Encryption-Based Multilevel Model for DBMS - Multilevel Security for Relational Databases - page 97

• If there are no attributes of the primary key in the SET clause,
for attributes in the SET clause:
• Encrypt the attribute value and update the tuple.
• If there is a tuple that has an attribute that depends on the
attribute in the updated tuple, the value of this attribute will
be encrypted and updated.
• If there are attributes of the primary key in the SET clause:
• Encrypt the attribute value and update the tuple.
• If the primary key class is equal to the class of the subject
that executes the UPDATE statement, all tuples that have
the same primary key will be deleted.
Figure 5.4 illustrates the flow chart for the update operation in the
encryption-based multilevel database model.
5.3.5 he UPLEVEL Statement
The UPLEVEL statement executed by a user with security class level
L has the following general form:
UPLEVEL R GET [A 1 ,A 2 ,...,A n ] FROM [C 1 ,C 2 ,...,C n ]
WHERE P
where R is the relation name; A 1 ,A 2 ,...,A n are data attribute
names; C 1 ,C 2 ,...,C n are values of classification levels for
A 1 ,A 2 ,...,A n , respectively; and P is a predicate expression that
may include conditions involving the classification attributes and
tuple-class attributes, in addition to the usual data attributes. Only
tuples t ∈ r with t [ TC ] ≤ L will be decrypted by key according to
the classification level of the tuple Key [ TC ] and will be taken into
the calculation of P.
For decrypted tuples that have at least one tuple t ′ ∈ r that satisfies
the predicate P, an L-tuple, t, is constructed as follows:
• Create a temporary tuple for decrypted data to store the deleted
tuple during the execution of the UPLEVEL statement.
• If A i is in the GET clause, encrypt the data in the tuple with
class equal to the security class level in the FROM clause.
• If A i is not in the GET clause, set the data value to null.