Public-Key Encryption with Simulation-Based Selective-Opening Security and Compact Ciphertexts

In a selective-opening (SO) attack on an encryption scheme, an adversary A gets a number of ciphertexts (with possibly related plaintexts), and can then adaptively select a subset of those ciphertexts. The selected ciphertexts are then opened for A (which means that A gets to see the plaintexts and the corresponding encryption random coins), and A tries to break the security of the unopened ciphertexts. Two main flavors of SO security notions exist: indistinguishability-based (IND-SO) and simulation-based (SIM-SO) ones. Whereas IND-SO security allows for simple and efficient instantiations, its usefulness in larger constructions is somewhat limited, since it is restricted to special types of plaintext distributions. On the other hand, SIM-SO security does not suffer from this restriction, but turns out to be significantly harder to achieve. In fact, all known SIM-SO secure encryption schemes either require O(|m|) group elements in the ciphertext to encrypt |m|-bit plaintexts, or use specific algebraic properties available in the DCR setting. In this work, we present the first SIM-SO secure PKE schemes in the discrete-log setting with compact ciphertexts (whose size is O(1) group elements plus plaintext size). The SIM-SO security of our constructions can be based on, e.g., the k-linear assumption for any k. Technically, our schemes extend previous IND-SO secure schemes by the property that simulated ciphertexts can be efficiently opened to arbitrary plaintexts. We do so by encrypting the plaintext in a bitwise fashion, but such that each encrypted bit leads only to a single ciphertext bit (plus O(1) group elements that can be shared across many bit encryptions). Our approach leads to rather large public keys (of O(|m|2) group elements), but we also show how this public key size can be reduced (to O(|m|) group elements) in pairing-friendly groups

Efficient distributed tag-based encryption and its application to group signatures with efficient distributed traceability

In this work, we first formalize the notion of dynamic group signatures with distributed traceability, where the capability to trace signatures is distributed among n managers without requiring any interaction. This ensures that only the participation of all tracing managers permits tracing a signature, which reduces the trust placed in a single tracing manager. The threshold variant follows easily from our definitions and constructions. Our model offers strong security requirements. Our second contribution is a generic construction for the notion which has a concurrent join protocol, meets strong security requirements, and offers efficient traceability, i.e. without requiring tracing managers to produce expensive zero-knowledge proofs for tracing correctness. To dispense with the expensive zero-knowledge proofs required in the tracing, we deploy a distributed tag-based encryption with public verifiability. Finally, we provide some concrete instantiations, which, to the best of our knowledge, are the first efficient provably secure realizations in the standard model simultaneously offering all the aforementioned properties. To realize our constructions efficiently, we construct an efficient distributed (and threshold) tag-based encryption scheme that works in the efficient Type-III asymmetric bilinear groups. Our distributed tag-based encryption scheme yields short ciphertexts (only 1280 bits at 128-bit security), and is secure under an existing variant of the standard decisional linear assumption. Our tag-based encryption scheme is of independent interest and is useful for many applications beyond the scope of this paper. As a special case of our distributed tag-based encryption scheme, we get an efficient tag-based encryption scheme in Type-III asymmetric bilinear groups that is secure in the standard model

Practical Dual-Receiver Encryption---Soundness, Complete Non-Malleability, and Applications

We reformalize and recast dual-receiver encryption (DRE) proposed in CCS \u2704, a public-key encryption (PKE) scheme for encrypting to two independent recipients in one shot. We start by defining the crucial soundness property for DRE, which ensures that two recipients will get the same decryption result. While conceptually simple, DRE with soundness turns out to be a powerful primitive for various goals for PKE, such as complete non-malleability (CNM) and plaintext-awareness (PA). We then construct practical DRE schemes without random oracles under the Bilinear Decisional Diffie-Hellman assumption, while prior approaches rely on random oracles or inefficient non-interactive zero-knowledge proofs. Finally, we investigate further applications or extensions of DRE, including DRE with CNM, combined use of DRE and PKE, strengthening two types of PKE schemes with plaintext equality test, off-the-record messaging with a stronger notion of deniability, etc

素因数分解に基づく暗号における新たな手法

学位の種別: 課程博士審査委員会委員 : （主査）東京大学准教授 國廣 昇, 東京大学教授 山本 博資, 東京大学教授 津田 宏治, 東京大学講師 佐藤 一誠, 東京工業大学教授 田中 圭介University of Tokyo(東京大学

