Computer Security Security Policies 7/24/2019

Security Policies We view a computer system as a finite-state machine Definition A 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. 7/24/2019

Example s1 s2 s3 s4 An insecure system Authorized states are s1 and s2 Unauthorized states are s3 and s4 7/24/2019

Security Policies Definition A breach of security occurs when a system enters an unauthorized state. Let X be a set of entities I be some information. I has the property of confidentiality wrt X if no member of X can obtain information about I. I has the property of integrity wrt X if all members of X trust I. Let I be a resource. I has the property of availability wrt X if all member of X can access I. A security mechanism is an entity or procedure that enforces some part of a security policy. 7/24/2019

Types of Policies Definition Military security policies or governmental security policies. Commercial security policies Confidentiality policies Integrity policies Transaction policies Discuss issues regarding trust. 7/24/2019

The role of trust The role of trust is fundamental in understanding the nature of computer security. Examples –see textbook 7/24/2019

Types of Access Control Discretionary Access Control (DAC) or identity based access control. Mandatory Access Control (MAC) or rule-based access control. An originator access control (ORCON or ORGON) bases access on the creator of an object. 7/24/2019

Discretionary Access Control (DAC) Access control is left to the discretion of the owner. Based on the identity of the subject. Example –see textbook 7/24/2019

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 7/24/2019

ORiginator access CONtrol (ORCON or ORGON) The originator of the file (or its information) has control the dissemination of its information. Example –see textbook 7/24/2019

Policy languages High level policy languages: independent of the mechanisms used. Low level policy languages 7/24/2019

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. 7/24/2019

Low level policy languages A set of inputs or arguments to commands that set or check constraints on a system. For examples, see textbook. 7/24/2019

Security and Precision Earlier 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 . 7/24/2019

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. 7/24/2019

Security and Precision Definition A program p is a function p : I1 Χ … Χ In → R. p has n inputs ij e Ij and one output r e R We say that p encodes all available information about i1,…,in (observability postulate). Example Suppose p does not alter information but merely provides a view of its inputs. A confidentiality policy seeks to control what views are available. 7/24/2019

Security and Precision Definitions Let p : I1 Χ … Χ In → R. A protection mechanism m is a function m :I1Χ … ΧIn → R  E (E is an error message) for which, when ( i1,...,in ) e I1Χ … ΧIn, either a. m (i1,...,ik) = p (i1,...,ik) or b. m (i1,...,ik) e 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 that would impart confidential information. 7/24/2019

Security and Precision Definitions A confidentiality policy for the program p :I1 Χ … Χ In → R is a function c :I1 Χ …Χ In → A, where A is a subset of I1 Χ … Χ In . Here the set A corresponds to those inputs that may be revealed. The complement of A to the confidential inputs. 7/24/2019

Security and Precision Definitions Let c be a confidentiality policy for a confidentiality program p. Let m :I1 Χ··· Χ In → R  E be a security mechanism for p. The mechanism m is secure iff there is a function m’ :I1 Χ··· Χ In → R  E such that for all (i1,...,in ) eI1 Χ ··· Χ In: m (i1,...,ik) = m’(c (i1,...,ik)) . That is, given any set of inputs, the protection mechanism m returns values consistent with the stated policy c (here “secure” = “confidential” ) 7/24/2019

Security and Precision Definitions Let m1, m2 protection mechanisms for program p under policy c. m1 is as precise as m2 if for all inputs (i1,...,in) : m2 (i1,...,ik) = p (i1,...,ik) ═> m1 (i1,...,ik) = p (i1,...,ik) 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′ ) 7/24/2019

Security and Precision Theorems 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. There is no effective way that determines a (maximally) precise secure mechanism for any policy and program. 7/24/2019