Abstract: According to the MOV reduction theory, the identity-based encryption scheme which is based on the bilinear map will lose the high efficiency of elliptic curve. For this reason, a provably secure identity-based encryption scheme without the bilinear map is proposed, which is based on combined public-key scheme. Furthermore, by applying the pair public-key technology introduced by Katz and Wang, the security proof of the proposed identity-based encryption scheme has “tight” reduction in the random oracle model. For showing the good efficiency of the proposed identity-based encryption scheme, the degree of reduction in the security proof and the performance of time and space complexity are analyzed, and these terms are compared with other identity-based encryption schemes without the bilinear map. Finally, for overcoming the conspiracy attack of combined public-key scheme, the number of users in the proposed identity-based encryption scheme is confined, thus leading to the result that the proposed identity-based encryption scheme is inefficient when the number of user is too large. So, for keeping the efficiency of the proposed identity-based encryption scheme in the system having a lot of users, multi-field model in the Kerberos protocol is consulted with, and then a basic multi-field model based on the proposed identity-based encryption scheme is proposed.