Bell LaPadula Security Model

This an outdated and mostly useless formal model of security policy describes a set of access control rules. By conforming to a set of rules, the model tries to inductively proves that the system is secure.  The proof is wrong.

A subject's (usually a user's) access to an object (usually a file) is allowed or disallowed by comparing the object's security classification with the subject's security clearance. The three basic rules are the *-property (star property), the simple property, and the tranquility property.

1. *-property (star property) (Also called 'confinement property') A rule defined within the Bell La-Padula security model. A subject (usually a user) is only allowed write access to an object (usually a file) if the security level of the object is greater than or equal to the clearance level of the subject.  This makes it impossible for data from a highly cleared subject to become available to users with a lower security clearance in an object (file/directory) with a low security level.  The purpose is to confine sensitive data at its correct level. Without this rule, Alice (with a high security clearance) could copy sensitive data into Bob's (low security clearance) document - thus allowing 'confidential' data to move from a 'top secret' to an 'unclassified' level.
2. Simple Property  One of the three main properties of the Bell LaPadula security model (the others being the *-property (star property) and the tranquility property). The simple property states that a subject may only have read access to an object if the security level of the subject dominates that of the object. When you think about it, it's almost useless. It states that a user (a subject) may only read a file (an object) if he or she has a security level equal to or greater than that of the file. It means that someone with a 'secret' security level cannot read a file with a 'top secret' security level; but can read a file with a 'secret' or 'confidential' security level.  No revelations here :-)
3. Tranquility Property  One of the three main properties of the Bell LaPadula security model (the others being the *-property (star property) and the simple property). The tranquility property states that the security level of an object cannot be changed while it is being processed by a computer system.

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the Bell-LaPadula model

Alex,

I've cc: this to the firewalls mailing list since I it was there I first asked about the Bell-LaPadula model, just over four years ago. Perhaps this is a FAQ?

It's very hard to find information about the Bell-LaPadula model. The best reference I know about is a large part of a chapter in "Information Security Handbook, Caelli, Longley, & Slain, 1994, Macmillan Press, ISBN  0-333-59901-2" (a really great book!).

A reference is also in the Orange Book (also known as DoD 5200.28-STD, Department of Defense Trusted Computer System Evaluation Criteria (TCSEC)). Lots of copies of orange book exist over then net.

I don't truly understand the model, but I hope this is will get you started....

A Quick summary of Bell La-Padula:

Is a Mandatory Access Control which is governed by strict rules for subjects (an active entity) to access stored information or objects (sets of passive, protected entities), but have provision for Dicretionary  Access Control via an Access Permissino Matrix.

- military based
- concerned with the confidentiality of information
- requires subject and object definition
- requires clearance and classifications to be given to both subjects and
objects
- policies need to be defined for accesses made to objects by subjects
- assumes integrity of subjects
- applicable to not only somputer security - but to physical and procedural
security
- defines two security axioms:
        (1) simple security rule (no read up)
        a subject cannot read information for which it is not cleared
        (2) (star)*-property (no write down)
        subjects cannot move information from an object with a higher
        classification to an object with a lower classification
- four modes for current access set are defined:
        - execute
        - read-only
        - append
        - read-write
- hierarchy structure imposed on objects
- an object can have many children
- a child object can have only one parent
- the embodiment of the mandatory access control model is done by the level
function:
        - subjects and objects are given a security designation
        (classification/clearances, set of categories)
        - both classification and clearances have a coarse hierarchy
        (Top Secret -> Secret -> Confidential -> Unclassified)
        - uses a category set (to better fine tune the function)
        (subject can be designated as having the clearance
        of (secret{cryptology,finance}) for example)
        (however is requires a need for overlap in the categories
        sub-section)
        - the concept of dominance for security designation is introduced
        (the clearance of subject A would dominate over subject B, if
        and only if (1) subject A's clearance is >= subject B's clearance
        AND (2) the category set in subject A's level function set
        includes subject B's category set as a subset)
- discretionary access control via the access permission matrix
        - a large simple matrix (and sparse) is created with subjects
        and objects as rows and columns
        - a subject is able to give acces rights to objects owned
        to another subject
- in practice it is necessary to specify trusted systems that are allowed to
contravene the *-property in order to perform functions essential to the
operating system (classic example would be a print spooler).

Gavin.

P.S. I would would like the reference URL, so I can place some information
there since I've been getting requests every now and then.

Problems

A Comment on the "Basic Security Theorem" of Bell and LaPadula* John McLean Center for High Assurance Computer Systems Naval Research Laboratory  Washington, D.C. 20375

Many claim that the security model developed by Bell and LaPadula and used as a basis for numerous prototype military computer systems is superior to others partly because its authors prove a "Basic Security Theorem" that applies to it. This paper shows that the theorem does not support such claims since it can be proven for security models that are obviously not secure. Further, the theorem provides little help to those who design and implement secure systems.

1. Introduction

The security model developed by Bell and LaPadula [1] has been widely used as a basis for designing systems with specified security properties [2]. It has been argued that one reason developers should

have confidence in the security provided by systems based on this model is a theorem, called the "Basic Security Theorem" (BST) [1, p. 20], proven about a formalization of the model by its authors [1,p.90, corollary A1]. Several authors have proven similarly named theorems about related security models[3,4,5]. This note reviews the Bell-LaPadula model briefly and shows that the BST can be proven for systems that directly contradict the notion of security embodied in the Bell-LaPadula model. We conclude that the value of the BST is much overrated since there is a great deal more to security than it captures. Further, what is captured by the BST is so trivial that it is hard to imagine a realistic security model for which it doesn't hold.

2. Bell-LaPadula Model

The Bell-LaPadula model is based on a state machine in which subjects apply operations (rules) that may require access to objects. The state of the system includes a set of triples that define the current access

mode each subject has to each object in the system. Permissible access is determined partly by a security level (classification or clearance) associated with each object and subject. These security levels are partially ordered. Each subject also has a current security level that is bounded above by its clearance. There is also an access matrix that further constrains the access mode an arbitrary subject is allowed to have to an arbitrary object.

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Last modified: March, 12, 2019