Abstract. In PKC 2010, Herranz et al. proposed the first ciphertext policy attribute-based encryption (CP-ABE) scheme with constant size ciphertexts for threshold predicates. However, their scheme was only secure against chosen plaintext attacks (CPA), which was impossible to obtain security against chosen ciphertext attacks (CCA) in the standard model, and they left open the following three problems for CP-ABE schemes with constant size ciphertexts, i.e., how to achieve full security (i.e., not only the selective security), CCA security in the standard model, and security reduction to a more standard mathematical problem. In this paper, we answer the last two of these three problems affirmatively. Towards our goal, we first design a CPA secure threshold CP-ABE scheme, which can be further upgraded to the CCA security. The security of our schemes can be proved under the decisional q-Bilinear Diffie-Hellman Exponent (q-BDHE) assumption in the selective model. To the best of our knowledge, this is the first construction of CCA secure CP-ABE scheme with constant size ciphertexts that can support flexible threshold access structure in the standard model.

Sahai and Waters [21] proposed Attribute-Based Encryption (ABE) as a new paradigm of encryption algorithms that allow the sender to set a policy describing who can decrypt a particular ciphertext. In this paper, we first propose a ciphertext policy attribute-based encryption (CP-ABE) scheme from lattices, which supports flexible threshold access policies on literal (or boolean) attributes. Then we extend it to support multi-valued attributes without increasing the public key and ciphertext size. Our scheme's master secret key has only one matrix despite of the number of the system's attributes. The security of our schemes is based on the worstcase hardness on lattices. Namely, under the learning with errors (LWE) assumption, our CP-ABE schemes are secure against chosen plaintext attack (CPA) in the selective model.

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There exists a demerit in normal attribute based signature, whose length of the signature depends on the largest size of attributes set. In this paper, an attribute-based signature scheme with constant size is proposed. It is proved to be unforgeable and unconditionally anonymous in the standard model. The security of the scheme is based on Computational Diffie Hellman(CDH) problem.

Decentralized attribute-based encryption (ABE) is a special form of multiauthority ABE systems, in which no central authority and global coordination are required other than creating the common reference parameters. In this paper, we propose a new decentralized ABE in prime-order groups by using extended dual system groups. We formulate some assumptions used to prove the security of our scheme. Our proposed scheme is fully secure under the standard -Lin assumption in random oracle model and can support any monotone access structures. Compared with existing fully secure decentralized ABE systems, our construction has shorter ciphertexts and secret keys. Moreover, fast decryption is achieved in our system, in which ciphertexts can be decrypted with a constant number of pairings.

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