Discretionary Access Control Based on Granting and Revoking Privileges
The typical method of enforcing discretionary access control in a database system is based on the granting and revoking of privileges. Let us consider privileges in the context of a relational DBMS. In particular, we will discuss a system of privileges somewhat similar to the one originally developed for the SQL language (see Chapters 4 and 5). Many current relational DBMSs use some variation of this tech-nique. The main idea is to include statements in the query language that allow the DBA and selected users to grant and revoke privileges.
1. Types of Discretionary Privileges
In SQL2 and later versions, the concept of an authorization identifier is used to refer, roughly speaking, to a user account (or group of user accounts). For simplicity, we will use the words user or account interchangeably in place of authorization identifier. The DBMS must provide selective access to each relation in the database based on specific accounts. Operations may also be controlled; thus, having an account does not necessarily entitle the account holder to all the functionality provided by the DBMS. Informally, there are two levels for assigning privileges to use the database system:
The account level. At this level, the DBA specifies the particular privileges that each account holds independently of the relations in the database.
The relation (or table) level. At this level, the DBA can control the privilege to access each individual relation or view in the database.
References privilege on R. This gives the account the capability to reference (or refer to) a relation R when specifying integrity constraints. This privilege can also be restricted to specific attributes of R.
Notice that to create a view, the account must have the SELECT privilege on all relations involved in the view definition in order to specify the query that corresponds to the view.
2. Specifying Privileges through the Use of Views
The mechanism of views is an important discretionary authorization mechanism in its own right. For example, if the owner A of a relation R wants another account B to be able to retrieve only some fields of R, then A can create a view V of R that includes only those attributes and then grant SELECT on V to B. The same applies to limiting B to retrieving only certain tuples of R; a view V can be created by defining the view by means of a query that selects only those tuples from R that A wants to allow B to access. We will illustrate this discussion with the example given in Section 24.2.5.
3. Revoking of Privileges
In some cases it is desirable to grant a privilege to a user temporarily. For example, the owner of a relation may want to grant the SELECT privilege to a user for a specific task and then revoke that privilege once the task is completed. Hence, a mechanism for revoking privileges is needed. In SQL a REVOKE command is included for the purpose of canceling privileges. We will see how the REVOKE command is used in the example in Section 24.2.5.
4. Propagation of Privileges Using the GRANT OPTION
Whenever the owner A of a relation R grants a privilege on R to another account B, the privilege can be given to B with or without the GRANT OPTION. If the GRANT OPTION is given, this means that B can also grant that privilege on R to other accounts. Suppose that B is given the GRANT OPTION by A and that B then grants the privilege on R to a third account C, also with the GRANT OPTION. In this way, privileges on R can propagate to other accounts without the knowledge of the owner of R. If the owner account A now revokes the privilege granted to B, all the privileges that B propagated based on that privilege should automatically be revoked by the system.
It is possible for a user to receive a certain privilege from two or more sources. For example, A4 may receive a certain UPDATE R privilege from both A2 and A3. In such a case, if A2 revokes this privilege from A4, A4 will still continue to have the privilege by virtue of having been granted it from A3. If A3 later revokes the privilege from A4, A4 totally loses the privilege. Hence, a DBMS that allows propagation of privi-leges must keep track of how all the privileges were granted so that revoking of priv-ileges can be done correctly and completely.
5. An Example to Illustrate Granting and Revoking of Privileges
Suppose that the DBA creates four accounts—A1, A2, A3, and A4—and wants only A1 to be able to create base relations. To do this, the DBA must issue the following GRANT command in SQL:
GRANT CREATETAB TO A1;
The CREATETAB (create table) privilege gives account A1 the capability to create new database tables (base relations) and is hence an account privilege. This privilege was part of earlier versions of SQL but is now left to each individual system imple-mentation to define.
In SQL2 the same effect can be accomplished by having the DBA issue a CREATE SCHEMA command, as follows:
CREATE SCHEMA EXAMPLE AUTHORIZATION A1;
User account A1 can now create tables under the schema called EXAMPLE. To con-tinue our example, suppose that A1 creates the two base relations EMPLOYEE and DEPARTMENT shown in Figure 24.1; A1 is then the owner of these two relations and hence has all the relation privileges on each of them.
