Optimistic fair exchange is a kind of protocols to solve the problem of fair exchange between two parties Almost all the previous work on this topic are provably secure only in the random oracle model In PKC 2007, Dodis et al considered optimistic fair exchange in a multiuser setting, and showed that the security of an optimistic fair exchange in a single-user setting may no longer be secure in a multi-user setting Besides, they also proposed one and reviewed several previous construction paradigms and showed that they are secure in the multi-user setting However, their proofs are either in the random oracle model, or involving a complex and very inefficient NP-reduction Furthermore, they only considered schemes in the certified-key model in which each user has to show his knowledge of the private key corresponding to his public key

In this paper, we make the following contributions First, we consider a relaxed model called chosen-key model in the context of optimistic fair exchange, in which the adversary can arbitrarily choose public keys without showing the knowledge of the private keys We separate the security of optimistic fair exchange in the chosen-key model from the certified-key model by giving a concrete counterexample Second, we strengthen the previous static security model in the multi-user setting to a more practical one which allows an adversary to choose a key adaptively Third, we propose an efficient and generic optimistic fair exchange scheme in the multi-user setting and chosen-key model The security of our construction is proven without random oracles We also propose some efficient instantiations

Efficient optimistic fair exchange secure in the multi-user setting and chosen-key model without random oracles

In the cloud era people get used to store their data to the cloud server, and would use encryption technique to protect their sensitive data from leakage. However, encrypted data management is a challenging problem, for example, encrypted data classification. Besides, how to effectively control the access to the encrypted data is also an important problem. Ciphertext-policy attribute-based encryption with equality test (CP-ABEET) is an efficient solution to the aforementioned problems, which enjoys the advantage of attribute-based encryption, and in the meanwhile supports the test of whether two different ciphertexts contain the same message without the need of decryption. However, the existing CP-ABEET schemes suffer from high computation costs. In this paper, we study how to outsource the heavy computation in CP-ABEET scheme to a third-party server. We introduce the notion of CP-ABEET supporting outsourced decryption (OCP-ABEET), which saves a lot of local computation loads of CP-ABEET. We propose a concrete construction of OCP-ABEET, and prove its security based on a reasonable number-theoretic assumption in the random oracle model. Compared with the existing CP-ABEET schemes, our scheme is more computationally .

Outsourced Ciphertext-Policy Attribute-Based Encryption with Equality Test

Sensitive data would be encrypted before uploading to the cloud due to the privacy issue. However, how to compare the encrypted data efficiently becomes a problem. Public Key Encryption with Equality Test (PKEET) provides an efficient way to check whether two ciphertexts (of possibly different users) contain the same message without decryption. As an enhanced variant, Attribute-based Encryption with Equality Test (ABEET) provides a flexible mechanism of authorization on the equality test. Most of the existing ABEET schemes are only proved to be secure in the random oracle model. Their security, however, would not be guaranteed if random oracles are replaced with real-life hash functions. In this work, we propose a construction of CP-ABEET scheme and prove its security based on some reasonable assumptions in the standard model. We then show how to modify the scheme to outsource complex computations in decryption and equality test to a third-party server in order to support thin clients.

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Attribute-Based Equality Test Over Encrypted Data Without Random Oracles

Optimistic fair exchange (OFE) is a kind of protocols to solve the problem of fair exchange between two parties. Most of the previous work on this topic are provably secure in the random oracle model. In this work, we propose a new construction of OFE from another cryptographic primitive, called time capsule signature. The construction is efficient and brings almost no overhead other than the primitive itself. The security of our new construction is based on that of the underlying primitive without relying on the random oracle heuristic. Applying our generic construction to the time capsule signature scheme recently proposed by Libert and Quisquater, we obtain a new concrete and efficient OFE construction secure based on Computational Diffie–Hellman assumption in the standard model.

