2016
    Abstract

    We propose several efficient card-based cryptographic protocols for the millionaires’ problem by introducing a new operation called Private Permutation (PP) instead of the shuffle used in existing card-based cryptographic protocols. Shuffles are useful randomization techniques for designing card-based cryptographic protocols for logical gates, and this approach seems to be almost optimal. This fact, however, implies that there is room for improvements if we do not use logical gates as building blocks for secure computing, and we show that such an improvement is actually possible for the millionaires’ problem. Our key technique, PP, is a natural randomization operation for permuting a set of cards behind the player’s back, and hence, a shuffle can be decomposed into two PPs with one communication between them. Thus PP not only allows us to transform Yao’s seminal protocol into a card-based cryptographic protocol, but also enables us to propose entirely novel and efficient protocols by securely updating bitwise comparisons between two numbers. Furthermore, it is interesting to remark that one of the proposed protocols has a remarkably deep connection to the well-known logical puzzle known as “The fork in the road”.

  • Probabilistic Generation of Trapdoors: Reducing Information Leakage of Searchable Symmetric Encryption
    Author(s)
    K. Hayasaka, Y. Kawai, Y. Koseki, T. Hirano, K. Ohta, and M. Iwamoto
    Conference
    CANS 2016
    Vol.
    LNCS 10052
    Pages
    500–517
    Publisher
    Springer
    Publication Year
    2016
    Abstract

    Searchable symmetric encryption (SSE) enables a user to outsource a collection of encrypted documents in the cloud and to perform keyword searching without revealing information about the contents of the documents and queries. On the other hand, the information (called search pattern) whether or not the same keyword is searched in each query is always leaked in almost all previous schemes whose trapdoors are generated deterministically. Therefore, reducing the search pattern leakage is outside the scope of almost all previous works. In this paper, we tackle to the leakage problem of search pattern, and study methodology to reduce this leakage. Especially, we discuss that it might be possible to reduce the search pattern leakage in cases where a trapdoor does not match any encrypted document. We also point out that the same search pattern is leaked regardless of probabilistic or deterministic generation of trapdoors when the user searches using a keyword which has already searched and matched a certain encrypted document. Thus, we further aim to construct SSE schemes with fast “re-search” process, in addition to reducing the search pattern leakage. In order to achieve the above, we introduce a new technique “trapdoor locked encryption” which can extract a deterministic trapdoor from a probabilistic trapdoor, and then propose a new SSE scheme which can generate trapdoors probabilistically and reduce the search pattern leakage. Our scheme is constructed by applying our technique to the well-known and influential scheme SSE-2 (ACM CCS 2006) and can be proved secure in the standard model.

  • Simple, Secure, and Efficient Searchable Symmetric Encryption with Multiple Encrypted Indexes
    Author(s)
    T. Hirano, M. Hattori, Y. Kawai, N. Matsuda, M. Iwamoto, K. Ohta, Y. Sakai, and T. Munaka
    Conference
    IWSEC 2016
    Vol.
    LNCS 9836
    Pages
    91–110
    Publisher
    Springer
    Publication Year
    2016
    Date Presented
    Sep., 2016
    Abstract

    In searchable symmetric encryption (SSE), adding documents to a database is an indispensable functionality in real situations, and there are two approaches for executing the process: One approach is to update the encrypted index, and the other is to generate a new encrypted index. The former approach is called dynamic SSE, which has been extensively studied recently due to its importance. The latter approach has an advantage such that it can be directly applied to any existing SSE scheme without degrading its original functionalities, but previous methods are not satisfactory from a viewpoint of security, storage size, or efficiency. In this paper, we propose a simple document adding method that resolve the problem occurred in the latter approach. Our method is quite generic, and therefore can be applied to any existing SSE scheme (e.g. non-dynamic one with useful functionalities). Our key idea is to utilize publicly available information and hash chains in construction of encrypted indexes. In order to exhibit the ability of our method, we present a concrete scheme which is led by applying our method to the well-known and influential scheme SSE-2 (ACM CCS 2006). Thanks to the simplicity of our method, the scheme can be easily proved secure under a naturally generalized setting of the most widely used security model.

  • Deep-Learning-Based Security Evaluation on Authentication Systems Using Arbiter PUF and Its Variants
    Author(s)
    R. Yashiro, T. Machida, M. Iwamoto, and K. Sakiyama
    Conference
    IWSEC 2016
    Vol.
    LNCS 9836
    Pages
    267–285
    Publisher
    Springer
    Publication Year
    2016
    Date Presented
    2016
    Abstract

    Fake integrated circuit (IC) chips are in circulation on the market, which is considered a serious threat in the era of the Internet of Things (IoTs). A physically unclonable function (PUF) is expected to be a fundamental technique to separate the fake IC chips from genuine ones. Recently, the arbiter PUF (APUF) and its variants are intensively researched aiming at using for a secure authentication system. However, vulnerability of APUFs against machine-learning attacks was reported. Upon the situation, the double arbiter PUF (DAPUF), which has a tolerance against support vector machine (SVM)-based machine-learning attacks, was proposed as another variant of APUF in 2014. In this paper, we perform a security evaluation for authentication systems using APUF and its variants against Deep-learning (DL)-based attacks. DL has attracted attention as a machine-learning method that produces better results than SVM in various research fields. Based on the experimental results, we show that these DAPUFs could be used as a core primitive in a secure authentication system if setting an appropriate threshold to distinguish a legitimate IC tags from fake ones.

  • Sequential Aggregate Authentication Codes with Information Theoretic Security
    Author(s)
    S. Tomita, Y. Watanabe, and J. Shikata
    Conference
    CISS 2016
    Pages
    192–197
    Publisher
    IEEE
    Publication Year
    2016
    Abstract

    Sequential aggregate signature (SAS) schemes provide a single, compact signature, which is generated from a number of signatures, that simultaneously ensures that each signature is legally generated from the corresponding message with a defined order. Although SAS schemes have various applications such as a secure border gateway protocol, all existing schemes are computationally secure (i.e., assuming computationally bounded adversaries). In this paper, we first propose sequential aggregate authentication codes (SAA-codes), which has similar functionality of SAS in the information theoretic security setting. Specifically, we give a model and security formalization of SAA-codes, derive lower bounds on sizes of secret keys and authenticators required in secure SAA-codes, and present two kinds of optimal constructions in the sense that each construction meets the lower bounds with equalities.