Next, suppose that account A1 wants to grant to account A2 the privilege to insert and delete tuples in both of these relations. However, A1 does not want A2 to be able to propagate these privileges to additional accounts. A1 can issue the following com-mand:
GRANT INSERT, DELETE ON EMPLOYEE, DEPARTMENT TO A2;
Notice that the owner account A1 of a relation automatically has the GRANT OPTION, allowing it to grant privileges on the relation to other accounts. However, account A2 cannot grant INSERT and DELETE privileges on the EMPLOYEE and DEPARTMENT tables because A2 was not given the GRANT OPTION in the preceding command.
Next, suppose that A1 wants to allow account A3 to retrieve information from either of the two tables and also to be able to propagate the SELECT privilege to other accounts. A1 can issue the following command:
GRANT SELECT ON EMPLOYEE, DEPARTMENT TO A3 WITH GRANT OPTION;
The clause WITH GRANT OPTION means that A3 can now propagate the privilege to other accounts by using GRANT. For example, A3 can grant the SELECT privilege on the EMPLOYEE relation to A4 by issuing the following command:
GRANT SELECT ON EMPLOYEE TO A4;
Notice that A4 cannot propagate the SELECT privilege to other accounts because the GRANT OPTION was not given to A4.
Now suppose that A1 decides to revoke the SELECT privilege on the EMPLOYEE relation from A3; A1 then can issue this command:
REVOKE SELECT ON EMPLOYEE FROM A3;
The DBMS must now revoke the SELECT privilege on EMPLOYEE from A3, and it must also automatically revoke the SELECT privilege on EMPLOYEE from A4. This is because A3 granted that privilege to A4, but A3 does not have the privilege any more.
Next, suppose that A1 wants to give back to A3 a limited capability to SELECT from the EMPLOYEE relation and wants to allow A3 to be able to propagate the privilege. The limitation is to retrieve only the Name, Bdate, and Address attributes and only for the tuples with Dno = 5. A1 then can create the following view:
CREATE VIEW A3EMPLOYEE AS
SELECT Name, Bdate, Address
WHERE Dno = 5;
After the view is created, A1 can grant SELECT on the view A3EMPLOYEE to A3 as follows:
GRANT SELECT ON A3EMPLOYEE TO A3 WITH GRANT OPTION;
Finally, suppose that A1 wants to allow A4 to update only the Salary attribute of EMPLOYEE; A1 can then issue the following command:
GRANT UPDATE ON EMPLOYEE (Salary) TO A4;
The UPDATE and INSERT privileges can specify particular attributes that may be updated or inserted in a relation. Other privileges (SELECT, DELETE) are not attrib-ute specific, because this specificity can easily be controlled by creating the appro-priate views that include only the desired attributes and granting the corresponding privileges on the views. However, because updating views is not always possible (see Chapter 5), the UPDATE and INSERT privileges are given the option to specify the particular attributes of a base relation that may be updated.
6. Specifying Limits on Propagation of Privileges
Techniques to limit the propagation of privileges have been developed, although they have not yet been implemented in most DBMSs and are not a part of SQL. Limiting horizontal propagation to an integer number i means that an account B given the GRANT OPTION can grant the privilege to at most i other accounts.
Vertical propagation is more complicated; it limits the depth of the granting of privileges. Granting a privilege with a vertical propagation of zero is equivalent to granting the privilege with no GRANT OPTION. If account A grants a privilege to account B with the vertical propagation set to an integer number j > 0, this means that the account B has the GRANT OPTION on that privilege, but B can grant the privilege to other accounts only with a vertical propagation less than j. In effect, vertical propagation limits the sequence of GRANT OPTIONS that can be given from one account to the next based on a single original grant of the privilege.
We briefly illustrate horizontal and vertical propagation limits—which are not available currently in SQL or other relational systems—with an example. Suppose that A1 grants SELECT to A2 on the EMPLOYEE relation with horizontal propagation equal to 1 and vertical propagation equal to 2. A2 can then grant SELECT to at most one account because the horizontal propagation limitation is set to 1. Additionally, A2 cannot grant the privilege to another account except with vertical propagation set to 0 (no GRANT OPTION) or 1; this is because A2 must reduce the vertical propagation by at least 1 when passing the privilege to others. In addition, the horizontal propagation must be less than or equal to the originally granted hor-izontal propagation. For example, if account A grants a privilege to account B with the horizontal propagation set to an integer number j > 0, this means that B can grant the privilege to other accounts only with a horizontal propagation less than or equal to j. As this example shows, horizontal and vertical propagation techniques are designed to limit the depth and breadth of propagation of privileges.
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