I know how to calculate the entropy of a key that relates to its selection process. For example if the key space is $1000$, entropy of a randomly chosen key is $1000$. Suppose now you have two keys $X$ and $Y$. The finale key $Z$ is a concatenation of $X$ and $Y$. There are two ways to combine. One is to use XOR and other way is to use a Key Based Key Derivation Function (KBKDF) such as HKDF. I want to know how to calculate the entropy of $Z$ when…
- $Z$ = $X$ XOR $Y$
- $Z$ = HKDF($X$,$Y$)
In the XOR case should I use mutual information of $X$ and $Y$?