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Computer SecuritySecurity Policies ..

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Security Policies

We view a computer system as a finite-state machine

DefinitionA security policy is a statement that partitions the states of a system into a set of authorized or secure states and a set of unauthorized or nonsecure states.A secure system is a system that starts in an authorized state and cannot enter an unauthorized state.

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Example

s1 s2 s3 s4

t1

t3

t2

t5t4

An insecure systemAuthorized states are s1 and s2

Unauthorized states are s3 and s4

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Security PoliciesDefinition

1. A breach of security occurs when a system enters an unauthorized state.

2. Let X be a set of entities and I be some information. I has the property of confidentiality with respect to X if no member of

X can obtain information about I. I has the property of integrity with respect to X if all members of X

trust I.3. Let I be a resource I has the property of availability with respect to

X if all member of X can access I.5. A security mechanism is an entity or procedure that enforces some

part of a security policy.

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Types of Policies

Definition

1. Military security policies or governmental security policies.2. Commercial security policies

• Confidentiality policies• Integrity policies• Transaction policies

Discuss issues regarding trust.

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The role of trust

The role of trust is fundamental in understanding the nature of computer security.Examples –see textbook [Example 1-2-3-4, pp 101-102 (high level)& 1-2-3-4 102-103 (low-level, formal)]

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Types of Access Control

• Discretionary Access Control (DAC) or identity based access control.• Mandatory Access Control (MAC) or role-based access control.

An originator access control (ORCON or ORGON) bases access on the creator of an object.

[Examples pp 103-104]

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Discretionary Access Control (DAC)

Access control is left to the discretion of the owner.Based on the identity of the subject.[Example –see textbook pp 104-105

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Mandatory Access Control (MAC)

The operating system enforces mandatory access controls.Neither the subject nor even the owner can determine access control.Example –see textbook

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ORiginator access CONtrol (ORCON or ORGON)

The originator of the file (or its information) has control the dissemination of its information.Example –see textbook

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Policy languages

• High level policy languages: independent of the mechanisms used.

• Low level policy languages

[Examples pp 104-105]

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High level policy languages

Express policy constraints on entities using abstraction and are independent of the security mechanisms.

This requires:•An unambiguous expression of policy•A mathematical or programming formulation

Details: see textbook. [Examples pp105-106]

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Low level policy languages

A set of inputs or arguments to commands that set or check constraints on a system.

For examples, see textbook

[Examples pp109-110]

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Security and PrecisionEarlier security and precision was defined in terms of the states of the system. We said that security policies were enforced by security mechanisms and that such mechanisms were either secure, precise or broad.Let P be the set of all states, Q the set of secure states and suppose that the mechanism restricts the system to the set of states R . A security mechanism was secure if R Q , precise if R = Q

and broad if there are states such that r R and r Q .

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Security and Precision

We now consider the possibility of devising a generic procedure for developing a mechanism that is security and precise.

For this, we will use programs, which will be viewed as abstract functions that “encode” the information that needs to be controlled.

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Security and PrecisionDefinitionA program p is a function p : I1 … In → R .

p has n inputs ij Ij and one output r R

Axiom (observability postulate)Suppose p does not alter information but merely provides a view of its inputs. We say that p encodes all available information about i1, …,inExample A confidentiality policy seeks to control what views are available.

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Security and PrecisionDefinitionLet p: I1 In → R be a function

1. A protection mechanism m for p is a function m : I1 In → R E

(E is an error message) for which, when ( i1in ) I1 In ,

either

a. m (i1ik = p (i1ik or

b. m (i1ik E .

That is, every “legal” input to m produces either the same value as p or an error message. The set of output values of p that are excluded by m are those outputs thatwould impart confidential information.[Examples p 115]

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Security and Precision

Definition2. A confidentiality policy for the program p :I1 In → R is

a function c :I1 In → A, where A is a subset of I1 In .

Here the set A corresponds to those inputs that may be revealed.The complement of A to the confidential inputs.

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Security and PrecisionDefinitions3. Let c be a confidentiality policy for a program p.

Let m : I1 In → R E be a security mechanism for p.

The mechanism m is secure iff there is a function

m’ : I1 In → R E such that for all (i1in ) I1 In

m (i1ik = m’(c (i1ik)) .

That is, given any set of inputs, the protection mechanism m

returns values consistent with the stated policy c

(here “secure” = “confidential” )

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Security and Precision

DefinitionsLet m1, m2 protection mechanisms for program p

under policy c.4. m1 is as precise as m2 if for all inputs (i1in) :

m2 (i1ik = p (i1ik ═> m1 (i1ik = p (i1ik

5. m1 is more precise than m2 if there is an input

(i1′in′ ) such that :

m2 (i1′in′ ) = p (i1′in′ ) & m1 (i1′in′ ) ≠ p (i1′in′ )

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Security and PrecisionTheorems

1. For any program p there exists a precise secure mechanism m* such that for all secure mechanisms m associated with p and c we have m* = m.

2. There is no effective way that determines a (maximally) precise secure mechanism for any policy and program.