[Intro. to Computer Security Course Note] Ch 4

Posted on Edited on Views: Word count in article: 7.2k Reading time ≈ 7 mins.

Ch4. Access Control

Access control: the central element of computer security

Principal Objectives of Computer Security

  • Prevent unauthorized users from gaining access to recources
  • Prevent legitimate users from accessing resources in an unauthorized manner
  • Enable legitimate users to access resources in an authorized manner

Access Control Principles

Access Control Context

  • Authentication: verification that user / system credentials are valid
  • Authorization: the granting of a right or permission to a system entity ro access a system resource
  • Audit: an independent examination of system records and activities
    • To test for adequacy of system controls
    • To ensure compliance with established policy and operational procedures
    • To detect breaches in security
    • To recommend any indicated changes in control, policy ans procedures

Access Control Policies

  • Discretionary access control (DAC)
    • Based on the identity of the requestor, and on access rules stating what requestors are (or are not) allowed to do
    • Why discretionary ? An entity might have access rights to enable another entity to access some resource
  • Mandatory access control (MAC)
    • Based on security clearances of system entities, and on security labels of resources
    • Why mandatory ? An entity that has clearance to access a resource may not enable another entity to access that resource
  • Role-based access control (RBAC)
    • Based on the roles that users have, and on rules stating what accesses are allowed to given roles
  • Attribute-based access control (ABAC)
    • Based on attributes of the user, the resource to be accessed, and current environmental conditions

Subjects, Objects, and Access Rights

Subject

  • An entity capable of accessing objects
  • 3 classes
    • Owner
    • Group
    • World (include all users)

Object

  • A resource to which access is controlled

Access Rights

  • Describes the way in which a subject may access an object
  • Could include
    • Read
    • Write
    • Execute
    • Delete
    • Create
    • Search

Discretionary(任意) Access Control

  • A general approach: access
    • Subjects vs. Objects
    • Each entry: access right

Decomposition Method 1

  • Access controls lists (ACL): decomposed by columns (objects)
    • For each object, an ACL lists users and their permitted access rights
    • Default set of rights: users that are not explicitly listed
    • Convenient: determining which subjects have which access rights to a particular resource
    • Inconvenient: determining the access rights available to a specific user

Decomposition Method 2

  • Capability tickets: decomposed by rows (subject)
    • A capability tickets specifies authorized objects and operations for a particular user
    • Convenient / inconvenient: opposite to ACLs
  • Have greater security problem than ACLs. Why ?
    • Tickets may be dispersed around the system
      • Integrity of the ticket must be protected, guaranteed, and unforgeable
    • 2 solutions
      • OS holds all tickets on behalf of users
      • An unforgeable token in the capability

Another Approach: Authorized Table

就資料庫的感覺

A General Access Control Model for DAC

  • 3 requirements
    • Representing the protection state
    • Enforcing access rights
    • Allowing subjects to alter the protection state in certain ways
  • Concepts
    • As usual: a set of subjects, objects, and rules
    • New: protection state
  • Protection states
    • Processes: delete, stop (block), and wake up
    • Devices: read / write, operation control, and block / unblock
    • Memory locations or regions: read / write
    • SubjectsL grant or delete access rights of objects

More Flexible Model: Protection Domains

  • A set of objects together with access rights to those objects
    • r.g., Access matrix
  • Recal security design principles: Least privilege
    • Every process and every user of the system should operate using the least set of privileges necessary to perform the task
  • More general concpet: minimize the access rights that any user of process has at any one time
  • Association between a process and a domain can be static or dynamic
    • e.g., Aprocess: a sequence of procedures require different access rights
  • One form: distinction mode in many OSes (e.g., UNIX)
    • User mode: certain areas of memory are protected amd certain instructions may not be executed
    • Kernel mode

Example: UNIX File Access Control

  • UNIX files are administered using inodes (index node)
  • Directories are structured in a hierarchical tree

