R. Thomas, R. Sandhu. 1994. "Conceptual Foundations for a Model of Task-based Authorizations"

Computer security models have a bias towards a centralized notion of computing.

• good for a computer security model, bad for an information-oriented model (higher level)

(first mention of task based authorization: R. Thomas, R. Sandhu. Towards a task-based paradigm for flexible and adaptable access control in distributed applications.)

	Abstraction stages	Security model stages	examples
What needs to be done? (people oriented)	Trust objectives	Organizational requirements	little work has been made
	External-interface requirement	Computer policy models	non-interference model, task-based authorization
	Internal requirements	Access control models	Bell-LaPadula model, HRU model, typed access matrix model
	Rules of operation	Implementation models
How to do it? (computer oriented)	Functional design	???

Paper based model
Authorization of transactions

• each source document should be authorized
  
• authorization can come in the form of signatures or authorization codes

Source documents (deposit slips, sales order forms) are also good for

• verification of data
• constructing audit trails
• recovery in the event that processed data is lost

Transaction processing in computerized information systems often do not involve source documents in the loop. (is this still true now?)




Transient objects

• documents - vouchers, purchase orders, sales slips
• issue a finite set of operations then leave the system

Persistent objects

• inventory databases
• bank accounts

Enforce controls primarily on the transient objects.
Transaction to be executed on persistent objects only as a side effect of executing transactions on transient objects.


Transaction control expressions
Example:

1. Clerk prepares a check
2. 3 separate supervisors approve the check
3. Check is issued by (a different) clerk

Transaction control expression for above is:
prepare * clerk;
3: approve * supervisor;
issue * clerk;

Each expression consists of a transaction and a role.
Separation of duty is achieved by requiring the users that execute different transactions in the expression to be distinct.
Provides support for linear sequences of authorizations.



Task-based authorizations
Authorization function


Each authorization function is a point in a network where responsibilities are accepted, and one from which associated obligations are discharged and new responsibilities created.


Issues
Abstraction and composition

• authorized-task-unit to model the authorizations associated with every authorization function
• an authorization unit may be composed of other smaller units called approval-steps

Dependencies

• must model dependencies between authorization-units as well as those internal to the authorization-unit
• temporal - relates to the order of execution of authorizations
• semantic - relates to the implied authorizations that may be needed as part of the authorization to be performed
• atomic - grant all or none

Incorporation of controls

• what constructs and mechanisms are needed to incorporate controls

Delegation and revocation
Authorization expiration
Authorization deadlines

• example, manager responsible for giving approvals may be available only for certain hours during the week

Failure and exception handling
Deadlocked authorizations



Groundwork for building a model


Authorization-task-unit(task name)

• Originating function
• Attributes: atomic, expiration, deadline
  
• Dependency specifications: {}
• Approval-steps: {}

Approval-step

• a tuple of the form {step-name, role, expiration, deadline}

Dependency specification

• {task-name, dependency type} OR
• {step-name, dependency type}