The Bell-LaPadula Model
is a state machine
model used for enforcing access control
in government and military applications. It was developed by David Elliott Bell and Len LaPadula, subsequent to strong guidance from Roger R. Schell
, Ph.D. 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 (e.g."Top Secret", down to the least sensitive, "Unclassified" or "Public")
The Bell-LaPadula model is an example of a model where there is no clear distinction of protection and security.
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 is 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 - 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) - a subject at a given security level must not write to any object at a lower security level (no write-down). The *-property is also known as the Confinement property.
- The Discretionary Security Property - use of 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 (i.e. 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 (i.e. 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 did not 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 is not 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.
The tranquility principle of the Bell-LaPadula model states that the classification of a subject or object does not change while it is being referenced. There are two forms to the tranquility principle: the "principle of strong tranquility" states that security levels do not change during the normal operation of the system and the "principle of weak tranquility" states that security levels do not 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 will not 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 does not violate the security policy during an operation.
- 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 is denied access.
- Sometimes, it is not sufficient to hide only the contents of objects. Their existence may have to be hidden, as well.
- Bishop, Matt (2003). Computer Security: Art and Science. Boston: Addison Wesley.
- Krutz, Ronald L.; Russell Dean Vines (2003). The CISSP Prep Guide. Gold Edition, Indianapolis, Indiana: Wiley Publishing.