Public-key encryption scheme with optimal continuous leakage resilience

The first practical public-key encryption scheme and digital signature scheme were proposed in 1978 by Rivest et al. [165]. While the original public-key encryption scheme would not be considered secure by modern standards, the RSARSA transform has been the basis of dozens of public-key encryption schemes and digital signature schemes. These schemes have proven very successful and have been very widely deployed in industry. However, despite being widely used in the design of public-key encryption and digital signature schemes, the RSARSA transform has not been widely used in the construction of signcryption schemes.

A leakage-resilient public-key encryption (PKE) scheme provides security even if an adversary obtains some information on the secret key. In recent years, much attention has been focused on designing provably secure PKE in the presence of key-leakage and almost all the constructions rely on an important building block namely hash proof system (HPS). However, in the setting of adaptive chosen-ciphertext attacks (CCA2), there are not many HPS-based leakage-resilient PKE schemes available. Moreover, most of them have an unsatisfactory leakage rate. In this study, the authors propose a new method of constructing leakage-resilient CCA2-secure PKE scheme from any tag-based strongly universal 2 HPS. The striking advantage of the authors scheme is the leakage rate, which is the best one among all known HPS-based indistinguishability key leakage CCA2-secure constructions. In particular, they present an instantiation under the n -linear assumption. In the cases of n = 1 (resp. n = 2), they actually obtain a decisional Diffie-Hellman (DDH)-based [resp. decisional linear (DLIN)-based] PKE scheme, where the leakage rate can be made to 1/4 (resp. 1/6). The authors DDH-based scheme achieves the best leakage rate among all known DDH-based (Cramer-Shoup-type) schemes. Their DLIN-based scheme is the first one that can achieve leakage of L /6 bits without pairing, where L is the length of the secret key.

Code-based public key encryption (PKE) is a popular choice to achieve post-quantum security, partly due to its capability to achieve fast encryption/decryption. However, code-based PKE has larger ciphertext and public key sizes in comparison to conventional PKE schemes (e.g., those based on RSA). In 2018, Lau and Tan proposed a new rank metric code-based PKE scheme, which has smaller public key and ciphertext sizes compared to other code-based PKE schemes. They also proved that their scheme achieves IND-CPA security, assuming the intractability of the decisional rank syndrome decoding problem. It is known that IND-CCA2 security is the strongest and most popular security assurance for PKE schemes. Therefore, in this paper, we obtain a new code-based PKE scheme from Lau and Tan’s scheme, in order to inherit the underlying small public key and ciphertext sizes. However, our new scheme is shown to achieve IND-CCA2 security, instead of the weaker IND-CPA security. Specifically, the respective public key size and ciphertext size in our new scheme are 15.06 KB and 1.37 KB under 141-bit security level, and 16.76 KB and 1.76 KB under 154-bit security level. We then present a use case for the proposed scheme, that is for secure cloud storage.

A public key encryption scheme based on a new variant of LWE with small cipher size

The Fujisaki-Okamoto (FO) conversion is a generic conversion to enhance a public key encryption scheme with security of one-way against chosen plaintext attacks (OW-CPA) to security of indistinguishable against adaptive chosen ciphertext attacks (IND-CCA) in the random oracle model. Existing works have shown that the FO conversion also can generically upgrade the security of the identity-based encryption (IBE) schemes or certificateless public key encryption (CL-PKE) schemes. However, it is still unknown if the same holds for certificate-based encryption (CBE) schemes. In this paper, we investigate the generic security of the CBE scheme obtained by applying the FO conversion to an arbitrary underlying OW-CBE-CPA secure CBE scheme and confirm that the FO conversion can generically convert any OW-CBE-CPA secure CBE into an IND-CBE-CCA secure CBE. We also note that the straightforward application of the FO conversion only leads to a CBE scheme with a loose reduction. To solve this problem, we propose an enhanced FO conversion which just introduces minor extra computation overhead, but results in considerably more efficient security reduction.

With the development of modern communication technology, an increasing number of people are choosing to communicate end-to-end over the network. However, the risk of information leakage and data tampering has also increased, and the issue of information confidentiality has become an increasing concern. Among the existing encryption algorithms, symmetric encryption algorithms cannot securely manage the key and share the key between communication parties; ECC and other public key encryption algorithms have problems such as slow encryption and decryption speed, low encryption strength, etc. In addition, key management is still not guaranteed to be very secure; conventional hybrid encryption algorithms need to manage two different types of keys, and key management may be very complicated and prone to key leakage or theft. Based on the problems of the above algorithms, this paper proposes RSA-Co-CP-ABE Architecture, which uses socket communication, encrypts the RSA public key passed from Client to Server with CP-ABE, encrypts the confidential message with the public key passed from Client at Server, and finally sends the encrypted message to Client. This scheme has higher flexibility than the traditional symmetric encryption and public key encryption schemes, and by using CP-ABE encryption, it can realize attribute-based access control. And RSA encryption can make the scheme make a better balance between encryption and decryption efficiency and encryption strength. In addition, because the communication is transmitted in full cipher text during the communication, the scheme can effectively resist tampering or stealing by man-in-the-middle attacks.

