Basic Concept of Multilevel Database Security - Multilevel Security for Relational Databases - page 22

In visible polyinstantiation, the multilevel relational database security
has three choices:

• Notifying the user that the new tuple exists at a lower secu-
rity level and therefore the insertion of the new tuple will
be rejected: This choice leads to the denial of service prob-
lem because the new tuple that needs to be inserted by the
user with high security is rejected by a user in the low level of
security.
• Replacing the existing tuple at a high security level with the
new tuple being inserted with a low security level: This choice
leads to the covert channel problem because the user with a
low security level gets unauthorized information at a high
level of security.
• Inserting the new tuple with a low security level without
modifying the existing tuple at the high security level: This
choice leads to the polyinstantiation of the tuple because there
are two tuples in the relation with the same primary key but
in different security levels.

2.3.3 Types of Polyinstantiation

There are two different types of polyinstantiation [21]:

• Entity polyinstantiation: In the multilevel relational database,
the entity polyinstantiation can occur when a relation con-
tains more than one tuple with the same primary key val-
ues, but with different access class values for the primary key.
As shown in Table 2.7, there are two tuples with the same
primary key (Ahmed) but with two different classes.
• Attribute polyinstantiation: In the multilevel relational data-
base, the attribute polyinstantiation can occur when a relation
contains two or more tuples with an identical primary key
and its security level values, but with different values for one

Table 2.7 Entity Polyinstantiation

EMPLOYEE DEPARTMENT SALARY

Ahmed U Accounting U 7,000 U

Ahmed S Sales S 10,000 S