Access Control list

Lists of Questions Answers and Short Study Notes

  • (1) What is Access Control list? Explain in brief..


Question-1 What is Access Control list? Explain in brief.

  • Most of domains have no access at all to most objects, so storing a very large, mostly empty, Access matrix is a waste of disk space.
  • Two methods are practical; the first is storing the matrix by rows and the second method is storing the matrix by columns, and then storing only the nonempty elements.
  • The two approaches are surprisingly different.
  • The first technique consists of associating with each object an (ordered) list containing all the domains that may access the object, and how.
  • This list is called the Access Control List or ACL and is illustrated in Fig below.
  • Here we see three processes A, b, and C, each belonging to a different domain. There are three files F1, F2, and F3.
  • For simplicity, we will assume that each domain corresponds to exactly one user, in this case, users are A, B, and C.
  • Often in the security literature, the users are called subjects or principals, to contrast them with the things owned, the objects, such us files.
  • Each file has an ACL associated with it. File F1 has two entries in its ACL (separated by a semicolon).
  • The first entry says that any process owned by user A may read and write the file. The second entry says that any process owned by user B may read the file.
  • All other accesses by these users and all accesses by other users are forbidden.
  • Note that the rights are granted by user, not by process.
  • As far as the protection system goes, any process owned by user A can read and write file F1. It does not matter if there is one such process or 100 of them. It is the owner,not the process ID that matters.
  • File F2 has three entries in its ACL: A, B, and C can all read the file, and in addition B can also write it.
  • No other accesses are allowed, File F3 is apparently an executable program, since B and C can both read and execute it. B can also write it.
  • Many systems support the concept of a group of users. Groups have names and can be included in ACLs.

access control list-1

  • The other way of slicing up the matrix of Fig above is by rows.
  • When this method is used, associated with each process is a list of objects that may be accessed, along with an indication of which operations are permitted on each, in other words, its domain.
  • This list is called a capability list or C-list and the individual items on it are called capabilities.