Access Control

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Steps in Authentication and Authorisation

Subject/ principle- an active entity

Object- resource being accessed

Access Operation

Reference Monitor- grants or denies access

Access control has 2 steps-

Authentication- who requested access?

Authorisation- who is allowed to access the resource?

Principals- entity that can be granted access to objects e.g. a user ID

Subject- an active entity within an IT system- e.g. process running under a user ID

Object- files/ resources e.g. memory, printers, directories

Access Modes:

Observe- subject may look at contents of an object

Alter- subject may change the contents of an object

Too simple, computer model- read/write/execute

Ownership- who is in charge of setting security policies

Access Control Structures:

Express policy

Often focused on efficient lookup

Access Control Matrix/ Capabilities/ Access Control Lists

Access Control Matrix (ACM)

Access rights are defined individually for each combination of subject and object

Allows very fine grain control

Not practical- large memory req

Access Control List (ACL)

Stored with an object itself, corresponds to a collumn of an ACM

Much less memory intensive

Quick to access

Management of individual subject cumbersome

Obtaining overview of permissions is challenging

Tedious to set up for all subjects and objects

UNIX

Simplifies the ACL structure to only user, group and others

User = current owner

Group = named group entity

Everyone else

Offers r/w/x access controls

Windows

Predominantly uses ACLs

Extends r/w/x with take ownership, change permissions and delete

Higher degree of control... but more complex

Capabilities

Access rights stored with a subject (user) not a resource

Every subject given a capability

Corresponds to row in a ACM

Typically associated with discretionary access control

Subjects can pass on their capabilities

Not widely used

Difficult to get overview of access rights on a file and revoke them

Intermediate Controls

Groups

Negative permissions- an operation that the user cannot perform

Privileges

Role based access control

Protection rings

Role based access control

Based on a role e.g. accountants should access certain financial files

Easier to scale and use

Role- collection of application specific operations or resource access

Subjects derive access rights from the role they perform

RBAC focuses on users and the jobs they perform

More applicable to large networks and orgs

Layers (between subjects and objects)

Layers consist of:

Roles- collection of procedures assigned to users

Procedures- high level access control methods

Datatypes- each object of certain data types

Roles vs groups

Groups are collections of users whereas roles are collections of permissions

Most OS's are user/group based so role based access can be provided using nested groups

Evaluating security policies

Evaluate against access control entry

Protection rings- hardware based access control, subject/object assigned no dep on importance. Decisions made by comparing subjects to objects numbers

Partial orderings- provide relations between subsets of groups

Lattices- e.g. military security- top secret/ secret/ confidential/ unclassified

Other systems

Sandboxing (e.g. JVM)

Virtualisation

Trusted computing- verifiable state of software and hardware- e.g. TPM