As data sharing has become one of the most popular services offered by cloud storage, designing public auditing mechanisms for integrity of shared data becomes more important. Two problems which arise in shared data auditing include preserving users identity and collusion resistant revocation of users. When data stored at the cloud is shared among a group of users, different users may modify and sign different data blocks which leaks signer identities to the public verifier. Also, when a user is revoked from the group, signatures generated by this user should be re-signed by the cloud server using re-signature keys. In addition, collusion of cloud server and the revoked user should leak no information about the private key of other users. In this paper, by employing a new proxy re-signature scheme, we propose a public shared data auditing mechanism that provides identity privacy and collusion resistant user revocation, simultaneously. The proposed protocol requires only lightweight computations at the user side for signing data blocks in real-time online phase. Moreover, our protocol supports large dynamic group of users, batch auditing and dynamic data operations. Experimental results demonstrate excellent efficiency of our scheme in comparison to the state of the art.

Concurrent signatures allow two entities to generate two signatures in such a way that both signatures are ambiguous till some information is revealed by one of the parties. This kind of signature is useful in auction protocols and in a wide range of scenarios in which involving participants are mutually distrustful. In this study, to have quantum‐attack‐resistant concurrent signatures as recommended by National Institute of Standards and Technology (NISTIR 8105), the first concurrent signature scheme based on coding theory is proposed. Then, its security is proved under Goppa Parameterized Bounded Decoding and the Goppa Code Distinguishing assumptions in the random oracle model. In addition, performance evaluation shows that the proposal is approximately as efficient as Dallot scheme. The authors should highlight that their proposal can be a post‐quantum candidate for fair exchange of signatures without a trusted third party in an efficient way (without a high degree of interactions).

The notions of identity-based multi-proxy signature, proxy multi-signature and multi-proxy multi-signature have been proposed to facilitate public key certificate management of these kinds of signatures by merely employing signer's identities in place of the public keys and their certificates. In the literature, most identity-based multi-proxy signature, proxy multi-signature and multi-proxy multi-signature schemes are based on bilinear pairings. Without incorporating bilinear pairings, Tiwari and Padhye proposed an identity-based proxy multi-signature scheme in 2011. Subsequently, an identity-based multi-proxy multi-signature scheme was proposed by Tiwari et al. in 2012. First, we review identity-based (multi)-proxy multi-signature schemes without bilinear pairings and show that unfortunately, they are insecure in their security models. Secondly, we propose an identity-based multi-proxy multi-signature scheme without bilinear pairings, where identity-based multi-proxy signature and proxy multi-signature schemes are its special cases. Then, we prove that they are secure under Rivest, Shamir and Adleman (RSA) assumption in the random oracle model by presenting a new Forking Lemma. The proposal and its special cases are the first identity-based multi-proxy signature, proxy multi-signature and multi-proxy multi-signature from RSA assumption.

Abstract Proxy signatures allow an entity to delegate its signing capability to a proxy which can sign messages on behalf of the delegator. We examine identity‐based versions of proxy signatures which employ identity strings in place of randomly generated public keys. First, we give a new generic construction of identity‐based proxy signatures from identity‐based standard signatures and show that our generic construction is secure if the underlying identity‐based standard signature is secure. In addition, we present the first identity‐based proxy signature from Rivest, Shamir and Adleman (RSA), secure under the one‐wayness of RSA in the random oracle model. We should highlight that the proxy key exposure attack cannot be applied to our proposed constructions. Copyright © 2015 John Wiley & Sons, Ltd.

Abstract 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 foridentity‐basedproxy ring signatures. We note that there exists no formal security model foridentity‐basedproxy ring signatures prior to our work. Second, we presentthe firstprovably secureidentity‐basedproxy ring signature scheme using a new paradigm called sequential aggregation. The construction is proved secure, under the one‐wayness assumption of RSA, in the random oracle model by presenting a new forking lemma. We should highlight that the proxy key exposure attack cannot be applied to our scheme. Further, in contrast to the existing schemes that are based on pairings, our scheme is based on RSA; therefore, it outperforms the existing schemes in terms of efficiency and practicality. Copyright © 2014 John Wiley & Sons, Ltd.