Abstract Recently, Qiao et al. proposed a novel construction of certificateless aggregate signature (CLAS) scheme to ensure the integrity and authenticity of medical data in healthcare wireless medical sensor networks (HWMSNs). They first created an underlying certificateless signature (CLS) scheme, and then proposed a CLAS scheme from the underlying CLS scheme by adding an aggregation algorithm and a verification algorithm. In this paper, we point out that their CLS scheme is insecure because the Type I adversary can forge valid signatures. That is, the unforgeability is not actually captured by their CLS scheme. Finally, we map our cryptanalysis to the practical application. That is, in the practical application of HWMSNs, the attacker can launch real attack to their CLS scheme using our cryptanalysis to forge signatures. Therefore, Qiao et al.’s CLS scheme can be totally broken.

Yang et al. proposed a privacy-preserving aggregation authentication scheme (PPAAS) for fog-cloud-based vehicular ad hoc network based on an anonymous certificateless aggregation signcryption. In their scheme, the vehicles’ public keys and pseudonyms are contained in the ciphertexts to provide sender anonymity. The receiver uses them after they are retrieved from the ciphertexts. However, this method contradicts the authentication. A pseudonym is essentially an encryption of the real identity. The receiver who does not know the encryption key cannot check the validity of the public keys with the pseudonyms. Thus, the attacker can contain public keys and pseudonyms selected by himself in the forged aggregated ciphertext. The receiver has to use them directly. Hence, the PPAAS can be broken in real life.

Abstract Recently, Yang et al. proposed the first certificateless proxy signature scheme against malicious-but-passive key generation center (MKGC) attacks. They proved that their scheme can resist the MKGC attacks in the standard model. In this paper, we point out that their scheme cannot achieve this security because the adversary can forge valid signatures.

In cloud environments, group data sharing has become a hot topic in recent years. How to share data securely and efficiently in cloud environments is an urgent problem to be solved. Recently, an anonymous and traceable group data sharing scheme was proposed by Shen et al. to address this issue. They constructed their scheme using a group signature scheme as the building block. In this comment, we discuss the security of their group signature scheme and point out that it does not achieve the anonymity which they claimed and give a corresponding attack.

Abstract Recently, Tseng et al. proposed a new notion for identity-based signature (IBS) scheme to resist ephemeral secret leakage (ESL) attacks, called leakage-free identity-based signature (leakage-free IBS), and devised the first secure leakage-free IBS scheme. However, they only considered the situation of the leakage of ephemeral secrets used for generating the signatures. Notice that the private key extraction procedure is probabilistic as well in their scheme, that is, there are ephemeral secrets used by the key generation center to generate the signers’ private keys. It is practical to consider that if the adversary comprises these ephemeral secrets, then he can reveal the master key of the system. Therefore, it is desired to introduce a new security notion for the leakage-free IBS schemes to consider the ESL attacks on both private key extraction and signing procedures. In this paper, we present such security notion. Moreover, we propose two IBS schemes that are proved to be secure under the new security notion.

Abstract Rastegari et al. recently proposed a certificateless signcryption (CL-SC) scheme. They claimed that their scheme is the first secure CL-SC scheme, which captures the known session-specific temporary information security (KSSTIS), in the standard model. In this paper, we point out that their scheme is insecure, which implies that how to construct a secure CL-SC scheme with KSSTIS in the standard model is still an open problem.

Abstract Recently, Zhang et al. proposed a new anonymous hierarchical identity-based encryption (anonymous HIBE) over prime order groups to achieve both constant size private key and constant size ciphertext. Moreover, a double exponent technique was used to provide anonymity. They proved that their scheme is secure and anonymous against chosen plaintext attacks in the standard model. In this paper, we point out that their scheme is insecure.

Certificateless signcryption (CLSC) has attracted much attention from the research community since it provides both confidentiality and unforgeability, and, at the same time, it does not suffer from the certificate management problem in traditional public key cryptography and the key escrow problem in identity-based cryptography. However, most CLSC schemes are based on the bilinear pairing which is still a time-costing operation although many efforts have been made to improve its efficiency. Recently, Yu et al. proposed a pairing-free CLSC scheme and proved its security. In this paper, we point out that their scheme can be totally broken since confidentiality and unforgeability actually are not captured.

