Master key updatable attribute-based encryption on the dual pairing vector space

In this paper, we focus on a "master key update" method for attribute-based encryption (ABE) schemes. In ABE system, there is a master key which is a pair of a public key and a master secret key in order to generate ciphertexts and decryption keys. The update of the master key (the public key and the master secret key) is necessary as similar as normal public key encryption. Since all ciphertexts are constructed by using the master key, all ciphertexts should be updated to new ciphertexts on a new master key when the master key is updated. In naive master key update method, in this situation, all ciphertexts must be decrypted and plaintexts are encrypted by using the new public key which is generated from the new master secret key. This naive method is called dec-then-enc method in this paper. When the master key is updated, all ciphertext must be decrypted if dec-then-enc method is adopted. Hence, if all ciphertexts are stored an external storage (e.g., cloud storages), a user should download all ciphertexts and executes dec-then-enc method for all ciphertexts. In order to overcome this problem, we introduce a new method that ciphertexts can be updated without decryption. We propose the ciphertext-policy ABE scheme with mater key update property based on the technique of the access structures of Okamoto-Takashima (CRYPTO 2010). The security is proven under Decisional Linear (DLIN) in the standard model as same as Okamoto-Takashima CP-ABE scheme.