Distributed computer security
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Distributed Computer Security. 8.2 Discretionary Access Control Models - Sai Phalgun Tatavarthy. Index. Security policies Access control Different types of ACM Distributed compartment - discom ACM implementations ACL vs CL RBAC Research 3D ACM ACM for grid networks.

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Distributed Computer Security

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Distributed computer security

Distributed Computer Security

8.2 Discretionary Access Control Models

- Sai Phalgun Tatavarthy



  • Security policies

  • Access control

  • Different types of ACM

  • Distributed compartment - discom

  • ACM implementations

  • ACL vsCL

  • RBAC

  • Research

    • 3D ACM

    • ACM for grid networks

Security policy

Security policy

  • There are two kinds of security policies:

    • Simple security policies

      • Access control matrix (ACM) models are widely used to enforce the simple security policies.

    • Complex Security policies

      • Security requirements how and when the accesses are performed( special constraints are involved).

      • Relevant to the distributed systems.

Access control

Access Control

  • An access control is a function that given a subject and object pair i.e. (s,o) and a requested operation r , from s to o , returns a true value if requested is permitted.

    R = P ( s , o )

    P – access matrix

    R – set of allowable operations.( ‘r’ is a particular operation belonging to set ‘R’ ).

    s – subject

    o – object

Access control1

Access Control

  • The process of access validation is performed by a ‘reference monitor’ with an ACM for all subjects and objects

  • Practically it is preferable to have separate reference monitors for different categories of subjects and objects.

Resource acm

Resource ACM

  • In a resource ACM subjects are users and objects are the files to be accessed.

  • Access Rights - “read”, “write”, “execute”, “append”.

  • Special privileges may be like “owner” privilege.

Process acm

Process ACM

  • In process ACM the subjects and objects are both processes.

  • Operations are basically related to communication and synchronization

Domain acm

Domain ACM

  • Set of objects with same access rights

Distributed computer security


  • Reducing the Size of Access Control Matrix

    • Subject rows in the ACM that have identical entries i.e. subjects that have similar access rights on common objects , could be merged into groups.

    • If a user belongs to more than one group, his access rights is the union of all access rights of all the groups he belongs to.

    • Similarly Object columns with same entries could be merged into ‘categories’.

Distributed compartment

Distributed compartment

Distributed compartment1

Distributed compartment

  • A solution to management of system resources and access control in a distributed computing environment.

  • The model consists of two parts

    • Distributed Handles, a means for user identification and access control

    • Distributed compartments, a method for allowing users to manage resources within a distributed system across computer systems

Distributed compartment2

Distributed compartment

  • The distributed compartment model has a number of advantages

    • The grouping of subjects and objects is logical and application specific.

    • The accesses are more transparent since they do not depend on the operating systems and administrative units.

    • Since the application manages the distributed handles, it allows different security policies to be implemented

Acm implementations

ACM implementations

  • For efficiency and organizational purposes , access control matrices need to be partitioned

  • The Linked list structure that contains all entries in a column for a particular object is called a Access control List (ACL) for the object - specifies the permissible rights that various subjects have on the object

  • Likewise all entries in a row for a subject is called a Capability List (CL) for the subject - CL specifies privileges to various objects held by a subject – like movie tickets

Acl vs cl


Comparison of acl cl

Comparison of ACL & CL

  • Comparison in terms of management functions

    • Authentication

    • Reviewing of Access Rights

    • Propagation of Access Rights

    • Revocation of Access Rights

    • Conversion between ACL and CL



  • ACL Authenticates subjects, which is performed by the system

  • While in CL, authentication is performed on capabilities of objects , by the object server.

  • Objects have knowledge of the capabilities ,but do not know the users or processors. This is one of the reasons why many Distributed implementations favour the CL approach

Review of access rights

Review of Access rights

  • To know which subjects are authorized to use a certain objects.

  • Easier to review ACL, because ACL contains exactly this information. For storage efficiency subject grouping, wildcards ,prohibitive rights could also be used.

  • It is difficult to review for a CL unless some type of activity log is kept for all subjects that are given the capability

Propagation of access rights

Propagation of access rights

  • Access rights must be replicable to facilitate sharing.

  • Propagation is Duplication of some or all the privileges from one subject to the others.

  • Propagation is not transfer of rights, it is only duplication.

  • In ACL, propagation of rights is explicitly initiated by a request to the object server, which modifies or adds an entry to its ACL.

Propagation of access rights1

Propagation of access rights

  • Propagation of rights must adhere to the principle of least principles, i.e. Only the minimum privileges required to perform the tasks are given when propagating the rights

  • In CL, theoretically it is possible to propagate rights between subjects without intervention of object server.

  • This could result in an uncontrollable system and hence is avoided.

Revocation of access rights

Revocation of Access rights

  • Revocation is trivial in ACL because it is easy to delete subject entries from the ACL.

  • It is difficult for CL’s to revoke access selectively.

Conversion between acl cl

Conversion between ACL & CL

  • Interactions among processes involving different Access control models would require gateways for conversions.

  • Conversion to ACL is straightforward.

  • Consider example of processes in a CL requiring to access remote objects in ACL

    • Gateway Authenticates the process identifier.

    • It then verifies the operation in the capability list.

    • The request is then converted to ACL and is presented to the remote host

Conversion between acl cl1

Conversion between ACL & CL

  • Converting a ACL request to CL is slightly more complex

  • Requires a database with resource capabilities for the interacting processes

    • Gateway validates the ACL request

    • obtains the resource capability from the database server

    • Capability is then presented to capability based object server.

  • A system utilizing both ACL and CL suffers the drawback of both approaches7)

Role based access control rbac

Role-based Access Control (RBAC)

  • Access decisions are based on the roles that individual users have as part of an organization.

  • Users take on assigned roles (such as doctor, nurse, teller, manager). The operations that a user is permitted to perform are based on the user's role.

  • Role hierarchies can be established to provide for the natural structure of an enterprise.

  • Organizations establish the rules for the association of operations with roles.

Research work privacy enhanced access control models

Research work - Privacy enhanced access control models

  • It’s an enhancement to traditional 2D ACM.

  • We add one more dimension called privacy and make it a 3D ACM.

  • The access is granted depending on privacy, for example if it’s true grant access else deny.

  • Costly – so use sparingly.

Requirements of access control in data grid

Requirements of Access control in Data grid

  • Single sign on

  • Separation of duties

  • High efficiency

  • Centralized management and autonomy

  • Support QoS ( Quality of service).

Future work

Future Work

  • Absolute trust mechanism in access control system is an important subject of P2P security research.

  • Also secure efficient ACM for huge networks especially the data-centric networks will be effectively realized.

  • Excellent Access control methods for highly dynamic real time systems.



Thank you

Thank you



  • [1] Randy Chow & Theodore Johnson, 1997,“Distributed Operating Systems & Algorithms”, (Addison-Wesley), p. 271 to 278.

  • [2] http://en.wikipedia.org/wiki/Access_control

  • [3] Ninghui Li and Mahesh V. Tripunitara, IEEE Symposium on Security and Privacy, May 2005. "Safety in Discretionary Access Control".

  • [4] K. Gopinath, Communication System Software and Middleware, 2006, "Access Control in Communication Systems“

  • [5] FeiXu; Jingsha He; Xu Wu; Jing Xu; Networks Security, Wireless Communications and Trusted Computing, 2009. "A Privacy-Enhanced Access Control Model". NSWCTC '09. International Conference onVolume 2, 25-26 April 2009 Page(s):703 - 706.

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