  • Identity-Based Hierarchical Key-Insulated Encryption without Random Oracles
    Author(s)
    Y. Watanabe and J. Shikata
    Conference
    PKC 2016
    No.
    LNCS 9614
    Pages
    255–279
    Publisher
    Springer
    Publication Year
    2016
    Abstract

    Key-insulated encryption is one of the effective solutions to a key exposure problem. Recently, identity-based encryption (IBE) has been used as one of fundamental cryptographic primitives in a wide range of various applications, and it is considered that the identity-based key-insulated security has a huge influence on the resulting applications. At Asiacrypt’05, Hanaoka et al. proposed an identity-based hierarchical key-insulated encryption (hierarchical IKE) scheme. Although their scheme is secure in the random oracle model, it has a “hierarchical key-updating structure,” which is attractive functionality that enhances key exposure resistance.

  • Constructions of Unconditionally Secure Broadcast Encryption from Key Predistribution Systems with Trade-offs between Communication and Storage
    Author(s)
    Y. Watanabe and J. Shikata
    Conference
    ProvSec 2015
    Vol.
    LNCS 9451
    Pages
    489–502
    Publisher
    Springer
    Publication Year
    2015
    Abstract

    An (≤n,≤ω)-one-time secure broadcast encryption schemes (BESs) allows a sender to specify any subset of receivers so that only the specified recievers can decrypt a ciphertext. In this paper, we first show an efficient construction of a BES with general ciphertext sizes. Specifically, we propose a generic construction of a BES from key predistribution systems (KPSs) when its ciphertext size is equal to integer multiple of the plaintext size, and our construction includes all known constructions. However, there are many possible combinations of the KPSs to realize the BES in our construction methodology, and therefore, we show that which combination is the best one in the sense that secret-key size can be minimized. Deriving a tight bound on the secret-key size required for (≤n,≤ω)-one-time secure BES with any ciphertext size still remains an open problem.Our result also means that we first show an upper bound on the size of secret keys for general ciphertext sizes.

  • Keyword Revocable Searchable Encryption with Trapdoor Exposure Resistance and Re-Generateability
    Author(s)
    K. Emura, L. T. Phong, and Y. Watanabe
    Conference
    IEEE TrustCom 2015
    Pages
    167–174
    Publisher
    IEEE
    Publication Year
    2015
    Abstract

    In searchable encryption in the public key setting, a trapdoor is uploaded to a server, and the server runs the test algorithm by using the trapdoor. However, if trapdoors stored in the server will be exposed due to unexpected situations, then anyone can run the test algorithm. Therefore, the trapdoor revocation functionality is desirable in practice. Moreover, even certain keyword revocation functionality is supported, the impact of trapdoor exposure should be minimized. In addition to this, it seems difficult to assume that revoked keywords will never be used. Therefore, we need to consider the case where a new trapdoor can be generated even a trapdoor has been revoked before. In this paper, we give a formal definition of keyword revocable public key encryption with keyword search (KR-PEKS), and propose a generic construction of KR-PEKS from revocable identity-based encryption with a certain anonymity. Our construction is not only a generalization of revocable keyword search proposed by with Yu, Ni, Yang, Mu, and Susilo (Security and Communication Networks 2014), but also supports trapdoor exposure resistance which guarantees that an exposure of a trapdoor does not infect of other trapdoors, and trapdoor re-generateability which guarantee that a new trapdoor can be generated even a keyword has been revoked before.

  • Construction of Symmetric-Key Encryption with Guessing Secrecy
    Author(s)
    M. Iwamoto and J. Shikata
    Conference
    IEEE ISIT 2015
    Pages
    725–729
    Publisher
    IEEE
    Publication Year
    2015
    Date Presented
    Jun. 14–19, 2015
    Abstract

    Constructions of symmetric-key encryption with guessing secrecy are discussed. In the previous works, only a construction of symmetric-key encryption with average guessing secrecy is proposed for one-bit plaintexts. In this paper, we analyze a symmetric-key encryption with average guessing secrecy through OTP (one-time pad) constructions for a wide class of probability distributions of plaintexts and keys. As a result, we show a necessary and sufficient condition that such class of distributions satisfies average guessing secrecy in OTP constructions. On the other hand, we prove that optimal guessing secrecy is essentially equivalent to perfect secrecy under several natural restrictions. Therefore, only average guessing secrecy is meaningful for considering guessing secrecy other than perfect secrecy.

  • Implementation of Double Arbiter PUF and Its Performance Evaluation on FPGA
    Author(s)
    T. Machida, D. Yamamoto, M. Iwamoto, and K. Sakiyama
    Conference
    ASP-DAC 2015
    Pages
    6–7
    Publication Year
    2015
    Date Presented
    Jan. 19–22, 2015
    Abstract

    Low uniqueness and vulnerability to machine-learning attacks are known as two major problems of Arbiter-Based Physically Unclonable Function (APUF) implemented on FPGAs. In this paper, we implement Double APUF (DAPUF) that duplicates the original APUF in order to overcome the problems. From the experimental results on Xilinx Virtex-5, we show that the uniqueness of DAPUF becomes almost ideal, and the prediction rate of the machine-learning attack decreases from 86% to 57%.

  • Constructions of CCA-Secure Revocable Identity-Based Encryption
    Author(s)
    Y. Ishida, Y. Watanabe, and J. Shikata
    Conference
    ACISP 2015
    Vol.
    LNCS 9144
    Pages
    174–191
    Publisher
    Springer
    Publication Year
    2015
    Abstract

    Key revocation functionality is important for identity-based encryption (IBE) to manage users dynamically. Revocable IBE (RIBE) realizes such revocation functionality with scalability. In PKC 2013, Seo and Emura first considered decryption key exposure resistance (DKER) as a new realistic threat, and proposed the first RIBE scheme with DKER. Their RIBE scheme is adaptively secure against chosen plaintext attacks (CPA), and there is no concrete RIBE scheme adaptively secure against chosen ciphertext attacks (CCA) even without DKER so far. In this paper, we first propose two constructions of adaptively CCA-secure RIBE schemes with DKER. The first scheme is based on an existing transformation, which is called a BCHK transformation, that a CPA-secure hierarchical IBE scheme can be transformed into a CCA-secure scheme. The second scheme is constructed via the KEM/DEM framework. Specifically, we newly propose a revocable identity-based key encapsulation mechanism (RIB-KEM), and we show a generic construction of a CCA-secure RIBE scheme from the RIB-KEM and a data encapsulation mechanism (DEM). The second scheme is more efficient than the first one in terms of the ciphertext size.

