Establishing Databases and Data Warehousing
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Context-dependent access control is often discussed alongside content-dependent access
control because of the similarity of the terms. Context-dependent access control evaluates
the big picture to make access control decisions. The key factor in context-dependent access
control is how each object or packet or field relates to the overall activity or communica-
tion. Any single element may look innocuous by itself, but in a larger context that element
may be revealed to be benign or malign.
Administrators might employ database partitioning to subvert aggregation and infer-
ence vulnerabilities, which are discussed in the section “Aggregation” later in this chapter.
Database partitioning is the process of splitting a single database into multiple parts, each
with a unique and distinct security level or type of content.
Polyinstantiation
, in the context of databases, occurs when two or more rows in the
same relational database table appear to have identical primary key elements but contain
different data for use at differing classification levels. It is often used as a defense against
some types of inference attacks (see “Inference,” which was covered in Chapter 9).
Consider a database table containing the location of various naval ships on patrol.
Normally, this database contains the exact position of each ship stored at the secret clas-
sification level. However, one particular ship, the USS
UpToNoGood
, is on an undercover
mission to a top-secret location. Military commanders do not want anyone to know that
the ship deviated from its normal patrol. If the database administrators simply change the
classification of the
UpToNoGood
’s location to top secret, a user with a secret clearance
would know that something unusual was going on when they couldn’t query the location of
the ship. However, if polyinstantiation is used, two records could be inserted into the table.
The first one, classified at the top-secret level, would reflect the true location of the ship and
be available only to users with the appropriate top-secret security clearance. The second
record, classified at the secret level, would indicate that the ship was on routine patrol and
would be returned to users with a secret clearance.
Finally, administrators can insert false or misleading data into a DBMS in order to redi-
rect or thwart information confidentiality attacks. This is a concept known as noise and
perturbation. You must be extremely careful when using this technique to ensure that noise
inserted into the database does not affect business operations.
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