暗号要素技術の一般的構成を介した高い安全性・高度な機能を備えた暗号要素技術の構成

Recent years have witnessed an active research on cryptographic primitives with complex functionality beyond simple encryption or authentication. A cryptographic primitive is required to be proposed together with a formal model of its usage and a rigorous proof of security under that model.This approach has suffered from the two drawbacks: (1) security models are defined in a very specific manner for each primitive, which situation causes the relationship between these security models not to be very clear, and (2) no comprehensive ways to confirm that a formal model of security really captures every possible scenarios in practice.This research relaxes these two drawbacks by the following approach: (1) By observing the fact that a cryptographic primitive A should be crucial for constructing another primitive B, we identify an easy-to-understand approach for constructing various cryptographic primitives.(2) Consider a situation in which there are closely related cryptographic primitives A and B, and the primitive A has no known security requirement that corresponds to some wellknown security requirement (b) for the latter primitive B.We argue that this situation suggests that this unknown security requirement for A can capture some practical attack. This enables us to detect unknown threats for various cryptographic primitives that have been missed bythe current security models.Following this approach, we identify an overlooked security threat for a cryptographic primitive called group signature. Furthermore, we apply the methodology (2) to the “revocable”group signature and obtain a new extension of public-key encryption which allows to restrict a plaintext that can be securely encrypted.通常の暗号化や認証にとどまらず, 複雑な機能を備えた暗号要素技術の提案が活発になっている. 暗号要素技術の安全性は利用形態に応じて, セキュリティ上の脅威をモデル化して安全性要件を定め, 新方式はそれぞれ安全性定義を満たすことの証明と共に提案される.既存研究では, 次の問題があった: (1) 要素技術ごとに個別に安全性の定義を与えているため, 理論的な体系化が不十分であった. (2) 安全性定義が実用上の脅威を完全に捉えきれているかの検証が難しかった.本研究は上記の問題を次の考え方で解決する. (1) ある要素技術(A) を構成するには別の要素技術(B) を部品として用いることが不可欠であることに注目し, 各要素技術の安全性要件の関連を整理・体系化して, 新方式を見通し良く構成可能とする. (2) 要素技術(B)で考慮されていた安全性要件(b) に対応する要素技術(A) の安全性要件が未定義なら, それを(A) の新たな安全性要件(a) として定式化する. これにより未知の脅威の検出が容易になる.グループ署名と非対話開示機能付き公開鍵暗号という2 つの要素技術について上記の考え方を適用して, グループ署名について未知の脅威を指摘する.また, 証明書失効機能と呼ばれる拡張機能を持つグループ署名に上記の考え方を適用して, 公開鍵暗号についての新たな拡張機能である, 暗号化できる平文を制限できる公開鍵暗号の効率的な構成法を明らかにする.電気通信大学201

Efficient CCA-Secure PKE from Identity-Based Techniques

Office of Research, Singapore Management Universit

Non-Interactive Verifiable Secret Sharing For Monotone Circuits

We propose a computationally secure and non-interactive verifiable secret sharing scheme that can be efficiently constructed from any monotone Boolean circuit. By non-interactive we mean that the dealer needs to be active only once, where he posts a public message as well as a private message to each shareholder. In the random oracle model, we can even avoid interaction between shareholders. By efficient, we mean that we avoid generic zero-knowledge techniques. Such efficient constructions were previously only known from linear secret sharing schemes (LSSS). It is believed that the class of access structures that can be handled with polynomial size LSSS is incomparable to the class that can be recognized by polynomial size monotone circuits, so in this sense we extend the class of access structures with efficient and non-interactive VSS

Signature Schemes with Efficient Protocols and Dynamic Group Signatures from Lattice Assumptions

International audienceA recent line of works – initiated by Gordon, Katz and Vaikuntanathan (Asiacrypt 2010) – gave lattice-based realizations of privacy-preserving protocols allowing users to authenticate while remaining hidden in a crowd. Despite five years of efforts, known constructions remain limited to static populations of users, which cannot be dynamically updated. For example, none of the existing lattice-based group signatures seems easily extendable to the more realistic setting of dynamic groups. This work provides new tools enabling the design of anonymous authen-tication systems whereby new users can register and obtain credentials at any time. Our first contribution is a signature scheme with efficient protocols, which allows users to obtain a signature on a committed value and subsequently prove knowledge of a signature on a committed message. This construction, which builds on the lattice-based signature of Böhl et al. (Eurocrypt'13), is well-suited to the design of anonymous credentials and dynamic group signatures. As a second technical contribution, we provide a simple, round-optimal joining mechanism for introducing new members in a group. This mechanism consists of zero-knowledge arguments allowing registered group members to prove knowledge of a secret short vector of which the corresponding public syndrome was certified by the group manager. This method provides similar advantages to those of structure-preserving signatures in the realm of bilinear groups. Namely, it allows group members to generate their public key on their own without having to prove knowledge of the underlying secret key. This results in a two-round join protocol supporting concurrent enrollments, which can be used in other settings such as group encryption

Research Philosophy of Modern Cryptography

Proposing novel cryptography schemes (e.g., encryption, signatures, and protocols) is one of the main research goals in modern cryptography. In this paper, based on more than 800 research papers since 1976 that we have surveyed, we introduce the research philosophy of cryptography behind these papers. We use ``benefits and ``novelty as the keywords to introduce the research philosophy of proposing new schemes, assuming that there is already one scheme proposed for a cryptography notion. Next, we introduce how benefits were explored in the literature and we have categorized the methodology into 3 ways for benefits, 6 types of benefits, and 17 benefit areas. As examples, we introduce 40 research strategies within these benefit areas that were invented in the literature. The introduced research strategies have covered most cryptography schemes published in top-tier cryptography conferences

Universal Composition with Responsive Environments

In universal composability frameworks, adversaries (or environments) and protocols/ideal functionalities often have to exchange meta-information on the network interface, such as algorithms, keys, signatures, ciphertexts, signaling information, and corruption-related messages. For these purely modeling-related messages, which do not reflect actual network communication, it would often be very reasonable and natural for adversaries/environments to provide the requested information immediately or give control back to the protocol/functionality immediately after having received some information. However, in none of the existing models for universal composability is this guaranteed. We call this the \emph{non-responsiveness problem}. As we will discuss in the paper, while formally non-responsiveness does not invalidate any of the universal composability models, it has many disadvantages, such as unnecessarily complex specifications and less expressivity. Also, this problem has often been ignored in the literature, leading to ill-defined and flawed specifications. Protocol designers really should not have to care about this problem at all, but currently they have to: giving the adversary/environment the option to not respond immediately to modeling-related requests does not translate to any real attack scenario. This paper solves the non-responsiveness problem and its negative consequences completely, by avoiding this artificial modeling problem altogether. We propose the new concepts of responsive environments and adversaries. Such environments and adversaries must provide a valid response to modeling-related requests before any other protocol/functionality is activated. Hence, protocol designers do no longer have to worry about artifacts resulting from such requests not being answered promptly. Our concepts apply to all existing models for universal composability, as exemplified for the UC, GNUC, and IITM models, with full definitions and proofs (simulation relations, transitivity, equivalence of various simulation notions, and composition theorems) provided for the IITM model