NCSC-TG-030: A Guide to Understanding Covert Channel Analysis of Trusted Systems, November 1993. (Light Pink Book)
You can download it from the following link: https://packetstormsecurity.com/files/download/14022/NCSC-TG-030.txt
Source: https://packetstormsecurity.com/files/14022/NCSC-TG-030.txt.html