摘要：Recently Zhang et al. proposed a hierarchical identity-based encryption scheme which is the first efficient scheme where both ciphertexts and private keys achieve O(1)size, and is the best trade-off between private key size and ciphertext size at present. However, in this paper, it will be pointed out that their scheme exists an ambiguity or shortcoming which makes their scheme be insecure or non identity-based. Then, in order to overcome this problem, an improved hierarchical identity based encryption scheme is proposed with the same efficiency with Zhang et al.’s scheme (the private keys and ciphertexts of O(1)size). And, the security proof of the improved scheme also is given in the selective-identity model.
