Survey on Revocation in Ciphertext-Policy Attribute-Based Encryption <s> Introduction <s> As more sensitive data is shared and stored by third-party sites on the Internet, there will be a need to encrypt data stored at these sites. One drawback of encrypting data, is that it can be selectively shared only at a coarse-grained level (i.e., giving another party your private key). We develop a new cryptosystem for fine-grained sharing of encrypted data that we call Key-Policy Attribute-Based Encryption (KP-ABE). In our cryptosystem, ciphertexts are labeled with sets of attributes and private keys are associated with access structures that control which ciphertexts a user is able to decrypt. We demonstrate the applicability of our construction to sharing of audit-log information and broadcast encryption. Our construction supports delegation of private keys which subsumesHierarchical Identity-Based Encryption (HIBE). <s> BIB001 </s> Survey on Revocation in Ciphertext-Policy Attribute-Based Encryption <s> Introduction <s> In several distributed systems a user should only be able to access data if a user posses a certain set of credentials or attributes. Currently, the only method for enforcing such policies is to employ a trusted server to store the data and mediate access control. However, if any server storing the data is compromised, then the confidentiality of the data will be compromised. In this paper we present a system for realizing complex access control on encrypted data that we call ciphertext-policy attribute-based encryption. By using our techniques encrypted data can be kept confidential even if the storage server is untrusted; moreover, our methods are secure against collusion attacks. Previous attribute-based encryption systems used attributes to describe the encrypted data and built policies into user's keys; while in our system attributes are used to describe a user's credentials, and a party encrypting data determines a policy for who can decrypt. Thus, our methods are conceptually closer to traditional access control methods such as role-based access control (RBAC). In addition, we provide an implementation of our system and give performance measurements. <s> BIB002 </s> Survey on Revocation in Ciphertext-Policy Attribute-Based Encryption <s> Introduction <s> We present a new methodology for realizing Ciphertext-Policy Attribute Encryption (CP-ABE) under concrete and noninteractive cryptographic assumptions in the standard model. Our solutions allow any encryptor to specify access control in terms of any access formula over the attributes in the system. In our most efficient system, ciphertext size, encryption, and decryption time scales linearly with the complexity of the access formula. The only previous work to achieve these parameters was limited to a proof in the generic group model. ::: ::: We present three constructions within our framework. Our first system is proven selectively secure under a assumption that we call the decisional Parallel Bilinear Diffie-Hellman Exponent (PBDHE) assumption which can be viewed as a generalization of the BDHE assumption. Our next two constructions provide performance tradeoffs to achieve provable security respectively under the (weaker) decisional Bilinear-Diffie-Hellman Exponent and decisional Bilinear Diffie-Hellman assumptions. <s> BIB003 </s> Survey on Revocation in Ciphertext-Policy Attribute-Based Encryption <s> Introduction <s> The recent emergence of cloud computing has drastically altered everyone's perception of infrastructure architectures, software delivery and development models. Projecting as an evolutionary step, following the transition from mainframe computers to client/server deployment models, cloud computing encompasses elements from grid computing, utility computing and autonomic computing, into an innovative deployment architecture. This rapid transition towards the clouds, has fuelled concerns on a critical issue for the success of information systems, communication and information security. From a security perspective, a number of unchartered risks and challenges have been introduced from this relocation to the clouds, deteriorating much of the effectiveness of traditional protection mechanisms. As a result the aim of this paper is twofold; firstly to evaluate cloud security by identifying unique security requirements and secondly to attempt to present a viable solution that eliminates these potential threats. This paper proposes introducing a Trusted Third Party, tasked with assuring specific security characteristics within a cloud environment. The proposed solution calls upon cryptography, specifically Public Key Infrastructure operating in concert with SSO and LDAP, to ensure the authentication, integrity and confidentiality of involved data and communications. The solution, presents a horizontal level of service, available to all implicated entities, that realizes a security mesh, within which essential trust is maintained. <s> BIB004 </s> Survey on Revocation in Ciphertext-Policy Attribute-Based Encryption <s> Introduction <s> At Eurocrypt 2005, Sahai and Waters [7] introduced the concept of attribute-based encryption (ABE). ABE enables public key based one-to-many encryption and is envisioned as a promising cryptographic primitive for realizing scalable and fine-grained access control systems. There are two kinds of ABE schemes [1], key-policy ABE (KP-ABE) and ciphertext-policy ABE (CP-ABE) schemes. This paper, our concern is on the latter. <s> BIB005 </s> Survey on Revocation in Ciphertext-Policy Attribute-Based Encryption <s> Introduction <s> Data confidentiality in the Cloud Computing is a very challenging task. Encryption is one of the most secure methods ensuring this task, and searchable encryption techniques are used to search on encrypted data without the need for decryption. But, despite this secure measure some leaks may appear when searching over data. In this article, we propose to improve confidentiality of outsourced data. We are particularly interested in reinforcing the access control on the search result, when the search is performed over encrypted data. The property behind this aspect of security is known as ACAS (Access Control Aware Search) principle. We present a hybridization of Searchable Encryption and Attribute Based Encryption techniques in order to satisfy the ACAS property. The proposed model supports a personalized and secure multi-user access to outsourced data, presenting high search performance. It deals with multi-keywords searches and is designed to speed up the search time by taking advantage of High Performance Computing, which is widely used in Cloud Computing. Two Attribute Based Encryption techniques are considered on the side of the Cloud and some conducted experiments show the efficiency of the proposed method. <s> BIB006
