The Bell–LaPadula Model (BLP) is a

state machine A finite-state machine (FSM) or finite-state automaton (FSA, plural: ''automata''), finite automaton, or simply a state machine, is a mathematical model of computation. It is an abstract machine that can be in exactly one of a finite number o ...

model used for enforcing

access control In the fields of physical security and information security, access control (AC) is the selective restriction of access to a place or other resource, while access management describes the process. The act of ''accessing'' may mean consuming ...

in government and military applications. It was developed by David Elliott Bell and Leonard J. LaPadula, subsequent to strong guidance from

Roger R. Schell Roger R. Schell is president of ÆSec, a company focused on appliances built on hardened platforms for secure, reliable e-business on the Internet. For several years he managed the development and delivery of security for several Novell releases o ...

, to formalize the

U.S. Department of Defense The United States Department of Defense (DoD, USDOD or DOD) is an executive branch department of the federal government charged with coordinating and supervising all agencies and functions of the government directly related to national secu ...

(DoD)

multilevel security Multilevel security or multiple levels of security (MLS) is the application of a computer system to process information with incompatible classifications (i.e., at different security levels), permit access by users with different security clearan ...

(MLS) policy. The model is a formal state transition model of

computer security policy A computer security policy defines the goals and elements of an organization's computer systems. The definition can be highly formal or informal. Security policies are enforced by organizational policies or security mechanisms. A technical impleme ...

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 (e.g., "Unclassified" or "Public"). The Bell–LaPadula model is an example of a model where there is no clear distinction between protection and security.

Features

The Bell–LaPadula model focuses on data

confidentiality Confidentiality involves a set of rules or a promise usually executed through confidentiality agreements that limits the access or places restrictions on certain types of information. Legal confidentiality By law, lawyers are often required ...

and controlled access to

classified information Classified information is material that a government body deems to be sensitive information that must be protected. Access is restricted by law or regulation to particular groups of people with the necessary security clearance and need to k ...

, in contrast to the Biba Integrity Model which describes rules for the protection of

data integrity Data integrity is the maintenance of, and the assurance of, data accuracy and consistency over its entire life-cycle and is a critical aspect to the design, implementation, and usage of any system that stores, processes, or retrieves data. The ter ...

. In this formal model, the entities in an

information system An information system (IS) is a formal, sociotechnical, organizational system designed to collect, process, store, and distribute information. From a sociotechnical perspective, information systems are composed by four components: task, people ...

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

with a set of allowable states in a computer 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 Security policy is a definition of what it means to ''be secure'' for a system, organization or other entity. For an organization, it addresses the constraints on behavior of its members as well as constraints imposed on adversaries by mechanism ...

. 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 one

discretionary access control In computer security, discretionary access control (DAC) is a type of access control In the fields of physical security and information security, access control (AC) is the selective restriction of access to a place or other resource, whil ...

(DAC) rule and two

mandatory access control In computer security, mandatory access control (MAC) refers to a type of access control by which the operating system or database constrains the ability of a ''subject'' or ''initiator'' to access or generally perform some sort of operation on an ...

(MAC) rules 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. # The * (Star) Security Property states that a subject at a given security level may not write to any object at a lower security level. # The Discretionary Security Property uses an access matrix to specify the discretionary access control. The transfer of information from a high-sensitivity document 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 Star-property. Trusted Subjects must be shown to be trustworthy with regard to the security policy. The Bell–LaPadula security model is directed toward access control and is characterized by the phrase "write up, read down" (WURD). Compare the Biba model, the

Clark–Wilson model The Clark–Wilson integrity model provides a foundation for specifying and analyzing an integrity policy for a computing system. The model is primarily concerned with formalizing the notion of information integrity. Information integrity is ...

, and the

Chinese Wall A Chinese wall or ethical wall is an information barrier protocol within an organization designed to prevent exchange of information or communication that could lead to conflicts of interest. For example, a Chinese wall may be established to sepa ...

model. 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 channel In computer security, a covert channel is a type of attack that creates a capability to transfer information objects between processes that are not supposed to be allowed to communicate by the computer security policy. The term, originated in 19 ...

s. 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 Star Property

The Strong Star 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 Star Property is usually discussed in the context of multilevel

database management system In computing, a database is an organized collection of data stored and accessed electronically. Small databases can be stored on a file system, while large databases are hosted on computer clusters or cloud storage. The design of databases span ...

s and is motivated by integrity concerns. This ''Strong Star Property'' was anticipated in the Biba model where it was shown that strong integrity in combination with the Bell–LaPadula 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 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. The "principle of weak tranquility" states that security levels may never change in such a way as to violate a defined security policy. Weak tranquility is desirable as it allows systems to observe the

principle of least privilege In information security, computer science, and other fields, the principle of least privilege (PoLP), also known as the principle of minimal privilege (PoMP) or the principle of least authority (PoLA), requires that in a particular abstraction la ...

. That is, processes start with a low clearance level regardless of their owners clearance, and progressively accumulate higher clearance levels as actions require it.

Limitations

* Only addresses confidentiality, control of writing (one form of integrity), *-property and discretionary access control * Covert channels are mentioned but are not addressed comprehensively * The tranquility principle limits its applicability to systems where security levels do not change dynamically. It allows controlled copying from high to low via trusted subjects. d. Not many systems using BLP include dynamic changes to object security levels.

Notes

References