Watermarking, as an effective method for information hidding, has evolved very quickly. Most watermarking systems available have only a secret key, which cannot be public. But in some applications, watermarking needs to be retrieved by public keys. How to generate public keys without weaken the performance of the private key is a key problem. In this paper, a secret key watermarking system is designed, in which a novel method of generating public keys is proposed. The identifier (ID) embedded can be reliably retrieved using public keys without resorting to the orginal data. Because only part of embedding information is used in public keys, the above problem is successfully solved. Expeimental results show its security and validity.