  • Secure (M+1)st-Price Auction with Automatic Tie-Break
    Author(s)
    T. Nishide, M. Iwamoto, A. Iwasaki, and K. Ohta
    Conference
    InTrust 2014
    Vol.
    LNCS 9473
    No.
    422–436
    Publisher
    Springer
    Publication Year
    2015
    Abstract

    In auction theory, little attention has been paid to a situation where the tie-break occurs because most of auction properties are not affected by the way the tie-break is processed. Meanwhile, in secure auctions where private information should remain hidden, the information of the tie can unnecessarily reveal something that should remain hidden. Nevertheless, in most of existing secure auctions, ties are handled outside the auctions, and all the winning candidates or only the non-tied partial bidders are identified in the case of ties, assuming that a subsequent additional selection (or auction) to finalize the winners is held publicly. However, for instance, in the case of the (M+1) st-price auction, the tied bidders in the (M+1)st-price need to be identified for such a selection, which implies that their bids (unnecessary private information) are revealed. Hence it is desirable that secure auctions reveal neither the existence of ties nor the losing tied bidders.
    To overcome these shortcomings, we propose a secure (M+1)st-price auction protocol with automatic tie-breaks and no leakage of the tie information by improving the bit-slice auction circuit without increasing much overhead.

  • A New Model of Client–Server Communications under Information Theoretic Security
    Author(s)
    M. Iwamoto, T. Omino, Y. Komano, and K. Ohta
    Conference
    IEEE ITW 2014
    Pages
    512–516
    Publisher
    IEEE
    Publication Year
    2014
    Date Presented
    Nov. 5, 2014
    Abstract

    A new model for a Client-Server Communication (CSC) system satisfying information theoretic security is proposed, and its fundamental properties are discussed. Our CSC allows n users to upload their respective messages to a server securely by using symmetric key encryptions with their own keys, and all ciphertexts are decrypted by the server. If we require all messages to be perfectly secure in CSC against the corrupted clients and adversaries without any keys, it is proved that a one time pad or more inefficient encryption must be used for each communication link between a client and the server. This means that, in order to realize more efficient CSC, it is necessary to leak out some information of each message. Based on these observations, we introduce a new model for such a secure CSC formally, and discuss its fundamental properties. In addition, we propose the optimal construction of CSC under several constraints on security parameters called security rates.

  • Cheating on a Visual Secret Sharing Scheme under a Realistic Scenario
    Author(s)
    P. Lumyong, M. Iwamoto, and K. Ohta
    Conference
    ISITA 2014
    Pages
    546–550
    Publisher
    IEEE
    Publication Year
    2014
    Date Presented
    Oct. 29, 2014
    Abstract

    Cheating on a (2, n)-threshold visual secret sharing (VSS) schemes is discussed under a realistic scenario. Horng et al. pointed out an ordinary VSS scheme is vulnerable against a certain kind of cheating, and they proposed a countermeasure against it. In their work, so-called Kerckhoffs's principle and availability of computing power are implicitly assumed in cheating detection. Namely, this work follows a scenario where a victim knows basis matrices and can use computational ability in cheating detection. Under this scenario, Horng et al. showed that their countermeasure attains negligible success probability of generating the victim's share. However, recalling the fact that the decryption of VSS schemes does not depend on computations but depends on human visual system, we can naturally assume a realistic scenario where the victim does not know the basis matrices and has no computing power. Under this scenario, we show that the cheaters can make the victim recover an arbitrary forged secret image in Horng et al.'s countermeasure with probability 1.

  • Timed-Release Computational Secret Sharing Scheme and Its Applications
    Author(s)
    Y. Watanabe and J. Shikata
    Conference
    ProvSec 2014
    Vol.
    LNCS 8782
    Pages
    326–333
    Publisher
    Springer
    Publication Year
    2014
    Abstract

    A secret sharing scheme is an important cryptographic primitive. In this paper, we focus on a computational secret sharing (CSS) scheme, which is a practical, simple secret sharing scheme, with timed-release functionality, which we call a timed-release computational secret sharing (TR-CSS) scheme. In TR-CSS, participants more than or equal to a threshold number can reconstruct a secret by using their shares only when the time specified by a dealer has come. Our TR-CSS can be regarded as a natural extension of Krawczyk’s CSS, and we finally succeed to add timed-release functionality to Krawczyk’s CSS with small overhead, which seems to be almost optimal. Moreover, we show our proposal of TR-CSS is important for constructing threshold encryption and multiple encryption with timed-release functionality in a generic and efficient way.

  • Timed-Release Secret Sharing Schemes with Information Theoretic Security
    Author(s)
    Y. Watanabe and J. Shikata
    Conference
    BalkanCryptSec 2014
    Vol.
    LNCS 9024
    Pages
    219–236
    Publisher
    Springer
    Publication Year
    2014
    Abstract

    In modern cryptography, the secret sharing scheme is an important cryptographic primitive and it is used in various situations. In this paper, timed-release secret sharing (TR-SS) schemes with information-theoretic security is first studied. TR-SS is a secret sharing scheme with the property that participants more than a threshold number can reconstruct a secret by using their shares only when the time specified by a dealer has come. Specifically, in this paper we first introduce models and formalization of security for two kinds of TR-SS based on the traditional secret sharing scheme and information-theoretic timed-release security. We also derive tight lower bounds on the sizes of shares, time-signals, and entities’ secret-keys required for each TR-SS scheme. In addition, we propose direct constructions for the TR-SS schemes. Each direct construction is optimal in the sense that the construction meets equality in each of our bounds, respectively. As a result, it is shown that the timed-release security can be realized without any additional redundancy on the share size.

  • A New Mode of Operation for Arbiter PUF to Improve Uniqueness on FPGA
    Author(s)
    T. Machida, D. Yamamoto, M. Iwamoto, and K. Sakiyama
    Conference
    EAIS 2014
    Pages
    877–884
    Publisher
    IEEE
    Publication Year
    2014
    Date Presented
    Sep. 7–10, 2014
    Abstract

    Arbiter-based Physically Unclonable Function (PUF) is one kind of the delay-based PUFs that use the time difference of two delay-line signals. One of the previous work suggests that Arbiter PUFs implemented on Xilinx Virtex-5 FPGAs generate responses with almost no difference, i.e. with low uniqueness. In order to overcome this problem, Double Arbiter PUF was proposed, which is based on a novel technique for generating responses with high uniqueness from duplicated Arbiter PUFs on FPGAs. It needs the same costs as 2-XOR Arbiter PUF that XORs outputs of two Arbiter PUFs. Double Arbiter PUF is different from 2-XOR Arbiter PUF in terms of mode of operation for Arbiter PUF: the wire assignment between an arbiter and output signals from the final selectors located just before the arbiter. In this paper, we evaluate these PUFs as for uniqueness, randomness, and steadiness. We consider finding a new mode of operation for Arbiter PUF that can be realized on FPGA. In order to improve the uniqueness of responses, we propose 3-1 Double Arbiter PUF that has another duplicated Arbiter PUF, i.e. having 3 Arbiter PUFs and output 1-bit response. We compare 3-1 Double Arbiter PUF to 3-XOR Arbiter PUF according to the uniqueness, randomness, and steadiness, and show the difference between these PUFs by considering the mode of operation for Arbiter PUF. From our experimental results, the uniqueness of responses from 3-1 Double Arbiter PUF is approximately 50%, which is better than that from 3-XOR Arbiter PUF. We show that we can improve the uniqueness by using a new mode of operation for Arbiter PUF.

