Correlation immunity

In mathematics, the correlation immunity of a Boolean function is a measure of the degree to which its outputs are uncorrelated with some subset of its inputs. Specifically, a Boolean function is said to be correlation-immune of order m if every subset of m or fewer variables in $x_{1},x_{2},\ldots ,x_{n}$ is statistically independent of the value of $f(x_{1},x_{2},\ldots ,x_{n})$ .

Definition

A function $f:\mathbb {F} _{2}^{n}\rightarrow \mathbb {F} _{2}$ is $k$ -th order correlation immune if for any independent $n$ binary random variables $X_{0}\ldots X_{n-1}$ , the random variable $Z=f(X_{0},\ldots ,X_{n-1})$ is independent from any random vector $(X_{i_{1}}\ldots X_{i_{k}})$ with $0\leq i_{1}<\ldots .

Results in cryptography

When used in a stream cipher as a combining function for linear feedback shift registers, a Boolean function with low-order correlation-immunity is more susceptible to a correlation attack than a function with correlation immunity of high order.

Siegenthaler showed that the correlation immunity m of a Boolean function of algebraic degree d of n variables satisfies m + d ≤ n; for a given set of input variables, this means that a high algebraic degree will restrict the maximum possible correlation immunity. Furthermore, if the function is balanced then m + d ≤ n − 1.