Implementation of MLS/DBMS Models - Multilevel Security for Relational Databases - page 51

Get user class L(user)

Set i = 1

t i [PK] ∈ [A i ]*

No Ye s

Update t i

i = i + 1

t i [TC] ≥ L(user)

Ye s No

Update t´ i

Delete ti i

End

Figure 3.11 Flow chart for update operation in an MLR model.

where R is an MLS relation and P is a delete condition that defines the
tuples that are to be deleted. If a subject with the security class level L
runs a command to delete tuples from MLS relation R, for all t ∈ R,
if t satisfies P, the delete operation is implemented as follows [33]:

• Step 1: get the security level of the subject that runs the delete
operation.
• Step 2: delete all tuples from multilevel relations that satisfy
delete condition P in the delete statement and have a security
level equal to the security level of the user.
• Step 3: if there is a tuple that satisfies the delete condition P
in the delete statement and is borrowed by a high-level user,
the high-level user will be notified by exceptions or warnings
to delete this borrowed tuple.

Figure 3.12 illustrates the flow chart for the delete operation in the
MLR model.