Sharif Digital Repository

In this paper, we propose a new public key encryption scheme similar to ElGamal cryptosystem. We prove that it is provably secure, based on intractability of factoring problem and intuitively show that our scheme is as least as secure as RSA and ElGamal cryptosystems. We will also show that decryption in our scheme could be done four times faster than ElGamal  

This paper suggests a model and a definition for forward-secure authenticated key exchange (AKE) protocols, which can be satisfied without depending on the Diffie–Hellman assumption. The basic idea is to use keyevolving schemes (KES), where the long-term keys of the system get updated regularly and irreversibly. Protocols conforming to our model can be highly efficient, since they do not require the resource-intensive modular exponentiations of the Diffie–Hellman protocol. We also introduce a protocol, called FORSAKES, and prove rigorously that it is a forward-secure AKE protocol in our model. FORSAKES is a very efficient protocol, and can be implemented by merely using hash functions  

Temporary keyword search on confidential data in a cloud environment is the main focus of this research. The cloud providers are not fully trusted. So, it is necessary to outsource data in the encrypted form. In the attribute-based keyword search (ABKS) schemes, the authorized users can generate some search tokens and send them to the cloud for running the search operation. These search tokens can be used to extract all the ciphertexts which are produced at any time and contain the corresponding keyword. Since this may lead to some information leakage, it is more secure to propose a scheme in which the search tokens can only extract the ciphertexts generated in a specified time interval. To... 

We present an efficient and provably-secure e-voting protocol, which is a variant of the JCJ e-votingprotocol (Juels et al., 2010). It decreases the total number of JCJ's operations from O(n2) to O(n), where n is the number of votes or voters (whichever is the maximum). Note that since the operations under consideration are time-consuming (e.g., public-key encryption), the improvement is quite substantial. As a rough comparison, consider a nation-wide election with around ten million voters/votes. Assuming each operation takes one microsecond, and no parallelization is used, one can see a huge difference: our protocol tallies the votes in 10 seconds, while the JCJ protocol requires over 3... 

In this study, we review attribute-based ring signature schemes. Our proposal presents two contributions; the first one is security analysis of Wenqiang et al.'s scheme in 2009 by presenting an attack in which we show that their scheme is forgeable. The second contribution is an efficient attribute-based ring signature scheme with constant size and constant number of pairing operations. Our scheme is unconditionally anonymous and unforgeable in the standard model. The security of our proposal is based on Static Diffie Hellman (SDH) problem. Compared with the existing attribute-based ring signature scheme with constant size, the length of the signature and the pairing operations in our scheme... 

In current ring signature schemes, there is no distinguishing about the ranks of members of the ring. This paper proposes a ring signature scheme which considers the members' rank values. We show our scheme is anonymous against full key exposure attack and unforgeable with respect to insider corruption in the standard model with the computational Diffie Hellman (CDH) and Subgroup Hiding (SGH) assumptions in bilinear groups  

Identity-based proxy ring signature concept was introduced by Cheng et al. in 2004. This primitive is useful where the privacy of proxy signers is required. In this paper, the first short provably secure identity-based proxy ring signature scheme from RSA assumption has been proposed. In addition, the security of the proposed scheme tightly reduces to the RSA assumption, and therefore, the proposed scheme has a proper advantage in security reduction compared to the ones from RSA. The proposed scheme not only outperforms the existing schemes in terms of efficiency and practicality, but also does not suffer from the proxy key exposure attack due to the use of the sequential aggregation... 

Designing an efficient and secure electronic voting (e-voting) protocol without the presence of trusted authorities, known as decentralized voting protocols, is one of the most interesting and challenging problems in cryptography. In these protocols the outcome of the protocol is computed by voters collaborating with each other. We provide a rigorous proof of security of a decentralized e-voting protocol proposed by Khader et al. in the Universal Composability (UC) Framework. This protocol is the state-of-the-art decentralized e-voting protocol in terms of efficiency and security, whose security has only been justified against a set of desired properties required in e-voting protocols. For... 

Proxy ring (anonymous proxy) signatures allow an entity to delegate its signing capability to a group of entities (proxy group) such that only one of the members in the proxy group can generate a proxy signature on behalf of the delegator, while privacy of the proxy signer is protected. Identity-based versions of proxy ring signatures employ identity strings in place of randomly generated public keys. Our contribution is twofold. First, we formalize a security model for identity-based proxy ring signatures. We note that there exists no formal security model for identity-based proxy ring signatures prior to our work. Second, we present the first provably secure identity-based proxy ring... 

Strong Designated Verifier Signature (SDVS) provides authentication for the signer and the verifier such that the signer is assured that what s/he has signed, is only verified by the designated verifier. Currently, the security of most of the SDVS schemes is based on hard problems in the number theory. As it is proved that all kinds of cryptosystems which are based on the number theory will not tolerate quantum attacks, here, an SDVS scheme based on hard problems in coding theory is presented. We have proved that the suggested scheme is unforgeable under a chosen message attack in Random Oracle Model. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd  

Signatures with partially message recovery in which some parts of messages are not transmitted with signatures to make them shorter are helpful where bandwidth is one of the critical concern. This primitive is especially used for signing short messages in applications such as time stamping, certified email services, and identity-based cryptosystems. In this paper, to have quantum-attack-resistant short signatures, the first signature scheme with partially message recovery based on coding theory is presented. Next, it is shown that the proposal is secure under Goppa Parametrized Bounded Decoding and the Goppa Code Distinguishing assumptions in the random oracle model. Relying on the partially...