Unlimited cloud storage and data outsourcing services provide data owners and enterprises with capacities for storing and processing a massive amount of data. These high quality services enable easy accessibility, high scalability, and availability  . Despite all the advantages, serious concerns about the data security and confidentiality in such a cloud environment have been raised BIB004 . To preserve data confidentiality, many techniques support searching on the outsourced data stored in the cloud environment. However, these techniques cannot regulate access to specific stored data record or enforce access policies BIB006 . In an untrusted cloud environment, preserving data confidentiality, making an appropriate decision on data access, and enforcing access policies are core challenges. Therefore, many system models and techniques for access control based on cryptographic operations have been characterized and described by researchers to provide secure and efficient cloud access control. However, cryptographic approaches for lightweight devices have established their own paradigm. Although such techniques enable data sharing among a large number of users, some open issues still need to be addressed, particularly for Internet of Things (IoT) applications. Firstly, access control policies should be expressive enough increase in the amount of data sent via the secret channel. In addition, this system does not consider dynamic updates such as deletion, insertion and revocation processes. On the other hand, expressiveness is fulfilled by introducing attribute-based access control, which grants access privileges to users based on their attributes, e.g., doctor or researcher and health science center, specifies that only doctors or researchers in the Health Science center can access the related data. To perform attribute-based access control while preserving data confidentiality, Attribute-based Encryption (ABE) has been proposed, which is an advanced asymmetric cryptographic technique invented to leverage the merits of the symmetric encryption (e.g., high efficiency) and solve its key management and distribution problems. Such a system is based on elliptic curve cryptography. This makes the discrete logarithm problem hard with a reasonable security level by using a small secret key size (about 160 bits). Two variants of ABE were discussed in Goyal et al.'s scheme BIB001 . These variants are ciphertext policy attribute based-encryption (CP-ABE) and key policy attribute-based encryption (KP-ABE). The major difference between them lies in how to associate a secret key and an access policy with relevant data and attributes. In KP-ABE, an access policy is associated with a secret key and a set of attributes are associated with the data encrypted with the key. Conversely, in CP-ABE, each encrypted data item is assigned with a specific access policy and a user's secret key for the data decryption is assigned with a set of attributes. As a consequence of embedding an access policy into a user's secret key in KP-ABE, a data owner (who encrypts the data) can only select a set of attributes but will not be able to decide which user can access its ciphertext. To decrypt the ciphertext, a key generator (i.e., an attribute authority), which generates the decryption key for authorized users, will be responsible for granting or denying access to the key BIB002 . Due to this property, we pay little attention to KP-ABE and focus on CP-ABE in the rest of this paper, as our objective is to give data owners full control of their sensitive data: CP-ABE is more suitable for this purpose. However, despite the CP-ABE merits, the significant issues that the first proposed CP-ABE schemes BIB002 BIB003 did not consider are revocation and managing a wide range of attributes. In terms of revocation, in any access control system, this property is essential to emulate the dynamicity of cloud storage. For this reason, many researchers have paid a lot of attention to extend the proposed schemes and resolve the revocation issue with low computational costs to be applicable to IoT devices. To critically analyze the existing work based on the above stated challenges and requirements, we have to mention that most of the current CP-ABE work achieves data confidentiality and privacy, fine-grained access control, and expressive access structure BIB003 BIB005 . However, the challenge of performing an efficient revocation process is still open. Therefore, in this review, we put a strong emphasis on the main requirements that have not been efficiently fulfilled yet such as revocation and collusion resistance. To the best of our knowledge, this survey is the first work that examines the types of revocation techniques used in both single and multiauthority schemes built on the most popular variant of ABE, which is CP-ABE. Most of the existing reviews cover the common types of access policies (e.g., monotonic or nonmonotonic access policies) appeared in available literatures. In addition, these reviews are lack of information on research in revocation, although it is the most highlighted, well-known issue. This review is structured as follows. Section 2 presents the background of elliptic curve cryptography. ABE, its requirements, and the main problems of CP-ABE are defined in Section 3. Section 4 introduces existing single authority and multiauthority CP-ABE schemes. The research challenges and future directions are described in Section 5. Finally, Section 6 concludes the review.