Recently, a new framework, called secure server-designation public key encryption with keyword search (SPEKS), was proposed by Chen to withstand online keyword guessing attack. Moreover, Chen proposed a concrete scheme (IBE,TE)-2-SPEKS and proved that their scheme meets trapdoor indistinguishability. In this comment, we point out that the trapdoor, in fact, can be distinguished by the adversary.

Recently, Islam and Li proposed the first certificateless signcryption scheme without ephemeral secret leakage (ESL) attack, called leakage-free certificateless signcryption (leakage-free CLSC) scheme. However, we point out in this paper that the confidentiality property is not captured in their proposal. Moreover, our attack adheres to the security model proposed in the original paper. On the other hand, the security models proposed by Islam and Li are insufficient. In fact, the ESL attack is not involved in the security models since the ephemeral secret is not returned to the adversary when CLSC-Signcryption query and Challenge are issued. Finally, we give the amended security models.

Most cheating immune visual cryptography schemes (CIVCS) are based on a traditional visual cryptography scheme (VCS) and are designed to avoid cheating when the secret image of the original VCS is to be recovered. However, all the known CIVCS have some drawbacks. Most usual drawbacks include the following: the scheme needs an online trusted authority, or it requires additional shares for the purpose of verification, or it has to sacrifice the properties by means of pixel expansion and contrast reduction of the original VCS or it can only be based on such VCS with specific access structures. In this study, the authors propose a new CIVCS that can be based on any VCS, including those with a general access structure, and show that their CIVCS can avoid all the above drawbacks. Moreover, their CIVCS does not care about whether the underlying operation is OR or XOR.

Droste [CRYPTO’96] proposed a construction of threshold visual cryptography scheme (TVCS) under the visual cryptography model of Naor and Shamir, i.e. the visual cryptography model with the underlying operation OR. In this article, we give three extensions of TVCS. First, we prove that the TVCS proposed by Droste which was based on the OR operation is still a valid TVCS under the XOR operation, and then we propose a method to further reduce its pixel expansion. We then propose an interesting construction of TVCS with all shares being concolorous. Finally, we give a construction of threshold extended visual cryptography scheme (TEVCS) with the underlying operation OR or XOR. All of our schemes can be applied to the visual cryptography model introduced by Tuyls et al. (First Int. Conf. Security in Pervasive Computing 2004, International Patent with Application No.: PCT/IB2003/000261).

Two common drawbacks of the visual cryptography scheme (VCS) are the large pixel expansion of each share image and the small contrast of the recovered secret image. In this paper, we propose astep constructionto construct$\hbox{VCS}_{\rm OR}$and$\hbox{VCS}_{\rm XOR}$for general access structure by applying (2,2)-VCS recursively, where a participant may receive multiple share images. The proposed step construction generates$\hbox{VCS}_{\rm OR}$and$\hbox{VCS}_{\rm XOR}$which have optimal pixel expansion and contrast for each qualified set in the general access structure in most cases. Our scheme applies a technique to simplify the access structure, which can reduce the average pixel expansion (APE) in most cases compared with many of the results in the literature. Finally, we give some experimental results and comparisons to show the effectiveness of the proposed scheme.

In IEE Proceedings Communications, 2005, 152, (6), an efficient authenticated identity-based multicast scheme from bilinear pairing was proposed. Wang proved that the scheme is secure against the adversary who can forge a ciphertext, but it is shown that their scheme is not secure against an inside forger.

Visual cryptography scheme (VCS) is a kind of secret-sharing scheme which allows the encryption of a secret image into n shares that are distributed to n participants. The beauty of such a scheme is that, the decryption of the secret image requires neither the knowledge of cryptography nor complex computation. Colour visual cryptography becomes an interesting research topic after the formal introduction of visual cryptography by Naor and Shamir in 1995. The authors propose a colour (k, n)-VCS under the visual cryptography model of Naor and Shamir with no pixel expansion, and a colour (k, n)-extended visual cryptography scheme ((k, n)-EVCS) under the visual cryptography model of Naor and Shamir with pixel expansion the same as that of its corresponding black and white (k, n)-EVCS. Furthermore, the authors propose a black and white (k, n)-VCS and a black and white (k, n)-EVCS under the visual cryptography model of Tuyls. Based on the black and white schemes, the authors propose a colour (k, n)-VCS and a colour (k, n)-EVCS under the same visual cryptography model, of which the pixel expansions are the same as that of their corresponding black and white (k, n)-VCS and (k, n)-EVCS, respectively. The authors also give the experimental results of the proposed schemes, and compare the proposed scheme with known schemes in the literature.