  This paper focuses on several theoretical issues and principles in
steganography security, and defines four security levels by analyzing the
corresponding algorithm instances. In the theoretical analysis, we discuss the
differences between steganography security and watermarking security. The two
necessary conditions for the steganography security are obtained. Under the
current technology situation, we then analyze the indistinguishability of the
cover and stego-cover, and consider that the steganography security should rely
on the key secrecy with algorithms open. By specifying the role of key in
steganography, the necessary conditions for a secure steganography algorithm in
theory are formally presented. When analyzing the security instances, we have
classified the steganalysis attacks according to their variable access to the
steganography system, and then defined the four security levels. The higher
level security one has, the higher level attacks one can resist. We have also
presented algorithm instances based on current technical conditions, and
analyzed their data hiding process, security level, and practice requirements.
