Abstract: By making use of user's identity as his public key, identity based cryptosystems have many advantages over traditional PKI based cryptosystems. But identity based cryptosystems also have an inherent drawback of key escrow that the key generation center knows all private keys of users. To improve the security of keys in identity based encryp- tion, how to avoid key escrow problem in identity based encryption becomes a hot issue. A secure key issuing scheme for I3oneh-I3oyenl identity based encryption was proposed, in which multiple key privacy authorities are set in addition to the key generation center to protect the privacy of users' private keys. A rigorous security proof in standard model of our secure key issuing protocol was also given. Thus identity based encryption is more usable in practice.

Key words: Identity based cryptography, Key escrow, Secure key issuing, Key generation center, Bilinear pairings