Non-Zero Inner Product Encryption with Short Ciphertexts and Private Keys

International audienceWe describe two constructions of non-zero inner product encryption (NIPE) systems in the public index setting, both having ciphertexts and secret keys of constant size. Both schemes are obtained by tweaking the Boneh-Gentry-Waters broadcast encryption system (Crypto 2005) and are proved selectively secure without random oracles under previously considered assumptions in groups with a bilinear map. Our first realization builds on prime-order bilinear groups and is proved secure under the Decisional Bilinear Diffie-Hellman Exponent assumption, which is parameterized by the length n of vectors over which the inner product is defined. By moving to composite order bilinear groups, we are able to obtain security under static subgroup decision assumptions following the Déj a Q framework of Chase and Meiklejohn (Eurocrypt 2014) and its extension by Wee (TCC 2016). Our schemes are the first NIPE systems to achieve such parameters, even in the selective security setting. Moreover, they are the first proposals to feature optimally short private keys, which only consist of one group element. Our prime-order-group realization is also the first one with a deterministic key generation mechanism

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

Theory and Applications of Outsider Anonymity in Broadcast Encryption

Broadcast Encryption (BE) allows efficient one-to-many secret communication of data over a broadcast channel. In the standard setting of BE, information about receivers is transmitted in the clear together with ciphertexts. This could be a serious violation of recipient privacy since the identities of the users authorized to access the secret content in certain broadcast scenarios are as sensitive as the content itself. Anonymous Broadcast Encryption (AnoBe) prevents this leakage of recipient identities from ciphertexts but at a cost of a linear lower bound (in the number of receivers) on the length of ciphertexts. A linear ciphertext length is a highly undesirable bottleneck in any large-scale broadcast application. In this thesis, we propose a less stringent yet very meaningful notion of anonymity for anonymous broadcast encryption called Outsider-Anonymous Broadcast Encryption (oABE) that allows the creation of ciphertexts that are sublinear in the number of receivers. We construct several oABE schemes with varying security guarantees and levels of efficiency. We also present two very interesting cryptographic applications afforded by the efficiency of our oABE schemes. The first is Broadcast Steganography (BS), the extension of the state of the art setting of point-to-point steganography to the multi-recipient setting. The second is Oblivious Group Storage (OGS), the introduction of fine-grained data access control policies to the setting of multi-client oblivious cloud storage protocols

Design and Analysis of Opaque Signatures

Digital signatures were introduced to guarantee the authenticity and integrity of the underlying messages. A digital signature scheme comprises the key generation, the signature, and the verification algorithms. The key generation algorithm creates the signing and the verifying keys, called also the signer’s private and public keys respectively. The signature algorithm, which is run by the signer, produces a signature on the input message. Finally, the verification algorithm, run by anyone who knows the signer’s public key, checks whether a purported signature on some message is valid or not. The last property, namely the universal verification of digital signatures is undesirable in situations where the signed data is commercially or personally sensitive. Therefore, mechanisms which share most properties with digital signatures except for the universal verification were invented to respond to the aforementioned need; we call such mechanisms “opaque signatures”. In this thesis, we study the signatures where the verification cannot be achieved without the cooperation of a specific entity, namely the signer in case of undeniable signatures, or the confirmer in case of confirmer signatures; we make three main contributions. We first study the relationship between two security properties important for public key encryption, namely data privacy and key privacy. Our study is motivated by the fact that opaque signatures involve always an encryption layer that ensures their opacity. The properties required for this encryption vary according to whether we want to protect the identity (i.e. the key) of the signer or hide the validity of the signature. Therefore, it would be convenient to use existing work about the encryption scheme in order to derive one notion from the other. Next, we delve into the generic constructions of confirmer signatures from basic cryptographic primitives, e.g. digital signatures, encryption, or commitment schemes. In fact, generic constructions give easy-to-understand and easy-to-prove schemes, however, this convenience is often achieved at the expense of efficiency. In this contribution, which constitutes the core of this thesis, we first analyze the already existing constructions; our study concludes that the popular generic constructions of confirmer signatures necessitate strong security assumptions on the building blocks, which impacts negatively the efficiency of the resulting signatures. Next, we show that a small change in these constructionsmakes these assumptions drop drastically, allowing as a result constructions with instantiations that compete with the dedicated realizations of these signatures. Finally, we revisit two early undeniable signatures which were proposed with a conjectural security. We disprove the claimed security of the first scheme, and we provide a fix to it in order to achieve strong security properties. Next, we upgrade the second scheme so that it supports a iii desirable feature, and we provide a formal security treatment of the new scheme: we prove that it is secure assuming new reasonable assumptions on the underlying constituents