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Real-time bidding (RTB), one of the major trading mechanisms used for online advertising, allows the advertiser to make an impression-level bid decision. However, the security and privacy concerns of RTB are gaining increasing attention with the recent enforcement of the European Union General Data Protection Regulation (GDPR). In this study, we present a novel privacy-preserving RTB (Pri-RTB) protocol, which aims at preserving both data privacy and utility by performing additively homomorphic encryption on the user profile. To our knowledge, this is the first work to address the privacy issue of the RTB paradigm. We present a formal proof for the security of Pri-RTB under the assumption of the decisional Diffie–Hellman (DDH) problem. We analyze and elaborate the superiority of Pri-RTB over other related works. We also developed a prototype for Pri-RTB and conducted several experiments to evaluate its feasibility and efficiency under different parameters. Our experiments demonstrate that Pri-RTB can work highly efficiently in a practical setting (for example, a scenario with approximately 200 advertisers per auction).

Cloud-assisted wireless body area networks (WBANs) significantly facilitate efficient patient treatment of high quality, unfortunately in the meanwhile greatly challenge the patient’s data confidentiality and privacy. The existing work mainly focused on the traditional scenario where patients securely stay indoors. In this paper, we consider a more practical situation of cloud-assisted WBANs in m-healthcare social networks where patients traverse among blocks outdoors and WBANs are more vulnerable to sophisticated attacks including even node compromise attack. To solve the problem, a secure and privacy-preserving key management scheme resilient to both time-based and location-based mobile attacks is proposed by the cooperation of the mobile patients in the same social group for both hierarchical and distributed environment. It also protects patient’s identity privacy, sensor deployment privacy and location privacy by exploiting the blinding technique and embedding human body’s symmetric structure into Blom’s symmetric key mechanism with modified proactive secret sharing. Especially, the computationally-intensive privacy-preserving key material updating is outsourced to the cloud server and the unchanged pairwise keys after key material updating dramatically saves the resources for energy-constrained WBANs. Finally, the security analysis and simulation results show our scheme far outperforms the previous ones in terms of resisting mobile attacks and storage, computation and communication overhead.

Secure two-party computation allows two parties with private inputs to securely compute some function of their inputs, even in the presence of a malicious adversary. In this work, we revisit zero-knowledge proofs and focus on adaptive adversaries, which could corrupt an arbitrary number of parties and adaptively determine who and when to corrupt during the computation phase. Previous constructions could realize adaptive zero-knowledge proofs for all languages in NP (Lindell and Zarosim TCC'09) at the cost of a high round-complexity, i.e., super-constant number of rounds. In this work, assuming the existence of constant-round statistically hiding commitment schemes, we build efficient adaptive zero-knowledge proofs for all languages in NP, which only require constant number of communication rounds. The system is also a proof of knowledge. The construction relies on an adaptive instance-dependent commitment scheme, and the proof of security requires only the use of black-box techniques and is presented according to the real/ideal simulation paradigm.

Cloud computing emerges as a new computing paradigm that aims to provide reliable, customized and quality of service guaranteed computation environments for cloud users. Applications and databases are moved to the large centralized data centers, called cloud. Due to resource virtualization, global replication and migration, the physical absence of data and machine in the cloud, the stored data in the cloud and the computation results may not be well managed and fully trusted by the cloud users. Most of the previous work on the cloud security focuses on the storage security rather than taking the computation security into consideration together. In this paper, we propose a privacy cheating discouragement and secure computation auditing protocol, or SecCloud, which is a first protocol bridging secure storage and secure computation auditing in cloud and achieving privacy cheating discouragement by designated verifier signature, batch verification and probabilistic sampling techniques. The detailed analysis is given to obtain an optimal sampling size to minimize the cost. Another major contribution of this paper is that we build a practical secure-aware cloud computing experimental environment, or SecHDFS, as a test bed to implement SecCloud. Further experimental results have demonstrated the effectiveness and efficiency of the proposed SecCloud.

We present the concept of a certificate-based proxy decryption (CBPd) system with revocability, which has the following properties: (1) key-escrow freeness; (2) implicit certification; (3) revocability without public key change; and (4) fine-grained delegation. A concrete CBPd scheme is also proposed. The scheme is semantically secure against adaptive chosen plaintext attack (IND-CBPd-Rev-CPA) under the decisional bilinear Diffie-Hellman assumption in the standard model. We also give an enhanced scheme that is secure against adaptive chosen ciphertext attacks (IND-CBPd-Rev-CCA) in the standard model.

In 2009, Wu proposed a fast modular exponentiation algorithm and claimed that the proposed algorithm on average saved about 38.9% and 26.68% of single-precision multiplications as compared to Dusse-Kaliski's Montgomery algorithm and Ha-Moon's Montgomery algorithm, respectively. However, in this comment, we demonstrate that Wu's algorithm on average reduces the number of single-precision multiplications by at most 22.43% and 6.91%, when respectively compared with Dusse-Kaliski's version and Ha-Moon's version. That is, the computational efficiency of Wu's algorithm is obviously overestimated.