  • Secret Sharing Schemes Based on Min-Entropies
    Author(s)
    M. Iwamoto and J. Shikata
    Conference
    IEEE ISIT 2014
    Pages
    401–405
    Publisher
    IEEE
    Publication Year
    2014
    Abstract

    Fundamental results are clarified with respect to secret sharing schemes (SSSs) in which security and each share size are measured by (conditional) min-entropies. We first formalize a unified framework of SSS based on conditional Rényi entropies, which includes SSSs based on Shannon and min entropies etc., as special cases. By deriving the lower bound of share sizes in terms of Rényi entropies, we can derive the lower bounds of share sizes measured by Shannon and min entropies in a unified manner. Then, we focus on the existence of SSSs based on min-entropies for several important settings. In the traditional SSSs based on (conditional) Shannon entropies, it is known that; (1) there exists a SSS for arbitrary secret information and arbitrary access structure, and; (2) for every integers k and n (k ≤ n), the ideal (k,n)-threshold scheme exists when secret information is uniform or deterministic. Corresponding to these results, we clarify the following: (1') there exists a SSS for arbitrary binary secret information and arbitrary access structure, and; (2') for every integers k and n (k ≤ n), the ideal (k,n)-threshold scheme exists even if the secret is neither uniform nor deterministic.

  • Privacy-Preserving Smart Metering with Verifiability for Both Billing and Energy Management
    Author(s)
    K. Ohara, Y. Sakai, F. Yoshida, M. Iwamoto, and K. Ohta
    Conference
    ACM ASIAPKC 2014
    Pages
    23–32
    Publisher
    ACM
    Publication Year
    2014
    Abstract

    In smart grid systems, security and privacy prevention is great concerns. The suppliers of the power in smart grid systems demand to know the consumption of each customer for correctly calculating billing price and the total amount of consumption in a certain region for managing energy supply adopted real-time needs. On the other hand, the customer of the power desires to hide his/her own consumption profile, since it contains privacy information of the customer. However, hiding the consumption allows customers to reduce billing price. Previous privacy-preserving smart metering schemes provide only one of billing or energy management functionality, or even if both of them are achieved, these schemes cannot verify the integrity of the consumption issued by the smart meter. We propose a novel smart metering scheme that provides both of billing and energy management functionality, as well as verifiability of the integrity of total amount of the consumption or billing price.

  • An Automated Evaluation Tool for Improved Rebound Attack: New Distinguishers and Proposals of ShiftBytes Parameters for Grøstl
    Author(s)
    Y. Sasaki, Y. Tokushige, L. Wang, M. Iwamoto, and K. Ohta
    Conference
    CT-RSA 2014
    Vol.
    LNCS 8366
    Pages
    424–443
    Publisher
    Springer
    Publication Year
    2014
    Abstract

    In this paper, we study the security of AES-like permutations against the improved rebound attack proposed by Jean et al. at FSE 2012 which covers three full-active rounds in the inbound phase. The attack is very complicated and hard to verify its optimality when the state size is large and rectangle, namely the numbers of rows and columns are different. In the inbound phase of the improved rebound attack, several SuperSBoxes are generated for each of forward analysis and backward analysis. The attack searches for paired values that are consistent with all SuperSBoxes. The attack complexity depends on the order of the SuperSBoxes to be analyzed, and detecting the best order is hard. In this paper, we develop an automated complexity evaluation tool with several fast implementation techniques. The tool enables us to examine all the possible orders of the SuperSBoxes, and provides the best analysis order and complexity. We apply the tool to large block Rijndael in the known-key setting and the Grøstl-512 permutation. As a result, we obtain the first 9-round distinguisher for Rijndael-192 and Rijndael-224. It also shows the impossibility of the improved rebound attack against 9-round Rijndael-160 and 10-round Rijndael-256, and the optimality of the previous distinguisher against the 10-round Grøstl-512 permutation. Moreover, the efficiency of the improved rebound attack depends on the parameter of the ShiftRows operation. Our tool can exhaustively examine all the possible ShiftRows parameters to search for the ones that can resist the attack. We show new parameters for the Grøstl-512 permutation obtained by our tool, which can resist a 10-round improved rebound attack while the specification parameter cannot resist it.

  • Limited-Birthday Distinguishers for Hash Functions—Collisions beyond the Birthday Bound Can Be Meaningful
    Author(s)
    M. Iwamoto, T. Peyrin, and Y. Sasaki
    Conference
    ASIACRYPT 2013
    Vol.
    LNCS 8269
    Pages
    505–523
    Publication Year
    2013
    Abstract

    In this article, we investigate the use of limited-birthday distinguishers to the context of hash functions. We first provide a proper understanding of the limited-birthday problem and demonstrate its soundness by using a new security notion Differential Target Collision Resistance (dTCR) that is related to the classical Target Collision Resistance (TCR) notion. We then solve an open problem and close the existing security gap by proving that the best known generic attack proposed at FSE 2010 for the limited-birthday problem is indeed the best possible method.
    Moreover, we show that almost all known collision attacks are in fact more than just a collision finding algorithm, since the difference mask for the message input is usually fixed. A direct and surprising corollary is that these collision attacks are interesting for cryptanalysis even when their complexity goes beyond the 2n/2 birthday bound and up to the 2n preimage bound, and can be used to derive distinguishers using the limited-birthday problem. Interestingly, cryptanalysts can now search for collision attacks beyond the 2n/2 birthday bound.
    Finally, we describe a generic algorithm that turns a semi-free-start collision attack on a compression function (even if its complexity is beyond the birthday bound) into a distinguisher on the whole hash function when its internal state is not too wide. To the best of our knowledge, this is the first result that exploits classical semi-free-start collisions on the compression function to exhibit a weakness on the whole hash function. As an application of our findings, we provide distinguishers on reduced or full version of several hash functions, such as RIPEMD-128, SHA-256, Whirlpool, etc.

  • Information-Theoretically Secure Entity Authentication in the Multi-User Setting
    Author(s)
    S. Hajime, Y. Watanabe, and J. Shikata
    Conference
    ICISC 2013
    Vol.
    LNCS 8565
    Pages
    400–417
    Publisher
    Springer
    Publication Year
    2013
    Abstract

    In this paper, we study unilateral entity authentication protocols and mutual entity authentication protocols with information-theoretic security in the multi-user setting. To the best of our knowledge, only one paper by Kurosawa studied an entity authentication protocol with information-theoretic security, and an unilateral entity authentication protocol in the two-user setting was considered in his paper. In this paper, we extend the two-user unilateral entity authentication protocol to the multi-user one. In addition, we formally study an information-theoretically secure mutual entity authentication protocol in the multi-user setting for the first time. Specifically, we formalize a model and security definition, and derive tight lower bounds on size of users’ secret-keys, and we show an optimal direct construction.