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A new efficient optimistic fair exchange protocol without random oracles

Fog computing is increasingly popular partly due to its capability to minimize data transfer and latency requirements, for example by moving some of the computational operations away from the cloud servers and closer to the users. To achieve fine-grained access control in fog-enabled application scenarios to guarantee data security and user privacy, one could use ciphertext-policy attribute-based encryption (CP-ABE). However, the lack of an effective mechanism to carry out access right revocation in conventional CP-ABE schemes limits the deployment of such schemes in practice. Thus, we propose an efficient CP-ABE scheme with attribute revocation capability, designed to construct a fine-grained access control system in fog-enabled E-health (referred to as AC-FEH). In our AC-FEH system, fog nodes undertake data encryption and decryption operations; thus, computational costs for data owners and users are minimized. In comparison to several other competing access control schemes based on CP-ABE, our AC-FEH system reduces the computational costs associated with encryption and decryption. We also prove the selective security of the underlying CP-ABE scheme under the intractability assumption of the $q$ -parallel BDHE problem.

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An Efficient Access Control Scheme With Outsourcing and Attribute Revocation for Fog-Enabled E-Health

Ciphertext-Policy Attribute-Based Encrypted Data Equality Test and Classification


 Thanks to the ease of access and low expenses, it is now popular for people to store data in cloud servers. To protect sensitive data from being leaked to the outside, people usually encrypt the data in the cloud. However, management of these encrypted data becomes a challenging problem, e.g. data classification. Besides, how to selectively share data with other users is also an important and interesting problem in cloud storage. In this paper, we focus on ciphertext-policy attribute based encryption with equality test (CP-ABEET). People can use CP-ABEET to implement not only flexible authorization for the access to encrypted data, but also efficient data label classification, i.e. test of whether two encrypted data contain the same message. We construct an efficient CP-ABEET scheme, and prove its security based on a reasonable number-theoretic assumption. Compared with the only existing CP-ABEET scheme, our construction is more efficient in key generation, and has shorter attribute-related secret keys and better security.

/pdf/ciphertext-policy-attribute-based-encrypted-data-equality-3h8g4sbr98.pdf

Ciphertext-Policy Attribute-Based Encrypted Data Equality Test and Classification.

Time Capsule Signature, first formalized by Dodis and Yum in Financial Cryptography 2005, is a digital signature scheme which allows a signature to bear a (future) time t so that the signature will only be valid at time t or later, when a trusted third party called time server releases time-dependent information for checking the validity of a time capsule signature. Also, the actual signer of a time capsule signature has the privilege to make the signature valid before time t.

In this paper, we provide a new security model of time capsule signature such that time server is not required to be fully trusted. Moreover, we provide two efficient constructions in random oracle model and standard model. Our improved security model and proven secure constructions have the potential to build some new E-Commerce applications.

/pdf/time-capsule-signature-efficient-and-provably-secure-37flye67gh.pdf

Time capsule signature: efficient and provably secure constructions

Time Capsule Signature, first formalized by Dodis and Yum in Financial Cryptography 2005, is a digital signature scheme which allows a signature to bear a (future) time t so that the signature will only be valid at time t or later, when a trusted third party called time server releases time-dependent information for checking the validity of a time capsule signature. Also, the actual signer of a time capsule signature has the privilege to make the signature valid before time t. In this paper, we provide a new security model of time capsule signature such that time server is not required to be fully trusted. Moreover, we provide two efficient constructions in random oracle model and standard model. Our improved security model and proven secure constructions have the potential to build some new E-Commerce applications.

/pdf/time-capsule-signature-efficient-and-provably-secure-1bfehjfwlx.pdf

Time Capsule Signature: Efficient and Provably Secure Constructions.

Nowadays in the cyber world, massive amounts of data are being collected, transmitted, and stored by different organizations and individuals.[...]

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Qiong Huang

Papers

Ciphertext-Policy Attribute-Based Encrypted Data Equality Test and Classification

Ciphertext-Policy Attribute-Based Encrypted Data Equality Test and Classification.

Time capsule signature: efficient and provably secure constructions

Time Capsule Signature: Efficient and Provably Secure Constructions.

A Summary of the Special Issue "Cybersecurity and Cryptography"