At ACNS 2007, Ateniese and Green proposed the concept of ID-based proxy re-encryption (IBPRE), where a semi-trusted proxy with some information (a.k.a. re-encryption key), can transform a ciphertext under an identity to another ciphertext under another identity with the same plaintext. However, the proxy cannot obtain the plaintext. Recently, Wang et al. revisited this primitive by allowing the chosen ciphertext attack when the ciphertext could be transformed several times. They also proposed a concrete multi-use unidirectional ID-based proxy re-encryption (MUIBPRE) scheme, which is proven secure in the random oracle. Nonetheless, they left the problem of constructing a multi-use unidirectional IBPRE scheme in the standard model. What is worse, their scheme cannot resist the collusion attack, which is an important security requirement for unidirectional proxy re-encryption. To solve this problem, in this paper we present a conversion from non-anonymous hierarchical identity-based encryption (NaHIBE) with strongly CPA security to CCA-secure and collusion-resistant MUIBPRE. Furthermore, based on the NaHIBE scheme proposed by Waters at Crypto'09, we give the first CCA-secure and collusion-resistant MUIBPRE scheme in the standard model.

In a proxy re-encryption scheme, a semi-trusted proxy is given special power that allows it to transform a ciphertext for Alice into a ciphertext for Bob without learning any information about the messages encrypted under either key. When a proxy re-encryption scheme is constructed in an identity-based setting, it means that a proxy converts a ciphertext encrypted under Alice's identity into a ciphertext under Bob's. Proxy re-encryption has become more and more popular these years due to the fact that it has many practical applications. In this paper, we present an IND-CCA2 secure identity-based proxy re-encryption scheme which has several useful properties, including, multi-use, unidirectionality, etc. Finding a unidirectional, multi-use, and CCA2-secure proxy re-encryption scheme is presented as an open problem by Green et al. Fortunately, our identity-based proxy re-encryption scheme is a solution to this problem. As a middleware for fulfilling our main goal, we also propose a new construction of identity-based encryption using random padding techniques. The security of our schemes is based on the standard decisional bilinear Diffie-Hellman assumption in the random oracle model.

We introduce a new cryptographic primitive, called proxy re-encryption with keyword search, which is motivated by the following scenario in email systems: Charlie sends an encrypted email, which contains some keywords, such as ''urgent'', to Alice under Alice's public key, and Alice delegates her decryption rights to Bob via her mail server. The desired situations are: (1) Bob can decrypt mails delegated from Alice by using only his private key, (2) Bob's mail gateway, with a trapdoor from Bob, can test whether the email delegated from Alice contains some keywords, such as ''urgent'', (3) Alice and Bob do not wish to give the mail server or mail gateway the access to the content of emails. The function of proxy re-encryption with keyword search (PRES) is the combination of proxy re-encryption (PRE) and public key encryption with keyword search (PEKS). However, a PRES scheme cannot be obtained by directly combining those two schemes, since the resulting scheme is no longer proven secure in our security model. In this paper, a concrete construction is proposed, which is proven secure in the random oracle model, based on the modified Decisional Bilinear Diffie-Hellman assumption.

An attribute based encryption scheme (ABE) is a cryptographic primitive in which every user is identified by a set of attributes, and some function of these attributes is used to determine the ability to decrypt each ciphertext. Chase proposed the first multi authority ABE scheme which requires a fully trusted central authority who has the ability to decrypt each ciphertext in the system. This central authority would endanger the whole system if it is corrupted. This paper provides a threshold multi authority fuzzy identity based encryption (MA-FIBE) scheme without a central authority for the first time. An encrypter can encrypt a message such that a user could only decrypt if he has at least d"k of the given attributes about the message for at least t+1,t=

In a proxy multi-signature scheme, a designated proxy signer can generate the signature on behalf of a group of original signers. To our best knowledge, most of existing proxy multi-signature schemes are proposed in public key infrastructure setting, which may be the bottleneck due to its complexity. To deduce the complexity, several proxy multi-signature schemes are proposed in the ID-based setting. However, no formal definitions on ID-based proxy are proposed until now. To fill the gap, this paper proposes the formal definition. Furthermore, we present a proven secure ID-based proxy multi-signature scheme, which is more efficient than existing schemes in term of computational cost.

Directed signature scheme allows only a designated verifier to check the validity of the signature issued to him; and at the time of trouble or if necessary, any third party can verify the signature with the help of the signer or the designated verifier as well. Due to its merits, directed signature scheme is widely used in situations where the receiver's privacy should be protected. Threshold directed signature is an extension of the standard directed signature, in which several signers may be required to cooperatively sign messages for sharing the responsibility and authority. To the best of our knowledge, threshold directed signature has not been well studied till now. Therefore, in this paper, we would like to formalize the threshold directed signature and its security model, then present a new (t,n) threshold directed signature scheme from bilinear pairings and use the techniques from provable security to analyze its security.

Proxy cryptosystem, first proposed by Mambo and Okamoto [M.Mambo, E. Okamoto, Proxy cryptosystem: delegation of a power to decrypt ciphertexts, IEICE Trans. Fundam. Electron. Commun. Comput. Sci. E80-A/1 (1997) 54-63], allows the original decryptor to delegate his decrypting capability to the proxies. However, until now, no practical proxy cryptosystem modes are proposed. Therefore, in this paper, we present a novel proxy cryptosystem model: proxy cryptosystem based on time segmentation. Under this mode, a security analysis model will be proposed. Furthermore, a proxy cryptosystem scheme is presented as an example. We will show that the proposed scheme is proven security in the proposed security analysis model. Finally, we will give the ID-based version of this construction.

We analyze the relationship between the notion of certificateless public key cryptography (CL-PKC) and identity-based schemes without a trusted private key generator (PKG), formally define the security of certificateless threshold signatures, and propose a concrete implementation based on bilinear pairings. To exhibit the security of our proposal, we develop the theory of simulatability and relationship between the certificateless threshold signatures and the underlying (non-threshold) ID-based signatures. We show that the proposed scheme is robust and existentially unforgeable against adaptively chosen message attacks under CDH assumption in the random oracle model.