Constructions Secure against Receiver Selective Opening and Chosen Ciphertext Attacks

In this paper we study public key encryption schemes of indistinguishability security against receiver selective opening (IND-RSO) attacks, where the attacker can corrupt some receivers and get the corresponding secret keys in the multi-party setting. Concretely: -We present a general construction of RSO security against chosen ciphertext attacks (RSO-CCA) by combining any RSO secure scheme against chosen plaintext attacks (RSO-CPA) with any regular CCA secure scheme, along with an appropriate non-interactive zero-knowledge proof. -We show that the leakage-resistant construction given by Hazay \emph{et al.} in Eurocrypt 2013 from weak hash proof system (wHPS) is RSO-CPA secure. -We further show that the CCA secure construction given by Cramer and Shoup in Eurocrypt 2002 based on the universal HPS is RSO-CCA secure, hence obtain a more efficient paradigm for RSO-CCA security

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

Encryption schemes secure against chosen-ciphertext selective opening attacks

Imagine many small devices send data to a single receiver, encrypted using the receiver's public key. Assume an adversary that has the power to adaptively corrupt a subset of these devices. Given the information obtained from these corruptions, do the ciphertexts from uncorrupted devices remain secure? Recent results suggest that conventional security notions for encryption schemes (like IND-CCA security) do not suffice in this setting. To fill this gap, the notion of security against selective-opening attacks (SOA security) has been introduced. It has been shown that lossy encryption implies SOA security against a passive, i.e., only eavesdropping and corrupting, adversary (SO-CPA). However, the known results on SOA security against an active adversary (SO-CCA) are rather limited. Namely, while there exist feasibility results, the (time and space) complexity of currently known SO-C

Trapdoor commitment schemes and their applications

Informally, commitment schemes can be described by lockable steely boxes. In the commitment phase, the sender puts a message into the box, locks the box and hands it over to the receiver. On one hand, the receiver does not learn anything about the message. On the other hand, the sender cannot change the message in the box anymore. In the decommitment phase the sender gives the receiver the key, and the receiver then opens the box and retrieves the message. One application of such schemes are digital auctions where each participant places his secret bid into a box and submits it to the auctioneer. In this thesis we investigate trapdoor commitment schemes. Following the abstract viewpoint of lockable boxes, a trapdoor commitment is a box with a tiny secret door. If someone knows the secret door, then this person is still able to change the committed message in the box, even after the commitment phase. Such trapdoors turn out to be very useful for the design of secure cryptographic protocols involving commitment schemes. In the first part of the thesis, we formally introduce trapdoor commitments and extend the notion to identity-based trapdoors, where trapdoors can only be used in connection with certain identities. We then recall the most popular constructions of ordinary trapdoor protocols and present new solutions for identity-based trapdoors. In the second part of the thesis, we show the usefulness of trapdoors in commitment schemes. Deploying trapdoors we construct efficient non-malleable commitment schemes which basically guarantee indepency of commitments. Furthermore, applying (identity-based) trapdoor commitments we secure well-known identification protocols against a new kind of attack. And finally, by means of trapdoors, we show how to construct composable commitment schemes that can be securely executed as subprotocols within complex protocols

Simulation-Based Selective Opening Security for Receivers under Chosen-Ciphertext Attacks

Security against selective opening attack (SOA) for receivers requires that in a multi-user setting, even if an adversary has access to all ciphertexts, and adaptively corrupts some fraction of the users to obtain the decryption keys corresponding to some of the ciphertexts, the remaining (potentially related) ciphertexts retain their privacy. In this paper, we study simulation-based selective opening security for receivers of public key encryption (PKE) schemes under chosen-ciphertext attacks (RSIM-SO-CCA). Concretely, we first show that some known PKE schemes meet RSIM-SO-CCA security. Then, we introduce the notion of master-key SOA security for identity-based encryption (IBE), and extend the Canetti-Halevi-Katz (CHK) transformation to show generic PKE constructions achieving RSIM-SO-CCA security. Finally, we show how to construct an IBE scheme achieving master-key SOA security

Simulation-based Receiver Selective Opening CCA Secure PKE from Standard Computational Assumptions

In the situation where there are one sender and multiple receivers, a receiver selective opening (RSO) attack for a public key encryption (PKE) scheme considers adversaries that can corrupt some of the receivers and get their secret keys and plaintexts. Security against RSO attacks for a PKE scheme ensures confidentiality of ciphertexts of uncorrupted receivers. Simulation-based RSO security against chosen ciphertext attacks (SIM-RSO-CCA) is the strongest security notion in all RSO attack scenarios. Jia, Lu, and Li (INDOCRYPT 2016) proposed the first SIM-RSO-CCA secure PKE scheme. However, their scheme used indistinguishablility obfuscation, which is not known to be constructed from any standard computational assumption. In this paper, we give two contributions for constructing SIM-RSO-CCA secure PKE from standard computational assumptions. Firstly, we propose a generic construction of SIM-RSO-CCA secure PKE using an IND-CPA secure PKE scheme and a non-interactive zero-knowledge proof system satisfying one-time simulation soundness. Secondly, we propose an efficient and concrete construction of SIM-RSO-CCA secure PKE based on the decisional Diffie-Hellman (DDH) assumption. Moreover, we give a method for efficiently expanding the plaintext space of the DDH-based construction. By applying this method to the construction, we obtain the first DDH-based SIM-RSO-CCA secure PKE scheme supporting a super-polynomially large plaintext space with compact ciphertexts