  • Information Theoretic Security for Encryption Based on Conditional Rényi Entropies
    Author(s)
    M. Iwamoto and J. Shikata
    Conference
    ICITS 2013
    Vol.
    LNSC 8317
    Pages
    101–121
    Publisher
    Springer
    Publication Year
    2013
    Abstract

    In this paper, information theoretic cryptography is discussed based on conditional Rényi entropies. Our discussion focuses not only on cryptography but also on the definitions of conditional Rényi entropies and the related information theoretic inequalities. First, we revisit conditional Rényi entropies, and clarify what kind of properties are required and actually satisfied. Then, we propose security criteria based on Rényi entropies, which suggests us deep relations between (conditional) Rényi entropies and error probabilities by using several guessing strategies. Based on these results, unified proof of impossibility, namely, the lower bounds on key sizes are derived based on conditional Rényi entropies. Our model and lower bounds include the Shannon’s perfect secrecy, and the min-entropy based encryption presented by Dodis, and Alimomeni and Safavi-Naini at ICITS2012. Finally, a new optimal symmetric key encryption protocol achieving the lower bounds is proposed.

  • Unconditionally Secure Blind Authentication Codes in the Manual Channel Model
    Author(s)
    N. Takei, Y. Watanabe, and J. Shikata
    Conference
    3rd ISEEE
    Pages
    297–302
    Publication Year
    2013
    Date Presented
    2013
    Abstract

    In this paper, as a fundamental cryptographic protocol with information-theoretic security, we propose unconditionally secure blind authentication codes in the manual channel model. The blind authentication code is a protocol in which a user can obtain a signer's authenticator of a message with anonymity of a message and a verifier can verify the validity of it. To realize such a mechanism, it is known that each entity of the system, a user, a signer, and a verifier needs to have secret information. In our model of blind authentication codes in the manual channel model, a verifier can verify the validity of an authenticated message without any secret key. In this paper we propose a formal model and security formalization of blind authentication codes in the manual channel model. In addition, we present a construction of unconditionally secure blind authentication codes in the manual channel model.

  • Unconditionally Secure Anonymous Group Authentication with an Arbiter
    Author(s)
    T. Seito, Y. Watanabe, K. Kinose, and J. Shikata
    Conference
    3rd ISEEE
    Pages
    291–296
    Publication Year
    2013
    Abstract

    In cryptographic application, there is need for protecting privacy of users besides integrity of data transmitted in a public channel. In unconditional (or information-theoretic) security setting, a model of GA-codes (Group Authentication codes) which ensures the anonymity for senders like the computationally secure group signature was proposed. In this model, it is assumed that both the sender and the receiver are mutually trusted. In this paper, we remove the assumption and newly propose a model and security definition of the GA2-code (Group Authentication codes with an Arbiter) in which a trusted arbiter is provided so that the arbiter can resolve a dispute between the sender and the receiver. This model can be considered as extension of both the GA-codes and the traditional A2-codes (Authentication codes with an Arbiter). In addition, we propose a construction which meets our security definition of GA2-codes by using polynomials over finite fields. We also consider the case that the arbiter is not always honest and call this model GA3-codes (GA-codes with protecting against arbiter's attack), which is similar to the setting of the traditional A3-codes.

  • Information-Theoretically Secure Aggregate Authentication Code: Model, Bounds, and Constructions
    Author(s)
    A. Kubai, J. Shikata, and Y. Watanabe
    Conference
    CD-ARES Workshop, MoCrySEn 2013
    Vol.
    LNCS 8128
    Pages
    16–28
    Publisher
    Springer
    Publication Year
    2013
    Date Presented
    Sep. 2–6, 2013
    Abstract

    In authentication schemes where many users send authenticated messages to a receiver, it is desirable to aggregate them into a single short authenticated message in order to reduce communication complexity. In this paper, in order to realize such a mechanism in information-theoretic security setting, we first propose aggregate authentication codes. Specifically, we newly propose a model and a security definition for aggregate authentication codes. We also show tight lower bounds on sizes of entities’ secret-keys and (aggregated) tags. Furthermore, we present optimal (i.e., most efficient) constructions for aggregate authentication codes.

  • Meet-in-the-Middle Preimage Attacks Revisited: New Results on MD5 and HAVAL
    Author(s)
    Y. Sasaki, W. Komatsubara, Y. Sakai, L. Wang, M. Iwamoto, K Sakiyama, and K. Ohta
    Conference
    SECRYPT 2013
    Pages
    111–122
    Publication Year
    2013
    Date Presented
    Jul. 29–31, 2013
    Abstract

    In this paper, we revisit previous meet-in-the-middle preimage attacks on hash functions. We firstly present a technical improvement for the existing local-collision and initial-structure techniques. With applying some equivalent transformation, we can significantly reduce the memory requirement from the original proposals. We then revisit the previous preimage attacks on MD5 and HAVAL with recent techniques. Consequently, we can improve the memory complexity of the previous preimage attack on full MD5 from 245 to 213 and on full 4-pass HAVAL from 264 to 232 . Moreover, we extend the preimage attack on 5-pass HAVAL from 151 steps to 158 steps, and present the first preimage attack with a single block message for 3-pass HAVAL.

  • Key-Dependent Weakness of AES-Based Ciphers Under Clockwise Collision Distinguisher
    Author(s)
    T. Nakasone, Y. Li, Y. Sasaki, M. Iwamoto, K. Ohta, and K. Sakiyama
    Conference
    ICISC 2012
    Vol.
    LNCS 7839
    Pages
    395–409
    Publisher
    Springer
    Publication Year
    2012
    Date Presented
    Dec., 2012
    Abstract

    In 2011, Li et al. proposed a series of side-channel attacks that are related to a fundamental side-channel leakage source called clockwise collision. This paper discloses the fact that hardware implementations of AES-based ciphers could have weak keys assuming that the leakage of clockwise collision is distinguishable. In order to explain this, we firstly set up an evaluation method by introducing a threshold-based distinguisher that takes an advantage of the locality of ElectroMagnetic (EM) measurements. Secondly, we discuss that the probability of clockwise collision depends on the key values and the byte positions in the AES states. Thirdly, based on practical EM measurements and mathematical analysis, we quantitatively evaluate the relationship between the probability of clockwise collision and the vulnerability to the side-channel attack. Finally, the discussion is extended to the design methodology of AES-based ciphers, i.e., the parameter selection for S-box and ShiftRows.

