Chapter: Cryptography and Network Security

Trusted systems

1. Data access control 2. The concept of Trusted Systems 3. Reference Monitor concept

Trusted systems


One way to enhance the ability of a system to defend against intruders and malicious programs is to implement trusted system technology.


1. Data access control


Following successful logon, the user has been granted access to one or set of hosts and applications. This is generally not sufficient for a system that includes sensitive data in its database. Through the user access control procedure, a user can be identified to the system. Associated with each user, there can be a profile that specifies permissible operations and file accesses. The operating system can then enforce rules based on the user profile. The database management system, however, must control access to specific records or even portions of records. The operating system may grant a user permission to access a file or use an application, following which there are no further security checks, the database management system must make a decision on each individual access attempt. That decision will depend not only on the user‟s identity but also on the specific parts of the data being accessed and even on the information already divulged to the user.


A general model of access control as exercised by an file or database management system is that of an access matrix. The basic elements of the model are as follows:


·        Subject: An entity capable of accessing objects. Generally, the concept of subject equates with that of process.  


·         Object: Anything to which access is controlled. Examples include files, portion of files, programs, and segments of memory.  


·        Access right: The way in which the object is accessed by a subject. Examples are   read, write and execute.


One axis of the matrix consists of identified subjects that may attempt data access. Typically, this list will consist of individual users or user groups. The other axis lists the objects that may be accessed. Objects may be individual data fields. Each entry in the matrix indicates the access rights of that subject for that object. The matrix may be decomposed by columns, yielding access control lists. Thus, for each object, an access control list lists users and their permitted access rights. The access control list may contain a default, or public, entry.



Decomposition by rows yields capability tickets. A capability ticket specifies authorized objects and operations for a user. Each user has a number of tickets and may be authorized to loan or give them to others. Because tickets may be dispersed around the system, they present a greater security problem than access control lists. In particular, the ticket must be unforgeable. One way to accomplish this is to have the operating system hold all tickets on behalf of users. These tickets would have to be held in a region of memory inaccessible to users.


2. The concept of Trusted Systems


When multiple categories or levels of data are defined, the requirement is referred to as multilevel security. The general statement of the requirement for multilevel security is that a subject at a high level may not convey information to a subject at a lower or noncomparable level unless that flow accurately reflects the will of an authorized user. For implementation purposes, this requirement is in two parts and is simply stated. A multilevel secure system must enforce:


·        No read up: A subject can only read an object of less or equal security level. This is referred to as simple security property.  


·        No write down: A subject can only write into an object of greater or equal security level.

This is referred to as *-property (star property).  


These two rules, if properly enforced, provide multilevel security.  


3. Reference Monitor concept


The reference monitor is a controlling element in the hardware and operating system of a computer that regulates the access of subjects to objects on the basis of security parameters of the subject and object. The reference monitor has access to a file, known as the security kernel database that lists the access privileges (security clearance) of each subject and the protection attributes (classification level) of each object. The reference monitor enforces the security rules and has the following properties:


Complete  mediation:  The  security  rules  are  enforced  on  every  access,  not  just,  for example, when a file is opened.  


Isolation:  The  reference  monitor  and  database  are  protected  from  unauthorised modification.  


Verifiability: The reference monitor‟s correctness must be provable. That is, it  must be possible to demonstrate mathematically that the reference monitor enforces the security rules and provides complete mediation and isolation. Important security events, such as detected security violations and authorized changes to the security kernel database, are stored in the audit file


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