Information flow policies define the way information moves throughout a system. Typically, these policies are designed to preserve confidentiality of data or integrity of data. In the former, the policy's goal is to prevent information from flowing to a user not authorized to receive it. In the latter, information may flow only to processes that are no more trustworthy than the data.

Any confidentiality and integrity policy embodies an information flow policy.

Let x be a variable in a program. The notation x refers to the information flow class of x.

Intuitively, information flows from an object x to an object y if the application of a sequence of commands c causes the information initially in x to affect the information in y.

The command sequence c causes a flow of information from x to y if, after execution of c, some information about the value of x before c was executed can be deduced from the value of y after c was executed.

This definition views information flow in terms of the information that the value of y allows one to deduce about the value in x. For example, the statement

y := x; 

reveals the value of x in the initial state, so information about the value of x in the initial state can be deduced from the value of y after the statement is executed. The statement

y := x / z; 

reveals some information about x, but not as much as the first statement.

The final result of the sequence c must reveal information about the initial value of x for information to flow. The sequence

tmp := x;  y := tmp; 

has information flowing from x to y because the (unknown) value of x at the beginning of the sequence is revealed when the value of y is determined at the end of the sequence. However, no information flow occurs from tmp to x, because the initial value of tmp cannot be determined at the end of the sequence.

x := y + z; 

if x = 1 then y := 0;  else y := 1;  

An implicit flow of information occurs when information flows from x to y without an explicit assignment of the form y := f(x), where f(x) is an arithmetic expression with the variable x.

The flow of information occurs, not because of an assignment of the value of x, but because of a flow of control based on the value of x. This demonstrates that analyzing programs for assignments to detect information flows is not enough. To detect all flows of information, implicit flows must be examined.

Information Flow Models and Mechanisms

An information flow policy is a security policy that describes the authorized paths along which that information can flow. Each model associates a label, representing a security class, with information and with entities containing that information. Each model has rules about the conditions under which information can move throughout the system.

In this tutorial, we use the notation x <= y to mean that information can flow from an element of class x to an element of class y. Equivalently, this says that information with a label placing it in class x can flow into class y.

The models of information flow policies were lattices. We first consider nonlattice information flow policies and how their structures affect the analysis of information flow. We then turn to compiler-based information flow mechanisms and runtime mechanisms. We conclude with a look at flow controls in practice.