关键词: 隐密术, 载体选择, 安全性, 桥牌, 替换密码

Abstract: Due to the lack of the integrated knowledge of covertext，the security and capacity of steganography method using multimedia information can’t be observably improved.According to the difference of function between digital watermarking and steganography and the particularity of security and capacity in steganography，the character of covertext is proposed.Using bridge as covertext and combined with substitution cipher，a secure steganography method is presented.

Key words: steganography, selection of covertext, security, bridge, substitution cipher