In PKC 2003, Bellare, Boldyreva, and Staddon proposed the reproducibility test. The test determines whether a single-recipient public-key encryption scheme is adapted to transform into an efficient multi-recipient public-key encryption scheme. In this paper, we propose a new approach to design an efficient multi-recipient single-message public-key encryption scheme. We focus on a certain simulator which appears in the security proof of an ordinary (single-recipient) public-key encryption scheme. By considering the behavior of the simulator, we construct two efficient multi-recipient single-message public-key encryption schemes. These schemes show that there exist schemes which can be transformed into efficient multi-recipient schemes, even they do not pass the reproducibility test.Keywordspublic-key encryptionmulti-recipientbroadcast encryptionsimulatorIND-CCA

In the actual applications, an adversary can break the security of cryptography scheme through various leakage attacks (e.g. side-channel attacks, cold-boot attacks, etc.), even the continuous leakage attacks. That is, a practical cryptography scheme must maintain its claimed security in the continuous leakage setting. However, the previous constructions on the leakage-resilient identity-based encryption (IBE) scheme could tolerate a leakage that is bounded, and cannot resist the continuous leakage attacks. In order to further achieve the better security, a novel method to build the continuous leakage-resilient IBE scheme with tight security is presented in this paper, and the scheme’s security is proved, in the standard model, based on a stronger security assumption that depends on the number of queries made by the adversary. In addition, our proposal has several advantages over previous such constructions, e.g. shorter public parameters, higher communication efficiency, tight security, etc.

Leakage of private information including private key has become a threat to the security of computing systems. It has become a common security requirement that a cryptographic scheme should withstand various leakage attacks, including continuous leakage attacks. In order to obtain an Identity-based encryption (IBE) scheme which can keep its original security in the continuous leakage setting, we propose a new construction method of IBE scheme with Chosen-ciphertext attacks (CCA) security, which can tolerate continuous leakage attacks on many private keys of each identity, and whose security is proved based on the hardness of the classical Decisional bilinear Diffie-Hellman (DBDH) assumption in the standard model. The leakage parameter is independent of the plaintext space and has the constant size.

The concept of policy-based cryptography is a promising paradigm for trust establishment and authorization in largescale open environments like the Internet and Mobile Networks. It aims at providing a framework for performing cryptographic operations with respect to policies formalized as monotone Boolean expressions written in standard normal forms. A policy involves conjunctions and disjunctions of conditions where each condition is fulfilled by a digital credential representing the signature of a specific credential issuer on a set of statements about a certain entity. Therefore, an entity fulfills a policy if any only if it has been issued a set of credentials fulfilling the combination of conditions specified by the policy.In this work, we focus on policy-based encryption schemes which allow to encrypt a message according to a policy so that only entities fulfilling the policy are able to decrypt the message. More generally, policy-based encryption belongs to an emerging family of encryption schemes sharing the ability to integrate encryption with access control structures. This ability is mainly enabled by bilinear pairings over elliptic curves and allows for several interesting applications in different contexts.A policy-based encryption scheme has to fulfill two primary requirements: on one hand, provable security under well defined attack models. On the other hand, efficiency, especially when dealing with the conjunctions and disjunctions of credential-based conditions.The contributions of our research work are twofold:1. The standard acceptable notion of security for public-key encryption schemes is indistinguishability against chosen ciphertext attacks. Hence, it is natural to require that a policy-based encryption scheme also satisfies this strong notion of security. However, the definition of this security notion must be adapted to the policy-based setting. Our first contribution is the definition of policy-oriented security model for policy-based encryption schemes as well as the development of an efficient policy-based encryption scheme that is provably secure under our security model in the random oracle model.2. Policy-based encryption schemes may suffer from the key-escrow property i.e. in addition to the legitimate holder of the credentials fulfilling the encryption policy, any collusion of credential issuers who are able to issue a set of credentials fulfilling the policy can decrypt the message. Our second contribution is to address this issue through the notion of policy-based public-key encryption. The latter allows encrypting a message not only with respect to a policy but also according to a public-key so that only an entity fulfilling the policy and having access to the corresponding private-key is able to decrypt the message. We developed a policy-based public-key encryption scheme from bilinear pairings and proved its security under the corresponding security model. Our proposal improves related work in terms of both security and efficiency.

We introduce the notion of complementary-witness-soundness for a witness indistinguishable proof system. We then present a new general framework for public key encryption schemes, denoted as CWSWI-type encryption, based on a complementary-witness-sound witness indistinguishable (CWSWI) proof system. The construction of a CWSWI-type encryption scheme is relatively straight forward, while the security thereof against adaptive chosen cipher text attack (denoted CCA2) can be rigorously proven. Many existing CCA2 public key encryption schemes can be interpreted as being of CWSWI-type, including the schemes of Lindell, Cramer-Shoup and Peikert-Waters. Thus, our CWSWI approach provides a novel means for designing and analyzing CCA2 secure public key encryption schemes.

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

By a generic transformation by Canetti, Halevi, and Katz (CHK) every Identity-based encryption (IBE) scheme implies a chosen-ciphertext secure public-key encryption (PKE) scheme. In the same work it is claimed that this transformation maps the two existing IBE schemes to two new and different chosen-ciphertext secure encryption schemes, each with individual advantages over the other.In this work we reconsider one of the two specific instantiations of the CHK transformation (when applied to the “second Boneh/Boyen IBE scheme”). We demonstrate that by applying further simplifications the resulting scheme can be proven secure under a weaker assumption than the underlying IBE scheme.Surprisingly, our simplified scheme nearly converges to a recent encryption scheme due to Boyen, Mei, and Waters which itself was obtained from the other specific instantiation of the CHK transformation (when applied to the “first Boneh/Boyen IBE scheme”). We find this particularly interesting since the two underlying IBE schemes are completely different.The bottom line of this paper is that the claim made by Canetti, Halevi, and Katz needs to be reformulated to: the CHK transformation maps the two known IBE schemes to nearly one single encryption scheme.KeywordsEncryption SchemeMessage Authentication CodeChallenge CiphertextChoose Ciphertext AttackBilinear GroupThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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