Analysis and Improvements of Several (H)IBS/IBSC/PRBE Schemes

Constructing efficient (Hierarchical) identity based signature/signcryption ((H)IBS/IBSC) schemes in the standard model with full security remain as open problems for a long time. Ren et al. constructed efficient (H)IBS/IBSC schemes with full security without random oracle in ISDPE’07 and Chinacrypt’08, . They claimed their schemes can be proved to simultaneously achieve high efficiency, short public parameters and a tight reduction. But we shall show their schemes are not secure. Furthermore, we give improvements to these schemes which can resist the proposed attack. Proxy re-encryption is a primitive which allows the transformation from A’s ciphertext to be B’s ciphertext by using proxies, without the proxy knowing the corresponding plaintexts or secret keys of A or B. Proxy broadcast re-encryption aims at transforming ciphertext from one user to a group, which is a generalization of proxy re-encryption. Recently, Sun et al. proposed a CCA-secure unidirectional proxy broadcast re-encryption in the standard model, we also show their scheme has some flaws.