Encryption Schemes Secure against Chosen-Ciphertext Selective Opening Attacks

textabstractImagine many small devices send data to a single receiver, encrypted using the receiver's public key. Assume an adversary that has the power to adaptively corrupt a subset of these devices. Given the information obtained from these corruptions, do the ciphertexts from uncorrupted devices remain secure? Recent results suggest that conventional security notions for encryption schemes (like IND-CCA security) do not suffice in this setting. To fill this gap, the notion of security against selective-opening attacks (SOA security) has been introduced. It has been shown that lossy encryption implies SOA security against a passive, i.e., only eavesdropping and corrupting, adversary (SO-CPA). However, the known results on SOA security against an active adversary (SO-CCA) are rather limited. Namely, while there exist feasibility results, the (time and space) complexity of currently known SO-CCA secure schemes depends on the number of devices in the setting above. In this contribution, we devise a new solution to the selective opening problem that does not build on lossy encryption. Instead, we combine techniques from non-committing encryption and hash proof systems with a new technique (dubbed ``cross-authentication codes'') to glue several ciphertext parts together. The result is a rather practical SO-CCA secure public-key encryption scheme that does not suffer from the efficiency drawbacks of known schemes. Since we build upon hash proof systems, our scheme can be instantiated using standard number-theoretic assumptions such as decisional Diffie-Hellman (DDH), decisional composite residuosity (DCR), and quadratic residuosity (QR). Besides, we construct a conceptually very simple and comparatively efficient SO-CPA secure scheme from (slightly enhanced) trapdoor one-way permutations. We stress that our schemes are completely independent of the number of challenge ciphertexts, and we do not make assumptions about the underlying message distribution (beyond being efficiently samplable). In particular, we do not assume efficient conditional re-samplability of the message distribution. Hence, our schemes are secure in arbitrary settings, even if it is not known in advance how many ciphertexts might be considered for corruptions

Possibility and Impossibility Results for Receiver Selective Opening Secure PKE in the Multi-Challenge Setting

Public key encryption (PKE) schemes are usually deployed in an open system with numerous users. In practice, it is common that some users are corrupted. A PKE scheme is said to be receiver selective opening (RSO) secure if it can still protect messages transmitted to uncorrupted receivers after the adversary corrupts some receivers and learns their secret keys. This is usually defined by requiring the existence of a simulator that can simulate the view of the adversary given only the opened messages. Existing works construct RSO secure PKE schemes in a single-challenge setting, where the adversary can only obtain one challenge ciphertext for each public key. However, in practice, it is preferable to have a PKE scheme with RSO security in the multi-challenge setting, where public keys can be used to encrypt multiple messages. In this work, we explore the possibility of achieving PKE schemes with receiver selective opening security in the multi-challenge setting. Our contributions are threefold. First, we demonstrate that PKE schemes with RSO security in the single-challenge setting are not necessarily RSO secure in the multi-challenge setting. Then, we show that it is impossible to achieve RSO security for PKE schemes if the number of challenge ciphertexts under each public key is a priori unbounded. In particular, we prove that no PKE scheme can be RSO secure in the k-challenge setting (i.e., the adversary can obtain k challenge ciphertexts for each public key) if its secret key contains less than k bits. On the positive side, we give a concrete construction of PKE scheme with RSO security in the k-challenge setting, where the ratio of the secret key length to k approaches the lower bound 1

Key Dependent Message Security and Receiver Selective Opening Security for Identity-Based Encryption

We construct two identity-based encryption (IBE) schemes. The first one is IBE satisfying key dependent message (KDM) security for user secret keys. The second one is IBE satisfying simulation-based receiver selective opening (RSO) security. Both schemes are secure against adaptive-ID attacks and do not have any a-priori bound on the number of challenge identities queried by adversaries in the security games. They are the first constructions of IBE satisfying such levels of security. Our constructions of IBE are very simple. We construct our KDM secure IBE by transforming KDM secure secret-key encryption using IBE satisfying only ordinary indistinguishability against adaptive-ID attacks (IND-ID-CPA security). Our simulation-based RSO secure IBE is based only on IND-ID-CPA secure IBE. We also demonstrate that our construction technique for KDM secure IBE is used to construct KDM secure public-key encryption. More precisely, we show how to construct KDM secure public-key encryption from KDM secure secret-key encryption and public-key encryption satisfying only ordinary indistinguishability against chosen plaintext attacks