Attribute Based Encryption Schemes for Efficient Verifiable Data Sharing

Attribute-based encryption (ABE) with outsourced decryption not only enables finegrained sharing of encrypted data, but also overcomes the effective drawback of the standard ABE schemes. An Attribute based encryption scheme with outsourced decryption allows a third party (e.g., a cloud server) to renovate an ABE cipher text into a (short) El Gamal-type cipher text using a public transformation key provided by a user so that the last can be decrypted much more efficiently than the former by the user. However, a shortcoming of the original outsourced ABE scheme is that the correctness of the cloud server’s transformation cannot be verified by the user. That is, an end user could be cheated into accepting a wrong or maliciously transformed output. A security model of ABE with verifiable outsourced decryption is formalized by introducing a verification key in the output of the encryption algorithm. An approach to convert any ABE scheme with outsourced decryption into an ABE scheme with verifiable outsourced decryption is presented. The new approach is simple, general, and almost optimal. Compared with the original outsourced ABE, verifiable outsourced ABE neither increases the users and the cloud server’s computation costs except some non-dominant operations (e.g., hash computations), nor expands the cipher text size except adding a hash value (which is <20 byte for 80-bit security level).