Everything about The Bell-lapadula Model totally explained
The
Bell-LaPadula Model was developed by David Elliott Bell and Len LaPadula—subsequent to strong guidance from then-CAPT Roger R. Schell, Ph.D. (USAR, Ret.)—in 1973
to formalize the
U.S. Department of Defense (DoD)
multilevel security (MLS) policy.
The model is a formal
state transition model of
computer security policy that describes a set of
access control rules which use security labels on objects and clearances for subjects. Security labels range from the most sensitive,
for example, "Top Secret", down to the least sensitive,
for example, "Unclassified" or "Public."
The Bell-LaPadula model is an example of a model where there's no clear
distinction of protection and security.
Features
The Bell-LaPadula model focuses on data
confidentiality and access to
classified information, in contrast to the
Biba Integrity Model which describes rules for the protection of
data integrity.
In this formal model, the entities in an
information system are divided into subjects and objects. The notion of a "
secure state" is defined, and it's proven that each state transition preserves security by moving from secure state to secure state, thereby
inductively proving that the system satisfies the security objectives of the model. The Bell-LaPadula model is built on the concept of a
state machine with a set of allowable states in a system. The transition from one state to another state is defined by
transition functions.
A system state is defined to be "secure" if the only permitted access modes of subjects to objects are in accordance with a
security policy. To determine whether a specific access mode is allowed, the clearance of a subject is compared to the classification of the object (more precisely, to the combination of classification and set of compartments, making up the
security level) to determine if the subject is authorized for the specific access mode. The clearance/classification scheme is expressed in terms of a
lattice. The model defines two
mandatory access control (MAC) rules and one
discretionary access control (DAC) rule with three security properties:
- The Simple Security Property states that a subject at a given security level may not read an object at a higher security level (no read-up).
- The *-property (read star-property) states that a subject at a given security level must not write to any object at a lower security level (no write-down).
- The Discretionary Security Property uses an access matrix to specify the discretionary access control.
The transfer of information from a high-sensitivity paragraph to a lower-sensitivity document may happen in the Bell-LaPadula model via the concept of
trusted subjects. Trusted Subjects are not restricted by the *-property. Untrusted subjects are. Trusted Subjects must be shown to be trustworthy with regard to the security policy.
This security model is directed toward access control and is characterized by the phrase:
no read up, no write down. Compare the Biba model, the
Clark-Wilson model and the
Chinese Wall.
With Bell-LaPadula, users can
create content only at or above their own security level (
Secret researchers can create
Secret or
Top-Secret files but may not create
Public files):
no write-down. Conversely, users can
view content only at or below their own security level (
Secret researchers can view
Public or
Secret files, but may not view
Top-Secret files):
no read-up.
The Bell-LaPadula model explicitly defined its scope. It didn't treat the following extensively:
Covert channels. Passing information via pre-arranged actions was described briefly.
Networks of systems. Later modeling work did address this topic.
Policies outside multilevel security. Work in the early 1990s showed that MLS is one version of boolean policies, as are all other published policies.
Strong * Property
The Strong * Property is an alternative to the *-property in which subjects may write to objects with only a matching security level. Thus, the write up operation permitted in the usual *-property isn't present, only a write to same operation. The Strong * Property is usually discussed in the context of multilevel database management systems and is motivated by integrity concerns.
This Strong * Property was anticipated in the Biba model where it was shown that strong integrity in combination with the Bell-La Padula model resulted in reading and writing at a single level.
Tranquility principle
The tranquility principle of the Bell-LaPadula model states that the classification of a subject or object doesn't change while it's being referenced.
There are two forms to the tranquility principle:
1) The "principle of strong tranquility" states that security levels don't change during the normal operation of the system
2) The "principle of weak tranquility" states that security levels don't change in a way that violates the rules of a given security policy.
Another interpretation of the tranquility principles is that they both apply only to the period of time during which an operation involving an object or subject is occurring. That is, the strong tranquility principle means that an object's security level/label won't change during an operation (such as read or write); the weak tranquility principle means that an object's security level/label may change in a way that doesn't violate the security policy during an operation.
Limitations
Restricted to Confidentiality.
No policies for changing access rights; a complete general downgrade is secure; intended for systems with static security levels.
Contains covert channels: a low subject can detect the existence of high objects when it's denied access.
Sometimes, it isn't sufficient to hide only the contents of objects. Their existence may have to be hidden, as well.Further Information
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