Survey on Revocation in Ciphertext-Policy Attribute-Based Encryption <s> Attribute-Based Encryption <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> Attribute-Based Encryption <s> In this work, we aim to make attribute-based encryption ABE more suitable for access control to data stored in the cloud. For this purpose, we concentrate on giving to the encryptor full control over the access rights, providing feasible key management even in case of multiple independent authorities, and enabling viable user revocation, which is essential in practice. Our main result is an extension of the decentralized CP-ABE scheme of Lewko and Waters [6] with identity-based user revocation. Our revocation system is made feasible by removing the computational burden of a revocation event from the cloud service provider, at the expense of some permanent, yet acceptable overhead of the encryption and decryption algorithms run by the users. Thus, the computation overhead is distributed over a potentially large number of users, instead of putting it on a single party e.g., a proxy server, which would easily lead to a performance bottleneck. The formal security proof of our scheme is given in the generic bilinear group and random oracle models. <s> BIB002
Attribute-based encryption (ABE) is one of the advanced cryptographic techniques for one-to-many encryption that overcomes the limited functionalities of the traditional public key cryptographic techniques. This scheme was proposed by Sahai and Waters  as an application of fuzzy identity-based encryption, which uses human-intelligible identities (such as unique name, IP address, email address, etc.) as public keys, where a data sender directly encrypts its data with the receiver's identity. Later, Goyal et al. BIB001 presented a more general construction of ABE, in which attributes have been utilized to issue a public key and to generate a logical expression of these attributes called an access policy. Both the public key and access policy are used for encrypting data. In contrast with the traditional cryptographic systems which encrypt data for one particular user or group that knows the decryption key, there is no more need to share the same private key or store several versions of the ciphertext encrypted with different keys BIB002 . Moreover, ABE has no restriction on the number of users in the system. Based on these considerations, this scheme has been leveraged to regulate users' access to cloud data by using attributes as an access policy. To apply ABE, a data owner encrypts its data using a symmetric encryption algorithm with a symmetric key and then encrypts the key using an ABE scheme with a public key. The encrypted key is distributed to a group of recipients/users as a ciphertext. Each user obtains the private key for the encrypted key decryption from a key generator that calculates the key according to the user's attributes. In this case, the data owner does not need to know the identities of the legitimate users and their dynamicity. Figure 1 illustrates the above-mentioned operational process. Applying ABE to the two variants (i.e., CP-ABE and KP-ABE) follows the same procedure as shown in Figures 2 and 3 . The main difference is, in KP-ABE, users' secret keys are issued using an access policy that defines the access privileges of the authorized user, and the symmetric key is encrypted over a set of attributes. However, CP-ABE uses access policies to encrypt data (i.e., symmetric key) and secret keys of the legitimate users are generated over a set of attributes. In addition, some evaluative requirements need to be satisfied by any ABE scheme