科研之友
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摘要
As Hierarchical Identity Based Encryption (HIBE) system usually maps the true institutional structure of an organization or entity relationship between objects in real world, It is important that computation &communication complexity of private key, ciphertext, cryptographic computations and so on related to an entity in the hierarchy is independent to the hierarchy depth of the entity. Moreover, key escrow problem that any non-leaf entity in a hierarchical identity based cryptosystem can derive private keys for its descendants with use of its private key should be resolved, in order to prevent any entity from behaving on behalf of its descendants. In this paper, a new technique is introduced for composing a private key for each individual entity in HIBE system by differentiating between non-local identifiers and local identifiers of the identity of the entity. That we call Identifier Discrimination. With the technique, A selective identity secure HIBE system is constructed under Decisional Bilinear Diffie-Hellman (DBDH) assumption without using random oracles, where the private key and the ciphertext consist of constant number of group elements, and decryption requires only three bilinear map computations, regardless of the identity hierarchy depth. Moreover, in contrast to previous HIBE constructions, where private key for an entity can be derived by its ancestors with direct use of their private keys, key escrow problem inherent in identity based cryptosystems is resolved in our HIBE construction. Privilege of deriving private keys for an entity can be delegated to any of its ancestors through authorization by distributing specifically crafted values to the ancestor in our HIBE system, that we call Authorized Delegation.
