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 this end, in this paper, we introduce a new cryptographic primitive called key-policy attribute-based temporary keyword search (KP-ABTKS) which provide this property. To evaluate the security of our scheme, we formally prove that our proposed scheme achieves the keyword secrecy property and is secure against selectively chosen keyword attack (SCKA) both in the random oracle model and under the hardness of Decisional Bilinear Diffie-Hellman (DBDH) assumption. Furthermore, we show that the complexity of the encryption algorithm is linear with respect to the number of the involved attributes. Performance evaluation shows our scheme's practicality.

Abstract In this paper, we propose the first provable secure certificate‐based proxy signature (CBPS) with message recovery without bilinear pairing. In 2003, Gentry introduced the concept of certificate‐based cryptography to solve certificate management and key escrow problems in public key infrastructure and identity‐based cryptosystems, respectively. Because the overall cost of pairing computation is higher than scalar multiplication (e.g., over elliptic curve group), consequently, the CBPS schemes without pairings would be more efficient. According to the available research in this regard, our scheme is the first provable secure CBPS scheme with message recovery that is based on the elliptic curve discrete logarithm problem. We prove the security of the presented scheme in the random oracle model against existential forgery under adaptive chosen message and ID attacks. Moreover, the paper will also show how it would be possible to convert this scheme to the CBPS scheme without message recovery. This scheme has more applications in cases with constrain power and restricted bandwidth. Copyright © 2016 John Wiley & Sons, Ltd.

In order to provide anonymity, universal cryptosystems have been used in various applications, including mixnets with multiple receivers. Unlike ordinary re-encryption cryptosystems, universal cryptosystems for re-encryption of ciphertexts do not require knowledge of the public key of the receiver. Golle et al. introduced universal cryptosystems for public key cryptography. Contrary to public key cryptography, in ID-based cryptography, a public key infrastructure is not needed, which makes it suitable for situations where it is not cost-effective to distribute certificates or establish a public key infrastructure. In this paper, we first generalize the definition of universal cryptosystems and the notion of universal semantic security to ID-based cryptography, then we propose two ID-based universal cryptosystem schemes, and lastly we prove the security of these two schemes. The first scheme has higher efficiency in terms of computation and communication while the second scheme has proof of security in the standard model and has a tighter security band. Copyright © 2015 John Wiley & Sons, Ltd.

In this paper, we present a framework for biclique cryptanalysis of block ciphers which extremely requires a low amount of data. To that end, we enjoy a new representation of biclique attack based on a new concept of cutset that describes our attack more clearly. Then, an algorithm for choosing two differential characteristics is presented to simultaneously minimize the data complexity and control the computational complexity. Then, we characterize those block ciphers that are vulnerable to this technique and among them, we apply this attack on lightweight block ciphers Piccolo-80, Piccolo-128, and HIGHT. The data complexity of these attacks is only 16-plaintext-ciphertext pairs, which is considerably less than the existing cryptanalytic results. In all the attacks, the computational complexity remains the same as the previous ones or even it is slightly improved.

Recently, AIDA/cube testers have been revealed to be useful in building distinguishers for several cryptography schemes. χ 2 tests, on the other hand, are well known and extensively used for distinguishing purposes. In this study, the notion of multi‐χ 2 test and AIDA/cube testers are utilised to introduce the superpoly algebraic normal form monomial test through which the output of reduced round Trivium is distinguished from being random. The test successfully distinguishes the keystream of Trivium with 830 out of 1152 initialisation rounds with a complexity of 2 39 operations, which is the most effective distinguisher on reduced Trivium thus far. Applying algebraic IV differential attack (AIDA)/cube testers to a system, one of the main concerns is the appropriate choice of the superpoly variables, in the sense that the complexity of distinguishing the system output becomes near minimal. In an effort to discover appropriate superpoly variables, the authors propose a heuristic method that determines weak combination set of bits. The notion of weak combinations is defined in this study as a property ofthe cube variables whose corresponding superpoly is distinguishable. This heuristic method is performed on reduced round Trivium and its strength on determining appropriate superpoly variables is verified for 730 and 760 rounds Trivium.

ABSTRACT In pseudonym systems, users by means of pseudonyms anonymously interact with organizations to obtain credentials. The credential scheme constructed by Lysyanskaya and Camenisch is among the most complete credential systems, in which “all‐or‐nothing” sharing scheme is used to prevent users sharing their credentials. If a user cannot directly show a credential issued by an organization, she or he has to give her or his own secret key to someone else as a proxy; afterward, the proxy can show the credential on behalf of the user. Thus, according to the all‐or‐nothing property of the system, having the user's secret key, the proxy can use all credentials of the user for itself. To solve this problem, in this paper, we present proxy zero‐knowledge proof and utilize it in Lysyanskaya and Camenisch anonymous credential system. In our proposed system, instead of giving the secret key to the proxy, the user generates a proxy key based on the desired credential particularly for the proxy. Therefore, the proxy neither is the owner of the user's credential nor uses his or her other credentials. Copyright © 2012 John Wiley & Sons, Ltd.

Trivium is a hardware profile finalist of eSTREAM project. It is a synchronous bit-oriented stream cipher. The cipher's internal state has 288 bits. Bivium is a simplified version of Trivium with a smaller internal state. Both algorithms provide the security level of 80 bits. In this paper we introduce a guess and determine attack on Trivium and Bivium. In our method, we first find the linear approximations for the updating functions. Then by using these approximations, we build a system of linear equations and internal state variables. In order to solve the system, some bits of the internal state should be guessed. Our attack on Trivium is not successful because of the large length of internal state therefore it is resistant to the method. It's complexity is of order O(290.67). But for recovering the state of Bivium, we need to guess only 27.55 bits and other bits will be determined. In order to complete the attack 243.99bits of key stream are needed. The complexity of the attack on Bivium is O(227.55), which is an improvement to the previous guess and determine attack with a complexity of order O(252.3).