  • Information-Theoretic Timed-Release Security: Key-Agreement, Encryption and Authentication Codes
    Author(s)
    Y. Watanabe, T. Seito, and J. Shikata
    Conference
    ICITS 2012
    Vol.
    LNCS 7412
    Pages
    167–186
    Publisher
    Springer
    Publication Year
    2012
    Abstract

    In this paper, we study timed-release cryptography with information-theoretic security. As fundamental cryptographic primitives with information-theoretic security, we can consider key-agreement, encryption, and authentication codes. Therefore, in this paper, we deal with information-theoretic timed-release security for all those primitives. Specifically, we propose models and formalizations of security for information-theoretic timed-release key-agreement, encryption, and authentication codes, and we present constructions of those ones. In particular, information-theoretic timed-release encryption and authentication codes can be constructed from information-theoretic timed-release key-agreement in a generic and simple way. Also, we derive tight lower bounds of sizes of secret-keys and show an optimal construction for information-theoretic timed-release key-agreement. Furthermore, we investigate a relationship of mechanisms between information-theoretic timed-release key-agreement and information-theoretic key-insulated key-agreement. It turns out that there exists a simple algorithm which converts the former into the latter, and vice versa. In the sense, we conclude that these two mechanisms are essentially close.

  • Variations of Information Theoretic Security Notions
    Author(s)
    M. Iwamoto and K. Ohta
    Conference
    AEW 7
    Pages
    73–76
    Publication Year
    2011
    Date Presented
    Jul., 2011
    Non-peer reviewed paper.
  • Uniqueness Enhancement of PUF Responses Based on the Locations of Random Outputting RS Latches
    Author(s)
    D. Yamamoto, K. Sakiyama, M. Iwamoto, K. Ohta, T. Ochiai, M. Takenaka, and K. Itoh
    Conference
    CHES 2011
    Vol.
    LNCS 6917
    Pages
    391–406
    Publisher
    Springer
    Publication Year
    2011
    Date Presented
    Oct., 2011
    Abstract

    Physical Unclonable Functions (PUFs) are expected to represent an important solution for secure ID generation and authentication etc. In general, PUFs are considered to be more secure the larger their output entropy. However, the entropy of conventional PUFs is lower than the output bit length, because some output bits are random numbers, which are regarded as unnecessary for ID generation and discarded. We propose a novel PUF structure based on a Butterfly PUF with multiple RS latches, which generates larger entropy by utilizing location information of the RS latches generating random numbers. More specifically, while conventional PUFs generate binary values (0/1), the proposed PUF generates ternary values (0/1/random) in order to increase entropy. We estimate the entropy of the proposed PUF. According to our experiment with 40 FPGAs, a Butterfly PUF with 128 RS latches can improve entropy from 116 bits to 192.7 bits, this being maximized when the frequency of each ternary value is equal. We also show the appropriate RS latch structure for satisfying this condition, and validate it through an FPGA experiment.

  • Security Notions for Information Theoretically Secure Encryptions
    Author(s)
    M. Iwamoto and K. Ohta
    Conference
    IEEE ISIT 2011
    Pages
    1743–1747
    Publisher
    IEEE
    Publication Year
    2011
    Date Presented
    Jul. 31–Aug. 5, 2011
    Abstract

    This paper is concerned with several security notions for information theoretically secure encryptions defined by the variational (statistical) distance. To ensure the perfect secrecy (PS), the mutual information is often used to evaluate the statistical independence between a message and a cryptogram. On the other hand, in order to recognize the information theoretically secure encryptions and computationally secure ones comprehensively, it is necessary to reconsider the notion of PS in terms of the variational distance. However, based on the variational distance, three kinds of definitions for PS are naturally introduced, but their relations are not known. In this paper, we clarify that one of three definitions for PS with the variational distance, which is a straightforward extension of Shannon's perfect secrecy, is stronger than the others, and the weaker two definitions of PS are essentially equivalent to the statistical versions of indistinguishability and semantic security.

  • Visual Secret Sharing Schemes for Multiple Secret Images Including Shifting Operation of Shares
    Author(s)
    A. Espejel-Trujillo, M. Nakano-Miyatake, and M. Iwamoto
    Conference
    CCE 2009
    Pages
    433–438
    Publication Year
    2009
    Date Presented
    Nov. 2009
    Abstract

    Visual Cryptography, proposed by Naor-Shamir in 1994, is also called a Visual Secret Sharing (VSS) scheme since it can be regarded as one realization of secret sharing scheme. In VSS schemes, an image is encrypted into a set of images called shares, which look like random noise. In decryption, the secret image is perceived from stacked shares by human visual system, and hence no extra computations and prior knowledge are required. The VSS scheme proposed by Naor-Shamir is a (k,n) or less threshold VSS scheme for binary image, where a secret image is decrypted by stacking arbitrary k out of n shares, but any (k−1) or less shares must not leak out any information of the secret image. In this paper a variant of the VSS Scheme is proposed, where three binary secret images are encrypted into two shares, at the same time these shares looks like innocent image. Furthermore The first secret image is decrypted by a typical stacking process, while the other two secret images are decrypted using the shifting, that is moving one of the shares respect to the other share in a appropriate position.

  • Coding Theorems for a (2, 2)–Threshold Scheme Secure against Impersonation by an Opponent
    Author(s)
    H. Koga, M. Iwamoto, and H. Yamamoto
    Conference
    IEEE ITW 2009
    Pages
    188–192
    Publisher
    IEEE
    Publication Year
    2009
    Date Presented
    Oct. 11–16, 2009
    Abstract

    In this paper, we focus on a (2,2)-threshold scheme in the presence of an opponent who impersonates one of the two participants. We consider an asymptotic setting where two shares are generated by an encoder blockwisely from an n-tuple of secrets generated from a stationary memoryless source and a uniform random number available only to the encoder. We introduce a notion of correlation level of the two shares and give coding theorems on the rates of the shares and the uniform random number. It is shown that, for any (2,2)-threshold scheme with correlation level r, none of the rates can be less than H(S) + r, where H(S) denotes the entropy of the source. We also show that the impersonation by the opponent is successful with probability at least 2-nr+o(n). In addition, we prove the existence of an encoder and a decoder of the (2, 2)-threshold scheme that asymptotically achieve all the bounds on the rates and the success probability of the impersonation.

  • A Coding Theorem for Cheating-Detectable (2, 2)-Threshold Blockwise Secret Sharing Schemes
    Author(s)
    M. Iwamoto, H. Yamamoto, and H. Koga
    Conference
    IEEE ISIT 2009
    Pages
    1308–1312
    Publisher
    IEEE
    Publication Year
    2009
    Date Presented
    Jun. 28–Jul. 3, 2009
    Abstract

    It is known that a secret sharing scheme (SSS) with perfect cheating detection cannot be realized because such a SSS requires infinite share rates. However, this impossibility comes from the fact that block coding is not used and any decoding error is not allowed in the SSS. Hence, in this paper, we consider a SSS constructed by block coding with an arbitrarily small decoding error probability. It is shown that the perfect cheating detection with finite rates is possible for the 2-out-of-2 SSS in a certain asymptotic sense. Furthermore, the supremum of the achievable exponent in the maximum success probability of impersonation attack turns out to be the mutual information between the two shares.

