Distributed computer security

8.2 Discretionary Access Control Models

-Ranjitha Shivarudraiah

Agenda

Concepts What are security policies? What is Access control? Different types of ACM. Concept of distributed compartment ACM implementations ACL vs CL

Research Case study: ACM for grid networks Example of an ACM for Mobile devices by virtualization.

Future expectations

CONCEPTS

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.

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

).s – subjecto – object

Access control

Access Control contd..

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

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

•Access Rights may include “read”, ”write”, ”execute”, ”append”.

•Special privileges may be the “owner” and copy privilege.[1]

[1]Randy Chow & Theodore Johnson, 1997,“[1]Randy Chow & Theodore Johnson, 1997,“Distributed Operating Distributed Operating Systems & AlgorithmsSystems & Algorithms”,”,

Process ACM

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

•Operations are basically related to communication and synchronization.[1]

[1]Randy Chow & Theodore Johnson, 1997,“[1]Randy Chow & Theodore Johnson, 1997,“Distributed Operating Systems & AlgorithmsDistributed Operating Systems & Algorithms”,”,

Domain ACM

Set of objects with same access rights.[1]

[1]Randy Chow & Theodore Johnson, 1997,“[1]Randy Chow & Theodore Johnson, 1997,“Distributed Operating Systems & AlgorithmsDistributed Operating Systems & Algorithms”,”,

ACM contd..

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, its access rights is the union of all access rights of all the groups it belongs to.

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

Distributed compartment.

Figure: Distributed Compartment [1]

[1] Randy 1997

Distributed compartment. Contd.. A distributed application with collaborating processes may consists of

subject users and object resources crossing the physical boundaries of physical resources.

Here, a logical ACM called a ‘distributed compartment’ that regulates access among the collaborating users would serve a better purpose.

Access to the distributed compartments are based on ‘distributed

handles’.

These handles are application oriented and they provide a protective wall around an application and are authenticated by the application

DCM contd..

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

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. An ACL 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 . A CL specifies privileges to various objects held by a subject

ACL vs CL

Comparison between ACLs and capabilities for protecting objects.

Using an ACL

Using capabilities.

Source: Randy 1997

Lock –Key implementation

ACL vs CL contd..

Comparison in terms of management functions Authentication Reviewing of Access Rights Propagation of Access Rights Revocation of Access Rights Conversion between ACL and CL

Authentication

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 favor the CL approach

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

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 rights contd..

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 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 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

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

Research

Case study: ACM for grid networks[2]

““Access control of global distributed storage system”(Dr Xie et Access control of global distributed storage system”(Dr Xie et al ,2004)al ,2004)

Local distributed storage system.

Source: “Access control of global distributed storage system” (Dr Xie et Source: “Access control of global distributed storage system” (Dr Xie et al -2004 )al -2004 )

Certification andAuthentication Server

Name Server

3. Requirements of Access Control in Data grid Environment

Single sign on Separation of duties High efficiency Centralized management and

autonomy Support Qos( Quality of service).

Source: “Access control of global distributed storage system” Dr Xie et al -Source: “Access control of global distributed storage system” Dr Xie et al -2004 )2004 )

Access control methods for mobile devices[5]

[5] ACM for mobile devices( Dr Lee et.al. 2008 ).

Root of trust. Rom has The master key.

Virtual Machine Monitor

ACM for Mobile devices.

Provides security

Provides Flexibilty

Future Expectations

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.

References

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

[2] Access control of global distributed storage systemChao Xie; Hai Jin; Song Wu; Shengli Li; Zhiping Wang;Computer and Information Technology, 2004. CIT '04 .

[3] Samarati, P.; Bertino, E.; Ciampichetti, A.; Jajodia, S.; “Information flow control in object-oriented systems”. Knowledge and Data Engineering, IEEE Transactions on Volume 9,  Issue 4,  July-Aug. 1997 Page(s):524 - 538

[4] Lin, Tsau Young (T. Y.); “Managing Information Flows on Discretionary Access Control Models” Systems, Man and Cybernetics, 2006. ICSMC '06. IEEE International Conference onVolume 6,  8-11 Oct. 2006 Page(s):4759 - 4762

[5]  A Multi-Layer Mandatory Access Control Mechanism for Mobile Devices Based on VirtualizationSung-Min Lee; Sang-bum Suh; Bokdeuk Jeong; Sangdok Mo;Consumer Communications and Networking Conference, 2008. CCNC 2008. 5th IEEE

[6] http://en.wikipedia.org/wiki/Access_control( Accessed on 27th October 2008).