Traditional UNIX File Access Control

  • UNIX user: a unique user identification number (user ID)
    • A member of a primary group, and possibly other groups
    • Each group is identified by a group ID
  • Each file / directory: 12 protection bits
    • FIrst 9 bits: read, write, execute
    • Last 3 bits: setUID, setGID, and sticky bit
  • SetUID / SetGID bits
    • Known as the effective user ID and effective group ID
    • System temporarily grants a real user with the rights of the file owner / group in addition to the real user’s rights
    • For directories
      • SetGID: newly created files will inherit the group of this directory, rather than the primary group ID of the user who created this file
      • SetUID is ignored
  • Sticky bit
    • Files: the system should retain the file contents in memory following execution (no longer used)
    • Directories: only the owner of any file in the directory can rename, move, or delete that file
  • superuser
  • Issues
    • No scalability, difficult to manage (user group 會弄得很冗)

Modern UNIX Access Control: Access Control Lists (ACLs)

  • Supported by many modern UNIX-based OSes
    • Extended ACL vs. minimal ACL (traditional)
  • FreeBSD
    • Any number of users and groups can be assigned to a file
      • Each with 3 protection bits
    • A file need not have an ACL, may be protected solely by traditional access control
    • An additional protection bit: whether the file has an extended ACL
  • Extended ACLs are used with the following strategies
    • Owner and other classes remain the same
    • Group class specifies the permissions for the owner group for this file
    • Additional named users and named groups may be associated with the file

Role-Based Access Control

  • Based on the roles that users assume, instead of their identities
  • Widespread commercial use and an area of active research
  • Many-to-many relationship
    • Users to roles
    • Roles to resources

RBAC Reference Models

  • 4 modes
    • RBAC0: minimum functionality
    • RBAC1: RBAC0 + role hierarchies
    • RBAC2: RBAC0 + constrains
    • RBAC3: RBAC0 + RBAC1 + RBAC2

RBAC0: Base Model

  • User: an individual that has access to this computer system
  • Role: a named job function (authority level)
  • Permission: an approval of a particular mode of access to one or more objects
  • Session: a mapping between a user and set of roles to which a user is assigned

RBAC1: Role Hierarchies

  • Roles with greater responsibility: greater authority to access resources

RBAC2: Constrains

  • Adapting RBAC to the specifics of administrative and security policies in an organization
    • Mutually exclusive roles
      • A user can be assigned to only one role in the set
      • Any permission can be granted to only one role in the set
      • Non-Overlapping permissions
    • Cardinality(基數)
    • Prerequesite role
      • e.g., a user can be assigned to a higher role only if it is already assigned an lower role

Attribute-Based Access Control

  • Define authorizations that express conditions on properties of both the resource and the subject
  • Strength: flexibility, expressive power
  • Drawback: 運算較多，花時間

ABAC Model: Attributes

  • Subject attributes
    • A subject is an active entity that causes info. to flow among objects or changes the system state
    • Attributes define the identity and characteristics of the subject
      • e.g., name, job title
  • Object attributes
    • An object (or resource) is a passive system-related entity containing or receiving info.
    • Objects have attributes that can be leveraged to make access control decisions
      • file name, file size, creator
  • Environment attributes
    • The operational, technical, and even situational environment or context in which the info. access occurs
      • e.g., current date, time
    • Have so far been largely ignored in most access control policies

ABAC Model: Distinguishable

  • Controls access to objects by evaluating rules against the attributes of entities (subject and object), operations, and the environment
  • Capable of enforcing DAC, RBAC, and MAC concepts
  • Fine-grained access control: allows an unlimited number of attributes to be combined to satisfy any access control rule

ABAC Logical Architecture

  • 4 independent sources of info. used for the access control decision
  • Powerful, flexible, but cost is large

ACL Trust Chain

ABAC Trust Chain

ABAC Policies

  • A policy is a set of rules and relationships that govern allowable behavior within an organization
    • Based on
      1. Privileges of subjects
      2. How resources or objects are to be protected
      3. Under which environment conditions
  • An ABAC policy model

Case Study: RBAC System for a Bank