Privacy protection technologies of automated trust negotiation have witnessed great development, among which hidden credentials is an admittedly more satisfactory one. However, for hidden credentials, there exists fatal weakness of blindly decryption, which results decryption execution costly. To solve the problem, using bilinear pairings over elliptic curves, a policy-based encryption for privacy protection is proposed. Under the assumption of BDH problem, the scheme is proven to be IND-Pol-CCA safety in the random prediction model. Compared with hidden credentials and other encryption solutions based on policy, this scheme is more efficient, secure and of shorter length of cipher text.