A certificateless (CL) signcryption scheme is a cryptographic primitive that provides user authentication and message confidentiality at the same time. CL signcryption schemes (as a type of certificateless encryption scheme) have solved problems concerning malicious server presentation, and the server who issues users’ partial private keys and certificates cannot obtain users’ signing keys. Therefore, the CL signcryption scheme is an excellent choice for protecting users’ signing keys and providing user authentication and message confidentiality. Moreover, signcryption schemes have lower computational costs than signature and encryption schemes. The present study presents a short and efficient CL signcryption scheme based on the hyperelliptic curve (HC). Applying HC as the calculation base for designing the presented CL signcryption scheme reduces key-length from 160 bits to 80. The presented CL signcryption scheme is shorter than other recently-proposed ones with regard to communication overhead with its less than one-third shorter length compared to the shortest of the others. Moreover, it is more efficient than other recently-proposed CL signcryption schemes in the user-side computational cost, including the key generation and user key generation phases that have been halved in total. Finally, the security of the presented CL signcryption scheme was analyzed in the random oracle (RO) model based on the hardness of the point factorization problem (PFP) on HC.

Certificateless public key cryptography (CL-PKC) is a useful method in order to solve the problems of traditional public key infrastructure (i.e., large amount of computation, storage and communication costs for managing certificates) and ID-based public key cryptography (i.e., key escrow problem), simultaneously. A signcryption scheme is an important primitive in cryptographic protocols which provides the goals of signing and encrypting, simultaneously. In 2010, Liuet al.presented the first certificateless signcryption (CLSC) scheme in the standard model, but their scheme is vulnerable against di erent attacks presented in the literature, till now. In this paper, we improve their scheme and propose a new CLSC scheme, which is semantically secure against adaptive chosen ciphertext attack under the (S2; 5) -BDHE-Set assumption and existentially unforgeable against adaptive chosen message attack under the 3-CDHE assumption in the standard model. Our scheme is more efficient than all other secure CLSC schemes in the standard model proposed up to now.

Wireless Body Area Networks (WBANs) have attracted a lot of attention in recent researches as they play a vital role in diagnosing, controlling and treating diseases. These networks can improve the quality of medical services by following the health status of people and providing online medical advice for them, momentarily. Despite the numerous advantages of these networks, they may cause irrecoverable problems for patients, if security considerations are not properly met. So, it is very important to find solutions for satisfying security requirements in these networks. A signcryption scheme can be considered as one of the most important cryptographic tools for providing the security requirements in WBANs. Recently, Kasyoka et al. proposed a signcryption scheme based on which they designed an access control protocol for WBANs. They proved the security of their proposals in the random oracle model (ROM). In this paper, we concentrate on Kasyoka et al. ’s proposals and show that their proposed signcryption scheme and consequently their proposed access control protocol for WBANs are vulnerable against various attacks, in contrast to their claims. Afterward, we fix the scheme to be secure against our proposed attacks.

The certi cateless public key cryptography (CL-PKC) setting, makes it possible to overcome the problems of the conventional public key infrastructure and the ID-Based public key cryptography, concurrently. A certi cateless signcryption (CL-SC) scheme is an important cryptographic primitive which provides the goals of a signature scheme and an encryption scheme both at once, in a certi cateless setting. In addition to the basic security requirements of a CL-SC scheme (i. e. the unforgeability and the con dentiality), a new security notion called as the known session speci c temporary information security (KSSTIS) has been proposed in the literature, recently. This security notion guarantees the con dentiality of the message even if the temporary information, used for creating the signcryption on the message, reveals. However, as discussed in the literature, there are not any secure CL-SC schemes in the standard model (i. e. without the assumption of random oracles) which guarantees the KSSTIS. In this paper, three recently proposed CL-SC schemes (Caixue, Shan and Ullah et al. 's schemes) are analyzed and it is shown that these schemes not only do not satisfy the KSSTIS, but also they do not even provide the basic security requirements of a CL-SC scheme. Furthermore, an enhanced secure CL-SC scheme is proposed in the standard model which satis es the KSSTIS.