  • Weakly Secure Visual Secret Sharing Schemes
    Author(s)
    M. Iwamoto
    Conference
    ISITA 2008
    Pages
    42–47
    Publication Year
    2008
    Date Presented
    Dec. 7–10, 2008
    Abstract

    We introduce a visual secret sharing (VSS) scheme with a new security condition, called a weakly secure VSS scheme, which is not unconditionally secure in general, but is designed to be secure for human eyesight. It is shown in this paper that the weakly secure VSS scheme is equivalent to the unconditional one for black-white binary secret images although they are different for color secret images. This fact implies that, at the sacrifice of security, the clearer color images can be reproduced by the weakly secure VSS schemes compared with the unconditional ones. Furthermore, some constructions of weakly secure VSS schemes are presented.

  • A Remark on Visual Secret Sharing Schemes Allowing the Rotation of Shares
    Author(s)
    M. Iwamoto, L. Wang, K. Yoneyama, N. Kunihiro, and K. Ohta
    Conference
    AEW 5
    Pages
    37–42
    Publication Year
    2006
    Date Presented
    Oct., 2006
    Non-peer-reviewed paper.
    Abstract

    Recently, a visual secret sharing scheme for q multiple secret images allowing the rotation of shares, a VSS-q-MI-R schemes for short, is proposed by Iwamoto et al. In this paper, another definition of VSS-q-MI-R schemes is given, which is simpler than that by Iwamoto et al.

  • Quantum Secret Sharing Schemes and Reversibility of Quantum Operations
    Author(s)
    T. Ogawa, A. Sasaki, M. Iwamoto, and H. Yamamoto
    Conference
    ISITA 2004
    Pages
    1440–1445
    Publication Year
    2004
    Date Presented
    Oct., 2004
    Abstract

    Quantum secret sharing schemes encrypting a quantum state into a multipartite entangled state are treated. The lower bound on the dimension of each share given by Gottesman [Phys. Rev. A 61, 042311 (2000)] is revisited based on a relation between the reversibility of quantum operations and the Holevo information. We also propose a threshold ramp quantum secret sharing scheme and evaluate its coding efficiency.

  • Strongly Secure Ramp Secret Sharing Schemes
    Author(s)
    M. Iwamoto and H. Yamamoto
    Conference
    IEEE ISIT 2005
    Pages
    1221–1225
    Publisher
    IEEE
    Publication Year
    2005
    Date Presented
    Sep. 4–9, 2005
    Abstract

    Ramp secret sharing (SS) schemes can be classified into strong ramp SS schemes and weak ramp SS schemes. The strong ramp SS schemes do not leak out any part of a secret explicitly even in the case where some information about the secret leaks from a non-qualified set of shares, and hence, they are more desirable than weak ramp SS schemes. However, it is not known how to construct the strong ramp SS schemes in the case of general access structures. In this paper, it is shown that a strong ramp SS scheme can always be constructed from a SS scheme with plural secrets for any feasible general access structure. As a byproduct, it is pointed out that threshold ramp SS schemes based on Shamir's polynomial interpolation method are not always strong.

  • Optimal Multiple Assignments Based on Integer Programming in Secret Sharing Schemes
    Author(s)
    M. Iwamoto, H. Yamamoto, and H. Ogawa
    Conference
    IEEE ISIT 2004
    Pages
    16
    Publisher
    IEEE
    Publication Year
    2004
    Date Presented
    Jun.–Jul., 2004
    Abstractt

    This paper shows the derivation procedure of optimal secret sharing scheme (SSS) for a given access structure in the multiple assignment schemes based on integer programming.

  • Visual Secret Sharing Schemes for Plural Secret Images
    Author(s)
    M. Iwamoto and H. Yamamoto
    Conference
    IEEE ISIT 2003
    Pages
    283
    Publisher
    IEEE
    Publication Year
    2003
    Date Presented
    Jun.–Jul., 2003
    Abstract

    Visual secret sharing schemes with q plural images, for short VSS-q-PI schemes, are studied for general access structures and gray-scale and/or color secret images.

  • Cache Line Impact on 3D PDE Solvers
    Author(s)
    M. Kondo, M. Iwamoto, and H. Nakamura
    Conference
    ISHPC 2002
    Vol.
    LNCS 2327
    Pages
    301–309
    Publisher
    Springer
    Publication Year
    2002
    Date Presented
    Oct., 2002
    Abstract

    Because performance disparity between processor and main memory is serious, it is necessary to reduce off-chip memory accesses by exploiting temporal locality. Loop tiling is a well-known optimization which enhances data locality. In this paper, we show a new cost model to select the best tile size in 3D partial differential equations. Our cost model carefully takes account of the effect of cache line. We present performance evaluation of our cost models. The evaluation results reveal the superiority of our cost model to other cost models proposed so far.

