Information Flow (information Theory) - Explicit Flows and Side Channels

Explicit Flows and Side Channels

Information flows can be divided in two major categories. The simplest one is explicit flow, where some secret is explicitly leaked to a publicly observable variable. In the following example, the secret in the variable h flows into the publicly observable variable l.

var l, h l := h

The other flows fall into the side channel category. For example, in the timing attack or in the power analysis attack, the system leaks information through, respectively, the time or power it takes to perform an action depending on a secret value.

In the following example, the attacker can deduce if the value of h is one or not by the time the program takes to finish:

var l, h if h = 1 then (* do some time-consuming work *) l := 0

Another side channel flow is the implicit information flow, which consists in leakage of information through the program control flow. The following program (implicitly) discloses the value of the secret variable h to the variable l. In this case, since the h variable is boolean, all the bits of the variable of h is disclosed (at the end of the program, l will be 3 if h is true, and 42 otherwise).

var l, h if h = true then l := 3 else l := 42

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