NCSC-TG-020-A: Trusted UNIX Working Group (TRUSIX) Rationale for Selecting Access Control List Features for the UNIX® System, 7 July 1989. (Silver Book)
You can download it from the following link: https://packetstormsecurity.com/files/download/14010/NCSC-TG-020-A.html
Source: https://packetstormsecurity.com/files/14010/NCSC-TG-020-A.html.html