Presentations

  • How to Improve the Private PEZ Protocol for General Functions
    Author(s)
    Y. Abe, M. Iwamoto, and K. Ohta
    Presenter
    Y. Abe
    Conference
    IWSEC 2019
    Venue
    Tokyo, Japan
    Type
    Poster
    Date
    2019/8/28
  • Key-Updatable Public-Key Encryption with Keyword Search: An Efficient Construction
    Author(s)
    H. Anada, A. Kanaoka, N. Matsuzaki, and Y. Watanabe
    Presenter
    N. Matsuzaki
    Conference
    IWSEC 2018
    Venue
    Sendai, Japan
    Type
    Poster
    Date
    Aug., 2018
  • Worst-Case Guessing Secrecy Is Meaningful in Secret Sharing Scheme
    Author(s)
    M. Iwamoto
    Presenter
    M. Iwamoto
    Conference
    ICITS 2017
    Publication Year
    2017
    Date Presented
    2017/12
    Venue
    Hong Kong
    Type
    Oral Presentation
    Date
    2017
  • Application of Joux-Lucks Search Algorithm for Multi-Collisions to MicroMint
    Author(s)
    Y. Kamoshida, M. Iwamoto, and K. Ohta
    Presenter
    Y. Kamoshida
    Conference
    IWSEC 2016
    Publication Year
    2016
    Date Presented
    2016
    Type
    Poster
  • How to Provide Long-Term Security and Required Functionality for Cloud Storage
    Author(s)
    Y. Watanabe, G. Hanaoka, and J. Shikata
    Presenter
    Y. Watanabe
    Conference
    PRIVAGEN 2015
    Venue
    Tokyo, Japan
    Type
    Poster
    Date
    2015
  • Constructions of Strongly Secure Revocable Identity-Based Encryption
    Author(s)
    Y. Ishida, Y. Watanabe, and J. Shikata
    Presenter
    Y. Ishida
    Conference
    Yokohama Environment and Information Sciences (YEIS) International Forum
    Venue
    Yokohama, Japan
    Type
    Poster
    Date
    2015
  • How to Provide Long-Term Security and Required Functionality for Cloud Storage
    Author(s)
    Y. Watanabe, G. Hanaoka, and J. Shikata
    Presenter
    Y. Watanabe
    Conference
    Yokohama Environment and Information Sciences (YEIS) International Forum
    Venue
    Yokohama, Japan
    Type
    Poster
    Date
    2015
  • Toward Reducing Shuffling in Card-Based Cryptographic Protocol for Millionaire Problem
    Author(s)
    T. Nakai, Y. Tokushige, M. Iwamoto, and K. Ohta
    Presenter
    T. Nakai
    Conference
    IWSEC 2015
    Publication Year
    2015
    Date Presented
    2015/08
    Venue
    Nara, Japan
    Type
    Poster
    Date
    Aug., 2015
  • Comparison of Security on Coded Signs with Public/Private Code Book
    Author(s)
    Y. Misawa, Y. Tokushige, M. Iwamoto, and K. Ohta
    Presenter
    Y. Misawa
    Conference
    IWSEC 2015
    Venue
    Nara, Japan
    Type
    Poster
    Date
    Aug., 2015
  • Information-Theoretically Secure Revocable-Storage Broadcast Encryption
    Author(s)
    Y. Watanabe and J. Shikata
    Presenter
    Y. Watanabe
    Conference
    IWSEC 2014
    Venue
    Hirosaki, Japan
    Type
    Poster
    Date
    2014
  • A New Model of Modeling Attacks against Arbiter PUF on FPGA
    Author(s)
    T. Machida, T. Nakasone, M. Iwamoto, and K. Sakiyama
    Presenter
    T. Machida
    Conference
    IWSEC 2013
    Publication Year
    2013
    Date Presented
    2013/11
    Type
    Poster
  • A t-Resilient Unconditionally Secure First-Price Auction Protocol
    Author(s)
    K. Ohara, Y. Sakai, M. Iwamoto, and K. Ohta
    Presenter
    K. Ohara
    Conference
    IWSEC 2012
    Publication Year
    2012
    Date Presented
    2012/11
    Type
    Poster
    Date
    Nov., 2012
  • Quantum Ramp Secret Sharing Schemes
    Author(s)
    T. Ogawa, A. Sasaki, M. Iwamoto, and H. Yamamoto
    Presenter
    M. Iwamoto
    Conference
    The 2004 workshop on information security research supported by MEXT Grant-in-aid scientific research on priority area, “informatics”
    Venue
    Tokyo, Japan
    Type
    Oral Presentation
    Date
    2004

Invited Talks

  • Recent Progress on Private PEZ Protocols
    Presenter
    Y. Abe
    Conference
    Workshop on Cryptography Using Physical Tools
    Venue
    Tokyo, Japan
    Date
    2019/12/17
  • Secret Sharing Schemes under Guessing Secrecy
    Presenter
    M. Iwamoto
    Conference
    Cryptographic Technologies for Securing Network Storage and Their Mathematical Modeling, MI Lecture Notes
    Venue
    Kyushu University, Japan
    Date
    Jun. 12, 2017
  • Unconditionally Secure Revocable Storage
    Presenter
    Y. Watanabe
    Conference
    IWSEC 2015
    Venue
    Nara, Japan
    Date
    2015
  • Timed-Release Cryptography –Two Theoretical Approaches to Achieve Security
    Presenter
    Y. Watanabe
    Conference
    JSPS-DST Asian Academic Seminar 2013 (AAS 2013)
    Venue
    Tokyo, Japan
    Date
    2013
  • Security Notions of Visual Secret Sharing Schemes
    Presenter
    M. Iwamoto
    Conference
    International Workshop on Advanced Image Technology (IWAIT2013)
    Date
    Jan., 2013

Invited Papers

  • Threshold Based Visual Cryptography: A Tutorial Review
    Author(s)
    M. Nakano, E. Escamilla, H. Pérez, and M. Iwamoto
    Journal/Conference
    Información Tecnológica
    Vol.
    22
    No.
    5
    Pages
    107–120
    Publisher
    SciELO
    Publication Year
    2011
    (in Spanish)
    Abstract

    In this paper, a review for threshold-based visual cryptography (VC), a visual representation of the secret sharing scheme is presented. The VC has potential applications in the electronic banking system, as well as in personal identification systems. En la VC a binary image such as letters, logotypes or halftone images are shared among a group of participants using a set of images, so called shares. Until now several VC schemes, such as threshold-based VC, the general access VC and the extended VC, have been proposed. The conditions that must be satisfied for a correct recovery of the secret message, as well as the most relevant security issues of this scheme are also analyzed.

Awards and Honors

Misc.

  • Efficient Identity-Based Encryption with Hierarchical Key-Insulation from HIBE
    Authors   K. Emura, A. Takayasu, and Y. Watanabe
    Abstract

    Hierarchical key-insulated identity-based encryption (HKIBE) is identity-based encryption (IBE) that allows users to update their secret keys to achieve (hierarchical) key-exposure resilience, which is an important notion in practice. However, existing HKIBE constructions have limitations in efficiency: sizes of ciphertexts and secret keys depend on the hierarchical depth. In this paper, we first triumph over the barrier by proposing simple but effective design methodologies to construct efficient HKIBE schemes. First, we show a generic construction from any hierarchical IBE (HIBE) scheme that satisfies a special requirement, called MSK evaluatability introduced by Emura et al. (ePrint, 2020). It provides several new and efficient instantiations since most pairing-based HIBE schemes satisfy the requirement. It is worth noting that it preserves all parameters' sizes of the underlying HIBE scheme, and hence we obtain several efficient HKIBE schemes under the k-linear assumption in the standard model. Since MSK evaluatability is dedicated to pairing-based HIBE schemes, the first construction restricts pairing-based instantiations. To realize efficient instantiation from various assumptions, we next propose a generic construction of an HKIBE scheme from any plain HIBE scheme. It is based on Hanaoka et al.'s HKIBE scheme (Asiacrypt 2005), and does not need any special properties. Therefore, we obtain new efficient instantiations from various assumptions other than pairing-oriented ones. Though the sizes of secret keys and ciphertexts are larger than those of the first construction, it is more efficient than Hanaoka et al.'s scheme in the sense of the sizes of master public/secret keys.