NCSC-TG-021: Trusted Database Management System Interpretation of the TCSEC (TDI), April 1991. (Purple Book)
You can download it from the following link: https://packetstormsecurity.com/files/download/14011/NCSC-TG-021.txt
Source: https://packetstormsecurity.com/files/14011/NCSC-TG-021.txt.html