Cross correlation of two Boolean functions
In theoretical computer science literature, boolean functions are defined as 
$$
f\colon \operatorname{GF}(2)^n \to \{\pm 1\},
$$
and the usual link with functions defined as
$$
\tilde{f}\colon \operatorname{GF}(2)^n \to \{0,1\}
$$
is via $$f(x)=(-1)^{\tilde{f}(x)}.$$

Edit Here's a standard reference from the Boolean function literature.