Partially wildcarded attribute-based encryption and its efficient construction

Many kinds of ciphertext-policy attribute-based encryption (CP-ABE) schemes have been proposed. In CP-ABE, the set of user attributes is associated with his/her secret key whereas a policy is associated with a ciphertext so that only users whose attributes satisfy the policy can decrypt the ciphertext. CP-ABE may be applied to a variety of services such as access control for file sharing systems and content distribution services. However, CP-ABE costs more for encryption and decryption in comparison with conventional public key encryption schemes since it can handle more flexible policies. In particular, wildcards, which mean that certain attributes are not relevant to the ciphertext policy, are not essential for a certain service. In this paper, we construct a partially wildcarded CP-ABE scheme with a lower decryption cost. In our scheme, the user's attributes are separated into those requiring wildcards and those not requiring wildcards. Our scheme hence embodies a CP-ABE scheme with a wildcard functionality and an efficient CP-ABE scheme without wildcard functionality. We compare our scheme with the conventional CP-ABE schemes and describe a content distribution service as an application of our scheme.