Excellent covertness is claimed.
They give examples of pictures of an aircraft, an airfield, and text, embedded in each other by replacing the four least significant bits of each pixel of the cover picture by the four most significant bits of the embedded picture. The loaded pictures cannot be distinguished by eye, and the quality of the recovered hidden pictures is acceptable for intelligence purposes. The author describes a system he developed to enable US military personnel review classified information for downgrading. It was inspired by emacs, but targeted at a Honeywell A1 machine and formally verified. This paper pointed out the existence of covert channels.