Until now, we have examined approaches to protecting a general object, no matter the object's nature or type. But some protection schemes are particular to the type. To see how they work, we focus in this section on file protection. The examples we present are only representative; they do not cover all possible means of file protection on the market.
Basic Forms of Protection
We noted earlier that all multiuser operating systems must provide some minimal protection to keep one user from maliciously or inadvertently accessing or modifying the files of another. As the number of users has grown, so also has the complexity of these protection schemes.

  1. All “None Protection: In the original IBM OS operating systems, files were by default public. Any user could read, modify, or delete a file belonging to any other user. Instead of software- or hardware-based protection, the principal protection involved trust combined with ignorance. System designers supposed that users could be trusted not to read or modify others' files, because the users would expect the same respect from others. Ignorance helped this situation, because a user could access a file only by name ; presumably users knew the names only of those files to which they had legitimate access. However, it was acknowledged that certain system files were sensitive and that the system administrator could protect them with a password. A normal user could exercise this feature, but passwords were viewed as most valuable for protecting operating system files. Two philosophies guided password use. Sometimes, passwords were used to control all accesses (read, write, or delete), giving the system administrator complete control over all files. But at other times passwords would control only write and delete accesses , because only these two actions affected other users. In either case, the password mechanism required a system operator's intervention each time access to the filebegan. However, this all-or-none protection is unacceptable for several reasons like Lack of trust, All or nothing, Rise of timesharing, Complexity, File listings.
  2. Group Protection: Because the all-or-nothing approach has so many drawbacks, researchers sought an improved way to protect files. They focused on identifying groups of users who had some common relationship. In a typical implementation, the world is divided into three classes: the user, a trusted working group associated with the user, and the rest of the users. For simplicity we can call these classes user, group, and world . This form of protection is used on some network systems and the Unix system. All authorized users are separated into groups. A group may consist of several members working on a common project, a department, a class, or a single user. The basis for group membership is need to share . The group members have some common interest and therefore are assumed to have files to share with the other group members. In this approach, no user belongs to more than one group. (Otherwise, a member belonging to groups A and B could pass along an A file to another B group member.) When creating a file, a user defines access rights to the file for the user, for other members of the same group, and for all other users in general. Typically, the choices for access rights are a limited set, such as {read, write, execute, delete}. For a particular file, a user might declare read-only access to the general world, read and write access to the group, and all rights to the user. This approach would be suitable for a paper being developed by a group, whereby the different members of the group might modify sections being written within the group. The paper itself should be available for people outside the group to review but not change. A key advantage of the group protection approach is its ease of implementation. A user is recognized by two identifiers (usually numbers ): a user ID and a group ID. These identifiers are stored in the file directory entry for each file and are obtained by the operating system when a user logs in. Therefore, the operating system can easily check whether a proposed access to a file is requested from someone whose group ID matches the group ID for the file to be accessed. Although this protection scheme overcomes some of the shortcomings of the all-or-nothing scheme, it introduces some new difficulties of its own like Group affiliation, Multiple personalities, All groups issue.

In spite of their drawbacks, the file protection schemes we have described are relatively simple and straightforward. The simplicity of implementing them suggests other easy-to-manage methods that provide finer degrees of security while associating permission with a single file including Password or Other Token, Temporary Acquired Permission


  1. P. Pfleeger, Shari Lawrence Pfleeger Charles: Security in Computing, PHI
  2. Notes: Veer Surendra Sai University of Technology (VSSUT)