A novel public key encryption scheme with multiple private keys is presented.The security of this encryption method depends on the difficulty of the decoding problem for block codes over finite fields.Based on the encryption method,a pirate tracing scheme is proposed for the copyright protection of digital data.In this tracing scheme,for each legal copy of digital data there is a codeword buried in the corresponding decryption software or box.The scheme can extract the codeword efficiently,and uncover all authorized users involved in making the illegal copy by using a proper decoding algorithm if the number of such users is not more than the error-correcting capacity of a given code.Compared with known tracing schemes,this scheme is efficient in performance,and easy in implementation.Potential applicable areas of the encryption method include the protection of copyrights of various forms of digital data such as computer software and audio/video products.