Involutory key for the substitution cipher

Question

I need to find all the involutory keys for the substitution cipher over $\mathbb{Z}_7$. I wasn't sure what can be the key for the substitution cipher. For example, for the affine cipher, $e(x) = kx + a$, $(k,a)$ is the key. However for the substitution cipher, I can't find the corresponding function.